Pollution

The cultural definition

The functions of pollution beliefs

Emotional aspects of pollution behavior

BIBLIOGRAPHY

One of the great puzzles in comparative studies of religion has been the reconciliation of the concept of pollution, or defilement, with that of holiness. In the last half of the nineteenth century, Robertson Smith asserted that the religion of primitive peoples developed out of the relation between a community and its gods, who were seen as just and benevolent. Dependent on a sociological approach to religion, Robertson Smith continued always to draw a line between religious behavior, concerned with ethics and gods, and nonreligious, magical behavior. He used the term taboo to describe nonreligious rules of conduct, especially those concerned with pollution, in order to distinguish them from the rules of holiness protecting sanctuaries, priests, and everything pertaining to gods. The latter behavior he held to be intelligible and praiseworthy and the former to be primitive, savage, and irrational—“magical superstition based on mere terror.”

He clearly felt that magic and superstition were not worth a scholar’s attention. But Sir James Frazer, who dedicated The Golden Bough to Robertson Smith, tried to classify and understand the nature of magical thinking. He formulated the two principles of sympathetic magic: action by contagion and action by likeness. Frazer followed Robertson Smith in assuming that magic was more primitive than religion, and he worked out an evolutionary scheme in which primitive man’s earliest thinking was oriented to mechanical ideas of contagion. Magic gradually gave way to another cosmology, the idea of a universe dominated by supernatural beings similar to man but greatly superior to him. Magic thus came to be accepted as a word for ritual which is not enacted within a cult of divine beings. But obviously there is an overlap between nonreligious ideas of contagion and rules of holiness. Robertson Smith accounted for this by making the distinction between holiness and uncleanness a criterion of the advanced religions:

The person under taboo is not regarded as holy, for he is separated from approach to the sanctuary as well as from contact with men, but his act or condition is somehow associated with supernatural dangers, arising, according to the common savage explanation, from the presence of formidable spirits which are shunned like an infectious disease. In most savage societies no sharp line seems to be drawn between the two kinds of taboo…and even in more advanced nations the notions of holiness and uncleanness often touch…[to] distinguish between the holy and the unclean, marks a real advance above savagery. ([1889] 1927, p. 153)

Frazer echoes the notion that confusion between uncleanness and holiness marks primitive thinking. In a long passage in which he considers the Syrian attitude to pigs, he concludes: “Some said this was because the pigs were unclean; others said it was because the pigs were sacred. This…points to a hazy state of religious thought in which the ideas of sanctity and uncleanness are not yet sharply distinguished, both being blent in a sort of vaporous solution to which we give the name of taboo’’ ([1890] 1955, vol. 2, part 5, p. 23).

The work of several modern-day students of comparative religion derives not directly from Frazer but from the earlier work of Durkheim, whose debt to Robertson Smith is obvious in many ways. On the one hand, Durkheim was content to ignore aspects of defilement which are not part of a religious cult. He developed the notion that magical injunctions are the consequence of primitive man’s attempt to explain the nature of the universe. Durkheim suggested that experimentation with magical injunctions, having thus arisen, has given way to medical science. But on the other hand, Durkheim tried to show that the contagiousness of the sacred is an inherent, necessary, and peculiar part of its character.

His idea of the sacred as the expression of society’s awareness of itself draws heavily on Robertson Smith’s thesis that man’s relation to the gods, his religious behavior, is an aspect of prescribed social behavior. It followed, for Durkheim, that religious ideas are different from other ideas. They are not referable to any ultimate material reality, since religious shrines and emblems are only themselves representations of abstract ideas. Religious experience is an experience of a coercive moral force. Consequently, religious ideas are volatile and fluid; they float in the mind, unattached, and are always likely to shift, or to merge into other contexts at the risk of losing their essential character: there is always the danger that the sacred will invade the profane and the profane invade the sacred. The sacred must be continually protected from the profane by interdictions. Thus, relations with the sacred are always expressed through rituals of separation and demarcation and are reinforced with beliefs in the danger of crossing forbidden boundaries. [See the biographies ofDurkheim; Frazer; Smith, William Robertson.]

If contemporary thinkers were not already well prepared to accept the idea that “religious” restrictions were utterly different from primitive superstitions about contagion, this circular distinction between two kinds of contagion could hardly have gone unchallenged. How can it be argued that contagiousness is the peculiar characteristic of ideas about the sacred when another kind of contagiousness has been bracketed away by definition as irrelevant?

This criticism of Durkheim’s treatment of sacred contagion is implicit in Levy-Bruhl’s massive work on primitive mentality (1922). Levy-Bruhl documented a special kind of outlook on the universe, one in which the power to act and to be acted upon regardless of restrictions of space and time is widely attributed to symbolical representations of persons and animals. He himself explained the belief in such remote contagion by the dominance of the idea of the supernatural in the primitive view of the world. And since he would expect “supernatural” to be equated with Durkheim’s “sacred,” he seems to have seen no conflict between his and the master’s views. [See the biography ofLevybruhl.]

We cannot accept Durkheim’s argument that there are two kinds of contagion, one the origin of primitive hygiene and the other intrinsic to ideas about the sacred, because it is circular. If we approach the problem of contagion in Levy-Bruhl’s terms, then the scope of the answer is broadened : there is not simply a residual area of magical behavior that remains to be explained after primitive religious behavior has been understood but rather a whole mentality, a view of how the universe is constituted. This view of the universe differs essentially from that of civilized man in that sympathetic magic provides the key to its control. Levy-Bruhl is open to criticism; his statement of the problem is oversimple. He bluntly contrasts primitive mentality with scientific thought, not fully appreciating what a rare and specialized activity scientific thinking is and in what well-defined and isolated conditions it takes place. His use of the word “prelogical” in his first formulation of primitive thinking was unfortunate, and he later discarded it. But although his work seems to be discredited at present, the general problem still stands. There is a whole class of cultures, call them what you will, in which great attention is paid to symbolic demarcation and separation of the sacred and the profane and in which dangerous consequences are expected to follow from neglect of the rituals of separation. In these cultures lustrations, fumigations, and purifications of various kinds are applied to avert the dangerous effect of breach of the rules, and symbolic actions based on likeness to real causes are used as instruments for creating positive effects.

The cultural definition

If we are not to follow Robertson Smith in treating the rules of uncleanness as irrational and beyond analysis, we need to clear away some of the barriers which divide up this whole field of inquiry. While the initial problem is posed by the difference between “our” kind of thinking and “theirs,” it is a mistake to treat “us” the moderns and “them” the ancients as utterly different. We can only approach primitive mentality through introspection and understanding of our own mentality. The distinction between religious behavior and secular behavior also tends to be misleadingly rigid. To solve the puzzle of sacred contagion we can start with more familiar ideas about secular contagion and defilement. In English-speaking cultures, the key word is the ancient, primitive, and still current “dirt.” Lord Chesterfield defined dirt as matter out of place. This implies only two conditions, a set of ordered relations and a contravention of that order. Thus the idea of dirt implies a structure of ideas. For us dirt is a kind of compendium category for all events which blur, smudge, contradict, or otherwise confuse accepted classifications. The underlying feeling is that a system of values which is habitually expressed in a given arrangement of things has been violated.

This definition of defilement avoids some historical peculiarities of Western civilization. For example, it says nothing about the relation between dirt and hygiene. We know that the discovery of pathogenic organisms is recent, but the idea of dirt antedates the idea of pathogenicity. It is therefore more likely to have universal application. If we treat all pollution behavior as the reaction to any event likely to confuse or contradict cherished classifications, we can bring two new approaches to bear on the problem: the work of psychologists on perception and of anthropologists on the structural analysis of culture.

Perception is a process in which the perceiver actively interprets and, in the course of his interpreting, adapts and even supplements his sensory experiences. Hebb has shown that in the process of perception, the perceiver imposes patterns of organization on the masses of sensory stimuli in the environment (1949; 1958). The imposed pattern organizes sequences into units–fills in missing events which would be necessary to justify the recognition of familiar units. The perceiver learns to adjust his response to allow for modification of stimuli according to changes in lighting, angle of regard, distance, and so forth. In this way the learner develops a scheme or structure of assumptions in the light of which new experiences are interpreted. Learning takes place when new experience lends itself to assimilation in the existing structure of assumption or when the scheme of past assumptions is modified in order to accommodate what is unfamiliar. In the normal process of interpretation, the existing scheme of assumptions tends to be protected from challenge, for the learner recognizes and absorbs cues which harmonize with past experience and usually ignores cues which are discordant. Thus, those assumptions which have worked well before are reinforced. Because the selection and treatment of new experiences validates the principles which have been learned, the structure of established assumptions can be applied quickly and automatically to current problems of interpretation. In animals this stabilizing, selective tendency serves the biological function of survival. In men the same tendency appears to govern learning. If every new experience laid all past interpretations open to doubt, no scheme of established assumptions could be developed and no learning could take place.

This approach may be extended to the learning of cultural phenomena. Language, for example, learned and spoken by individuals, is a social phenomenon produced by continuous interaction between individuals. The regular discriminations which constitute linguistic structure are the spontaneous outcome of continual control, exercised on an individual attempting to communicate with others. Expressions which are ambiguous or which deviate from the norm are less effective in communication, and speakers experience a direct feedback encouraging conformity. Language has more loosely and more strictly patterned domains in which ambiguity has either more or less serious repercussions on effective communication. Thus there are certain domains in which ambiguity can be better tolerated than in others (Osgood & Sebeok 1954, p. 129).

Similar pressures affect the discrimination of cultural themes. During the process of enculturation the individual is engaged in ordering newly received experiences and bringing them into conformity with those already absorbed. He is also interacting with other members of his community and striving to reduce dissonance between his structure of assumptions and theirs (Festinger 1957). Frenkel-Brunswik’s research among schoolchildren who had been variously exposed to racial prejudice illustrates the effects of ambiguity on learning at this level. The children listened to stories which they were afterwards asked to recall. In the stories the good and bad roles were not consistently allocated to white and Negro characters. When there was dissonance between their established pattern of assumptions about racial values and the actual stories they heard, an ambiguous effect was received. They were unable to recall the stories accurately. There are implications here for the extent to which a culture (in the sense of a consistent structure of themes, postulates, and evaluations) can tolerate ambiguity. It is now common to approach cultural behavior as if it were susceptible to structural analysis on lines similar to those used in linguistics (Levi-Strauss 1958; Leach 1961). For a culture to have any recognizable character, a process of discrimination and evaluation must have taken place very similar to the process of language development–with an important difference. For language the conditions requiring clear verbal communication provide the main control on the pattern which emerges, but for the wider culture in which any language is set, communication with others is not the only or principal function. The culture affords a hierarchy of goals and values which the community can apply as a general guide to action in a wide variety of contexts. Cultural interaction, like linguistic interaction, involves the individual in communication with others. But it also helps the individual to reflect upon and order his own experience.

The general processes by which language structure changes and resists change have their analogues at the higher level of cultural structure. The response to ambiguity is generally to encourage clearer discrimination of differences. As in language, there are different degrees of tolerance of ambiguity. Linguistic intolerance is expressed by avoidance of ambiguous utterances and by pressure to use well-discriminated forms where differences are important to interpretation and appropriate responses. Cultural intolerance of ambiguity is expressed by avoidance, by discrimination, and by pressure to conform.

The functions of pollution beliefs

To return to pollution behavior, we have already seen that the idea of dirt implies system. Dirt avoidance is a process of tidying up, ensuring that the order in external physical events conforms to the structure of ideas. Pollution rules can thus be seen as an extension of the perceptual process: insofar as they impose order on experience, they support clarification of forms and thus reduce dissonance.

Much attention has been paid to the sanctions by which pollution rules are enforced (see Steiner 1956, p. 22). Sometimes the breach is punished by political decree, sometimes by attack on the transgressor, and sometimes by grave or trivial sanctions; the sanction used reflects several aspects of the matter. We can assume that the community, insofar as it shares a common culture, is collectively interested in pressing for conformity to its norms. In some areas of organization the community is capable of punishing deviants directly, but in others this is not practicable. This may happen, for example, if political organization is not sufficiently developed or if it is developed in such a way as to make certain offenses inaccessible to police action. Homicide is a type of offense which is variously treated according to the relationship between killer and victim. If the offender is himself a member of the victim’s group and if this is the group which is normally entrusted with protection of its members’ interests, it may be held contradictory and impossible for the group to inflict punishment. Then the sanction is likely to be couched in terms of a misfortune that falls upon the offender without human intervention. This kind of homicide is treated as a pollution.

We would expect to find that the pollution beliefs of a culture are related to its moral values, since these form part of the structure of ideas for which pollution behavior is a protective device. But we would not expect to find any close correspondence between the gravity with which offenses are judged and the danger of pollution connected with them. Some moral failings are likely to be met with prompt and unpleasant social consequences. These self-punishing offenses are less likely to be sanctioned by pollution beliefs than by other moral rules. Pollution beliefs not only reinforce the cultural and social structure, but they can actively reduce ambiguity in the moral sphere. For example, if two moral standards are applied to adultery, so that it is condemned in women and tolerated in men, there will inevitably be some ambiguity in the moral judgment since adultery involves a man and a woman. A pollution belief can reduce the ambiguity. If the man is treated as dangerously contagious, his adulterous condition, while not in itself condemned, endangers the outraged husband or the children; moral support can be mustered against him. Alternatively, if attention is focused on the pollution aspect of the case, a rite of purification can mitigate the force of the moral condemnation.

This approach to pollution allows further applications of Durkheimian analysis. If we follow him in assuming that symbolism and ritual, whether strictly religious or not, express society’s awareness of its own configuration and necessities, and if we assume that pollution rules indicate the areas of greater systematization of ideas, then we have an additional instrument of sociological analysis. Durkheim held that the dangerous powers imputed to the gods are, in actual fact, powers vested in the social structure for defending itself, as a structure, against the deviant behavior of its members. His approach is strengthened by including all pollution rules and not merely those which form part of the religious cult. Indeed, deriving pollution behavior from processes similar to perception comes close to Durkheim’s intention of understanding society by developing a social theory of knowledge.

Pollution rules in essence prohibit physical contact. They tend to be applied to products or functions of human physiology; thus they regulate contact with blood, excreta, vomit, hair clippings, nail clippings, cooked food, and so on. But the anthropologist notes that the incidence of beliefs in physiological pollution varies from place to place. In some communities menstrual pollution is gravely feared and in others not at all; in some, pollution by contact with the dead is feared, in others pollution of food or blood. Since our common human condition does not give rise to a common pattern of pollution observances, the differences become interesting as an index of different cultural patterning. It seems that physiological pollutions become important as symbolic expressions of other undesirable contacts which would have repercussions on the structure of social or cosmological ideas. In some societies the social definition of the sexes is more important than in others. In some societies social units are more rigorously defined than in others. Then we find that physical contact between sexes or between social units is restricted even at second or third remove. Not only may social intercourse be restricted, but sitting on the same chair, sharing the same latrine, or using the same cooking utensils, spoons, or combs may be prohibited and negatively sanctioned by pollution beliefs. By such avoidances social definitions are clarified and maintained. Color bars and caste barriers are enforced by these means. As to the ordered relation of social units and the total structure of social life, this must depend on the clear definition of roles and allegiances. We would therefore expect to find pollution concepts guarding threatened disturbances of the social order. On this, nearly everything has been said by van Gennep. His metaphor of society as a kind of house divided into rooms and corridors, the compartments carefully isolated and the passages between them protected by ceremonial, shows insight into the social aspects of pollution. So also does his insistence on the relative character of the sacred:

Sacredness as an attribute is not absolute; it is brought into play by the nature of particular situations. . . . Thus the “magic circles” pivot, shifting as a person moves from one place in society to another. The categories and concepts which embody them operate in such a way that whoever passes through the various positions of a lifetime one day sees the sacred where before he has seen the profane, or vice versa. Such changes of condition do not occur without disturbing the life of society and the individual, and it is the function of rites of passage to reduce their harmful effects. (Gennep [1909] 1960, pp. 12–13)

Van Gennep saw that rites of transition treat all marginal or ill-defined social states as dangerous. His treatment of margins is fully compatible with the sociological approach to pollution. But van Gennep’s ideas must be vastly expanded. Not only marginal social states, but all margins, the edges of all boundaries which are used in ordering the social experience, are treated as dangerous and polluting.

Rites of passage are not purificatory but are prophylactic. They do not redefine and restore a lost former status or purify from the effect of contamination, but they define entrance to a new status. In this way the permanence and value of the classifications embracing all sections of society are emphasized.

When we come to consider cosmological pollution, we are again faced with the problem unresolved by Lévy-Bruhl. Cosmological pollution is to the Westerner the most elusive, yet the most interesting case. Our own culture has largely given up the attempt to unify, to interpenetrate, and to crossinterpret the various fields of knowledge it encompasses. Or rather, the task has been taken over by natural science. A major part of pollution behavior therefore lies outside the realm of our own experience: this is the violent reaction of condemnation provoked by anything which seems to defy the apparently implicit categories of the universe. Our culture trains us to believe that anomalies are only due to a temporarily inadequate formulation of general natural laws. We have to approach this kind of pollution behavior at second hand.

The obvious source of information on the place of cosmic abnormality in the mind of the primitive is again Levy-Bruhl. Earthquakes, typhoons, eclipses, and monstrous births defy the order of the universe. If something is thought to be frightening because it is abnormal or anomalous, this implies a conception of normality or at least of categories into which the monstrous portent does not fit. The more surprising that anomaly is taken to be, the clearer the evidence that the categories which it contradicts are deeply valued.

At this point we can take up again the question of how the culture of civilization differs from that which Lévy-Bruhl called primitive. Recalling that dirt implies system and that pollution beliefs indicate the areas of greatest systematization, we can assume that the answer must be along the same lines. The different elements in the primitive world view are closely integrated; the categories of social structure embrace the universe in a single, symbolic whole. In any primitive culture the urge to unify experience to create order and wholeness has been effectively at work. In “scientific culture” the apparent movement is the other way. We are led by our scientists to specialization and compartmentalism of spheres of knowledge. We suffer the continual breakup of established ideas. Levy-Bruhl, looking to define the distinction between the scientific and the primitive outlook, would have been well served if he had followed Kant’s famous passage on his own Copernican revolution. Here Kant describes each great advance in thought as a stage in the process of freeing “mind” from the shackles of its own subjective tendencies. In scientific work the thinker tries to be aware of the provisional and artificial character of the categories of thought which he uses. He is ready to reform or reject his concepts in the interests of making a more accurate statement.

Any culture which allows its guiding concepts to be continually under review is immune from cosmological pollutions. To the extent that we have no established world view, our ways of thinking are different from those of people living in primitive cultures. For the latter, by long and spontaneous evolution, have adapted their patterns of assumption from one context to another until the whole of experience is embraced. But such a comprehensive structure of ideas is precarious to the extent that it is an arbitrary selection from the range of possible structures in the same environment. Other ways of dividing up and evaluating reality are conceivable. Hence, pollution beliefs protect the most vulnerable domains, where ambiguity would most weaken the fragile structure.

Emotional aspects of pollution behavior

Pollution beliefs are often discussed in terms of the emotions which they are thought to express. But there is no justification for assuming that terror, or even mild anxiety, inspires them any more than it inspires the housewife’s daily tidying up. For pollution beliefs are cultural phenomena. They are institutions that can keep their forms only by bringing pressure to bear on deviant individuals. There is no reason to suppose that the individual in a primitive culture experiences fear, still less unreasoning terror, if his actions threaten to modify the form of the culture he shares. His position is exactly comparable to a speaker whose own linguistic deviations cause him to produce responses which vary with his success in communicating. The dangers and punishments attached to pollution act simply as means of enforcing conformity.

As to the question of the rational or irrational character of rules of uncleanness, Robertson Smith is shown to have been partly right. Pollution beliefs certainly derive from rational activity, from the process of classifying and ordering experience. They are, however, not produced by strictly rational or even conscious processes but rather as a spontaneous by-product of these processes.

Mary Douglas

[See alsoCaste; Communication; Magic; Myth and Symbol; Rltual.]

BIBLIOGRAPHY

Douglas, Mary 1966 Purity and Danger: A Comparative Study of Concepts of Pollution and Taboo. London: Routledge.

Durkheim, Émile (1912)1954 The Elementary Forms of the Religious Life. London: Allen & Unwin; New York: Macmillan. → First published as Les formes elementaires de la vie religieuse, le systeme totemique en Australie. A paperback edition was published in 1961 by Collier.

Eliade, Mircea (1957) 1959 The Sacred and the Profane: The Nature of Religion. New York: Harcourt. → First published in German. A paperback edition was published in 1961 by Harper.

Festinger, Leon 1957 A Theory of Cognitive Dissonance. Evanston, III.: Row.

Frazer, James (1890) 1955 The Golden Bough: A

Study in Magic and Religion. 3rd ed., rev. & enl. 13 vols. New York: St. Martins, London: Macmillan. → An abridged edition was published in 1922 and reprinted in 1955.

Frenkel-Brunswik, Else 1949 Intolerance of Ambiguity as an Emotional and Perceptual Personality Variable. Journal of Personality 18:108–143.

Gennep, Arnold Van (1909) 1960 The Rites of Passage. London: Routledge; Univ. of Chicago Press. → First published in French.

Hebb, Donald O. 1949 The Organization of Behavior: A Neuropsychological Theory. New York: Wiley. Hebb, Donald O. 1958 A Textbook of Psychology. Philadelphia: Saunders.

Leach, Edmund R. 1961 Rethinking Anthropology. London School of Economics and Political Science Monographs on Social Anthropology, No. 22. London: Athlone.

LÉvi-Strauss, Claude (1958) 1963 Structural Anthropology. New York: Basic Books. → First published in French.

LÉvy-Bruhl, Lucien (1910) 1926 How Natives Think. London: Allen & Unwin. → First published as Les fonctions mentales dans les societes primitives.

LÉvy-Bruhl, Lucien (1922) 1923 Primitive Mentality. Macmillan → First published in French.

Osgood, Charles E.; and Sebeok, Thomas A. (editors) (1954) 1965 Psycholinguistics: A Survey of Theory and Research Problems. Bloomington: Indiana Univ. Press.

Smith, William Robertson (1889) 1927 Lectures on the Religion of the Semites. 3d ed. New York: Mao millan.

Steiner, Franz 1956 Taboo. New York: Philosophical Library.

POLLUTION

Most often used in regard to the natural environment, the term pollute means to make foul or unclean, degrade ecological and/or human health, contaminate or defile, and, in a religious sense, render ceremonially impure or desecrate. The verb pollute derives from the Middle English polute, and this from the Latin pollūt(us), the past participle of polluere, which meant to soil, defile. Pollution generally denotes an undesirable condition, where there is too much of something (the pollutant or contaminant) in a natural or other beneficial system. It is, then, not an objectively determined state of affairs. Rather decisions about pollution require both science (for example, identification, monitoring, and classification) and ethics and politics (such as debate about what is undesirable, what is acceptable, who should monitor pollutants, and who should be held accountable). Although pollution is, in a sense, an unavoidable byproduct of human (and nonhuman) activity, it was not until the Industrial Revolution that it regularly occurred on a large-scale and became a public policy issue.

Measures have been taken to curb pollution, especially the public health activities in the 1800s and then again with the rise of the contemporary environmental movement in the 1960s. Largely due to political and economic incentives and advances in technology, many pollutants are declining. In several regions, however, pollution remains a serious problem threatening both human and environmental health. Pollution has long been seen as the most visible and costly reminder of a downside to the technological mastery of nature. The use of technologies to prevent and diminish pollution, however, may eventually eliminate this particular cause for technological pessimism.

Classifying and Describing Pollution

Environmental pollution can be either point source (such as emissions from factory smokestacks) or non-point source (for example, fertilizers and oil washed from lawns and parking lots into streams). It can occur suddenly, as in the massive radioactive plume released from a nuclear power plant in Chernobyl in 1986 or the 1.26 million barrels of oil spilled into Prince William Sound, Alaska, by the Exxon Valdez in 1989. However pollution usually stems from long-term emissions, as in the accumulation of carbon dioxide (CO₂) in the atmosphere resulting from fossil fuel combustion. Although most pollution is anthropogenic (human caused), some forms are naturally occurring. One example is radon gas that leaks from rocks into buildings and accounts for roughly 55 percent of the total radiation dose received by an average person in the United States. Anthropogenic and nonanthropogenic sources of pollution can also combine to produce deleterious environmental and health effects. One example is the combination of human-produced chlorofluorocarbons (CFCs) and naturally occurring ultraviolet radiation in the stratosphere, which initiates a reaction that depletes ozone. The methane gas produced by cows (and other farm animals) accounts for roughly 20 percent of all such global emissions. Although this is a natural source of pollution, the vast quantity of these animals would not exist without humans.

Any compound can be considered a pollutant if it is judged to exist either in excessive quantities or in the wrong place. For example, ozone in the stratosphere is regarded as beneficial, whereas ozone in the troposphere (the lowest layer of the atmosphere) is regarded as a pollutant because it contributes to smog, which causes harmful ecological, human health, and aesthetic effects. This is akin to exotic species, which can be considered pollutants, because they are located outside the boundaries of the area in which they evolved.

Pollution can be classified by economic sector (such as residential, industry, agriculture, transportation, and others), which can be helpful for regional governments implementing pollution reduction policies, but the relative contribution of each source varies markedly depending on the composition of regional economies. A more universalizable classificatory scheme groups pollutants according to the reservoir in which they are found: water, soil, air, and space. More detailed cataloging can then be carried out. Water pollution, for example, is typically sorted by type, including biological/pathogens, sedimentation, nutrients, toxic synthetic chemicals, and heat/cold. Water quality indicators include hardness (a measure of dissolved minerals), pH, temperature, dissolved oxygen, turbidity, and smell. The U.S. Environmental Protection Agency (EPA) has set national air quality standards based on six common (referred to as criteria) air pollutants: ozone, nitrogen dioxide, sulfur dioxide, particulate matter, carbon monoxide, and lead. Pollution in space (scraps from old satellites, rockets, or other spacecraft) is generally classified by size. The vast majority of the hundreds of thousands of human-produced particles in space are between one and ten centimeters in diameter, but even these small pieces have caused massive damage to satellites. Not included in the above list, but worthy of mention, is indoor air pollution (especially caused by smoking and the use of wood or coal-burning stoves), which poses grave health risks. Noise and even drugs can be considered pollutants insofar as they can have deleterious effects on the well-being of humans and other animals.

The severity of a pollutant depends upon its chemical nature, concentration, and persistence. One important equation used by environmental scientists to understand pollution derives from biogeochemical cycling: Residence time = Reservoir size/Sum of all fluxes (in or out of the reservoir). A reservoir is simply any "compartment" that can serve as a storage place for pollutants. Examples include the ocean, atmosphere, and biosphere. Reservoirs can be defined more precisely depending on the pollutant or other compound of interest. For example, scientists interested in particulate organic carbon may choose to focus only on the upper ocean (where the majority of carbon is located). Flux refers to the rate at which the pollutant (or other compound of interest) moves in and out of the reservoir. Residence time is how long the pollutant stays in the reservoir of interest. CO₂, for instance, has a long residence time in the atmosphere, such that even if all emissions were immediately stopped, CO₂ levels would drop only very slowly. Sulfur dioxide (SO₂), by contrast, is water soluble, and because water has a relatively short residence time in the atmosphere (less than five days), SO₂ will quickly precipitate out.

This explains why CO₂ emissions pose problems on a global scale (because it stays in the atmosphere long enough to thoroughly mix around the globe), whereas SO₂ emissions pose problems on a regional scale (because it will precipitate out of the atmosphere somewhere within five days downwind of the source). The equation also helps explain why groundwater pollution is so much more difficult to clean up than surface water pollution. Groundwater aquifers have very low fluxes, meaning residence times for pollutants are quite high. Surface water systems, for the most part, have high fluxes, meaning that pollutants can be quickly flushed through the system.

Ethics and Deciding How Much is Too Much

Classifying and describing the behavior of pollutants in natural systems still leaves many questions unanswered, including: How much pollution should be allowed? What should count as pollution? How should societies determine the relative values of risks to people's health and other matters of concern (for example, ecosystem integrity and aesthetics), or, how should they determine when there is too much of something, thus turning simple presence into pollution?

Pollution may result in injustice, because its effects can be disproportionately suffered by the poor. For example, poor people can often only afford to live in neighborhoods that are crowded with polluting industries, yet they seldom have the resources to challenge polluters in the court system (the Erin Brockovich case is an exception to this rule). Similarly global climate change resulting from CO₂ emissions (significantly produced by wealthy nations) may have the most devastating impacts on poor nations unable to adapt to rising sea levels and other effects. Welfare economists conceptualize pollution as a problem of establishing the proper costs so that its effects are fairly distributed.

Despite these injustices and the more general detrimental effects of pollution, several economic theorists and philosophers have made a strong case that the proper reaction is not to eliminate pollution, but rather find the optimal amount of pollution. Julian Simon (1981) and his successor Bjørn Lomborg (1998) argue that economics correctly views pollution as a trade-off between cost and cleanliness. This has two main implications. First, the goal is not pristine, pollution-free environments, but rather an environment that is optimally clean in the sense that, at this point, citizens would rather pay for some other service or good than more pollution abatement (this is the willingness-to-pay criterion for determining optimal pollution). Second, measuring the goal of optimal pollution would best be accomplished by some metric of human welfare such as life expectancy.

Both Lomborg and Simon argue that pollution does not undermine human well-being in the long run. Although air pollution continues to worsen in developing nations, they are just making the same trade-offs that developed nations did during the Industrial Revolution. Indeed, as Lomborg (1998) states, "the environment and economic prosperity are not opposing entities: without adequate environmental protection, growth is undermined, but environmental protection is unaffordable without growth" (p. 210). Thus following the path of developed nations, as the developing world achieves higher levels of income, it will choose and be able to afford an ever cleaner environment.

William Baxter (1974) echoes Simon and Lomborg by contending that only humans should count in the calculus of determining optimal pollution. This does not mean that other species will be wantonly destroyed, he maintains, because humans both depend on them and enjoy them for aesthetic and recreational reasons. It does mean, however, that the claim "DDT use is damaging penguin populations" does not automatically mean that people must stop the use of DDT. In order for this result to follow, Baxter claims that it must be shown that the well-being of people would be less impaired by discontinuing the use of DDT than by harming penguins. This conclusion is rejected by theorists such as Aldo Leopold (1949), who argued that humans must take the integrity, stability, and beauty of the biotic community directly into account when making decisions that impact the environment. Indeed perspectives and values play an enormous role in how one perceives pollution and the state of the planet. For example, the controversies aroused by the works of Simon and Lomborg show that measuring pollution is as much a political as a technical endeavor.

From a traditional economic standpoint, pollution can be classified as an externality, that is, an unintended and unaccounted for spillover effect on an unconsenting third party. A good example is industrial activities in the Midwestern United States leading to acid rain in the Northeastern United States and Canada. This definition logically leads to attempts to fix market failures (instances where not all costs are appropriately taken into account). Thus environmental economists attempt to quantify the costs of pollution and integrate them into market transactions. Many models use the willingness-to-pay criterion or cost-benefit analyses to establish these costs. The philosopher Mark Sagoff (1988) argued this form of economics and its narrow notion of physical spillover would not rule out many projects or policies that might seem appalling, for example, the attempted conversion of Mineral King Valley in California into a Disney resort. Such a narrow notion leaves no room for many aesthetic and ethical values.

This led economists, especially since the 1970s, to replace the notion of physical spillover with that of transaction or bargaining cost in evaluating the efficiency of a project or a policy concerning pollution. The focus was thereby widened to cover any unpriced benefit or cost (that is, anything a person might be willing to pay for) even if markets do not typically price it correctly. However, as Sagoff also argues, if the wider, more recent notion of transaction or bargaining cost is used, then economic calculations establish policy goals, in the process reducing factual, moral, and aesthetic judgments to mere preferences. But economists have replied that these economic analyses are assessment tools, not decision-making mechanisms. Whatever method of analysis, policymakers need information and tools that will allow them to examine more explicitly and precisely, whether quantitatively or qualitatively, what those affected value about their programs, and how the value of these programs can be assessed. Even if cost-benefit analysis and willingness-to-pay are inadequate, the question remains: How to value?

Sagoff argues that traditional economic methods for determining optimal pollution are insufficient because they place individuals in the role of mere consumers or bidders. Instead he claims each individual should play the role of citizen or trustee of one's own and others' health and well-being. On this view, questions should aim to determine not what individuals would be willing to pay for their health and well-being, but what they would exchange these things for. That is, a willingness-to-sell criterion should be used. This implies that citizens have property rights to an unpolluted environment, thus assigning them the role of sellers and not mere bidders. As such, they may be unwilling to sell those rights or willing to sell them only at a much higher price than they would have been willing to pay for them. One question is whether this leaves room for consent and respect of property rights.

Solving Pollution Problems

The natural reaction to pollution problems by polities has been to use command-and-control style regulations and legislation. Indeed, around 300 c.e. local Roman magistrates passed laws regulating certain sources of air pollution in York, England, and in 1272 Edward I banned the use of sea coal, while parliament ordered punishment by torture and hanging of people who sold and burned the outlawed coal. The rise in environmental consciousness in the United States in the early 1970s saw the continuation of this trend as government legislation and agencies multiplied to prevent and decrease pollution. Some examples include the National Environmental Policy Act, 1969; the creation of the EPA, 1970; Clean Air Act Amendments, 1970 and 1977; the Clean Water Act, 1972 and amended in 1977; the Endangered Species Act, 1973; and the Toxic Substances Control Act, 1976.

Pollution does not respect political borders, however, and transboundary issues have increasingly required international cooperation in the development of pollution regulations. One notable example is the UN Framework Convention on Climate Change (Framework Convention), formally established in March 1994, which is a constitutive body specifying rules for making decisions about global climate change. Its major outcome is the 1997 Kyoto Protocol, which attempted to prescribe legally binding targets and timetables for emissions reductions. The most successful example of international cooperation to control pollution is the 1987 Montreal Protocol on Substances that Deplete the Ozone Layer.

Although government approaches to pollution problems often result in important successes, they also betray the fact that there are governmental failures just as there are market failures. Several reasons for these failures exist. On the international level, bodies such as the Framework Convention often lack political power. Bureaucrats, like all people, are self-interested, and when governmental structures are not designed to link authority with responsibility for program outcomes, "decision makers have few incentives to consider the full social costs of their actions" (Baden and Stroup 1981, p. v). Furthermore decision makers have only a limited capacity to comprehend complex social and environmental interactions, which can constrain their ability to make wise regulatory decisions.

One response has been to improve the structure of government, but another reaction has been to improve the structure of markets by implementing what Terry Anderson and Donald Leal term Free Market Environmentalism (1991). The underlying philosophy of this regulatory approach is that markets and environmental concerns can be made compatible by internalizing costs and establishing the proper incentives. They write, "Instead of intentions, good resource stewardship depends on how well social institutions harness self-interest through individual incentives" (p. 4). Examples of utilizing market mechanisms for pollution abatement include green taxes, marketable emissions permits (for example, cap-and-trade systems), and the elimination of harmful government subsidies. Command-and-control and free market regulatory strategies can often be used in conjunction to achieve desired outcomes. One example is cost-effectiveness analysis, where courts or legislatures establish goals, but economists utilize cost-benefit analyses to establish the cheapest ways of attaining those independently set goals within the market.

Technological innovations have been a major force in pollution prevention and abatement as industry has been either forced to comply with regulations or more subtly incentivized to increase efficiencies and reduce pollution outputs. For example, smokestack scrubbers and catalytic converters in automobiles mitigate pollution problems originally caused by the technologies of electricity generation and automobile transportation. Although instances of technological fixes to pollution problems abound (as well as technological devices to monitor pollution), it is also true that technologies continually present novel pollution problems. This holds for the thousands of novel synthetic chemicals produced every year (of which very little is known about possible long-range health impacts) as well as potential future scenarios such as the emergence of grey goo, unrestrained nanobot replication, that could potentially wreak havoc on human and environmental health (see Joy 2000). Such devastating possibilities (not to mention the realities of disasters such as the deadly poison leaked from the Union Carbide insecticide plant in Bhopal, India in 1984) cause some to argue for the relinquishment of potentially harmful technologies or even the abandonment of industrial capitalism and the modern way of life (see for instance Bradford 2001). Others claim that society must develop defensive technologies in an arms race to stay ahead of destructive technologies. For example, Ray Kurzweil (2003) envisions blue goo, police nanobots that combat the bad nanobots, as the solution to potential unrestrained nanotechnology self-replication.

For the most part, society has come a long way from the 1952 killer smog in London, which caused an estimated 4,000 deaths in a three-day period. As Lomborg (1998) asserts, London has not been as clean as it is now since 1585. Systems thinking is also catching on in the form of industrial ecology, material flows assessments, and product life-cycle analyses. Yet all is not well. Developing nations are at least temporarily experiencing high levels of pollution as they begin to industrialize. Poor peoples, even in developed nations, continue to suffer disproportionate hazards from pollution. Radioactive waste and CO₂ emissions remain long-term issues with potentially disastrous outcomes. In both of these cases, it has become apparent that the political challenges of altering behavior, making trade-offs between competing goods, and finding common ground in contexts marked by a plurality of values is even more daunting than the technical challenges presented by pollution. Work is needed in crafting flexible, democratic mechanisms for deciding optimal levels of pollution.

A. PABLO IANNONE

SEE ALSO Automobiles;Aviation Regulatory Agencies;Conservation and Preservation;Dams;Ecology;Environmental Ethics;Environmental Regulatory Agencies;Global Climate Change;Three Gorges Dam;United Nations Environmental Program;Waste;Water.

BIBLIOGRAPHY

Anderson, Terry, and Donald Leal. (1991). Free Market Environmentalism. San Francisco: Westview Press. The first comprehensive treatment of the idea that free markets can achieve environmental goals.

Baden, John, and Richard Stroup. (1981). Bureaucracy vs. Environment: The Environmental Costs of Bureaucratic Governance. Ann Arbor: University of Michigan Press. Outlines the failures of bureaucratic solutions to environmental problems and offers suggestions for reforms based on aligning incentives with valued outcomes.

Baxter, William F. (1974). People or Penguins: The Case for Optimal Pollution. New York: Columbia University Press. Argues that a pollution-free society cannot exist without harming people.

Bradford, George. (2001). "We All Live in Bhopal." In Environmental Ethics: Readings in Theory and Application, 3rd edition, ed. Louis P. Pojman. Belmont, CA: Wadsworth.

Joy, Bill. (2000). "Why the Future Doesn't Need Us." Wired 8(4): 238–262. Also available from http://www.wired.com/wired/archive/8.04/joy.html. A pessimistic outlook on the impending loss of human control as genetics, nanotechnology, and robotics develop the ability to self-replicate.

Kurzweil, Ray. (2003). "Promise and Peril." In Living with the Genie: Essays on Technology and the Quest for Human Mastery, ed. Alan Lightman, Daniel Sarewitz, and Christina Desser. London: Island Press. Examines mid- and long-term hopes and dangers presented by technologies in the field of genetics, nanotechnology, and robotics.

Leopold, Aldo. (1949). A Sand County Almanac: And Sketches Here and There. London: Oxford University Press. Articulates the Land Ethic, which has inspired much thinking in nonanthropocentric or biocentric and systems-thinking in environmental ethics.

Lomborg, Bjørn. (1998). The Skeptical Environmentalist: Measuring the Real Sate of the World. Cambridge, UK: Cambridge University Press. A well-documented account of the positive trends in human welfare, including a section devoted to the decline in pollution problems.

Sagoff, Mark. (1988). The Economy of the Earth. Cambridge, UK: Cambridge University Press.

Simon, Julian. (1981). The Ultimate Resource. Princeton, NJ: Princeton University Press. Attacks the neo-Malthusians by arguing that human ingenuity effectively makes resources unlimited in the long run.

Pollution

Pollution can be defined as a change in the physical, chemical, or biological characteristics of the air, water, soil, or other parts of the environment that adversely affects the health, survival, or other activities of humans or other organisms. After pollution occurs, the polluted resource is no longer suitable for its intended use. Most pollutants are solid, liquid, or gaseous chemicals created as byproducts of the extractive or manufacturing industries. Pollution can also take the form of excessive heat, noise, light, or other electromagnetic radiation.

Pollution is also a complex political problem and affects people's lives in numerous ways other than health. Determining an acceptable level of pollution is often more of a political problem than a scientific problem. This is especially true when jobs are at stake, as when a manufacturing plant is forced to close or to modify its operations.

Air Pollution

Air pollution is the contamination of air by unwanted gases, smoke particles, and other substances. Air pollution has been around since the beginning of the Industrial Revolution. In the United States, smoke pollution was at its worst in the first half of the twentieth century. The smoke was so thick in the winter in some industrial cities that street lights had to be left on all day. However, air pollution was considered a local problem. The burning of fossil fuels produced the most smoke, so some cities restricted the type of coal that could be burned to hard coal, which burns cleaner than soft coal. More efficient burners were installed and devices were attached to smokestacks to remove soot. Diesel locomotives replaced steam locomotives, which had burned coal or oil to heat the water to make the steam. These changes all led to a gradual reduction in smoke pollution during the last half of the twentieth century.

However, a new type of air pollution, smog, became a problem beginning in the 1940s. Smog is not the same as smoke pollution, although this was not immediately apparent. When Los Angeles had its first major smog attack, it became obvious that some phenomenon other than smoke was responsible. Los Angeles did not burn coal or oil to generate heat or electricity, yet its smog problem worsened. Scientists now know that smog is the result of the action of sunlight on unburned hydrocarbons and other compounds in the air. These unburned hydrocarbons come from the exhaust of motor vehicles with internal combustion engines.

Rapid industrial growth, urban and suburban development, dependence on motor vehicles, construction and operation of large facilities using fossil fuels for generating electricity, production of iron and steel, petrochemicals, and petroleum refining converted what had been a local problem into a regional or national problem. Many areas of the United States now suffer seasonal episodes of unhealthy air, including smog, haze, and acid rain . A wide range of pollutants currently poses an ecological threat in cities all over the United States and in other industrialized nations. Since pollution does not recognize national borders, the problem can spread around the world.

Costs of Air Pollution

The costs of air pollution in the United State are difficult to estimate. The economic benefits from the control of pollution are even harder to estimate. Business and industry bear the direct costs of compliance with regulations designed to control air pollution. Ultimately, consumers bear these costs through increased prices and reduced stock dividends. However, the economic benefits of cleaner air may be returned to the consumer in the form of lower health costs and longer-lasting and more reliable products.

Types of Pollutants and Controls

There are six principal classes of air pollutants: particulate matter (soot), carbon monoxide, sulfur oxides, nitrogen oxides, unburned hydrocarbons, and ozone. Billions of metric tons of these compounds are discharged into the air each year.

Particulates.

Suspended particulate matter such as soot and aerosols, are particulates. These particles of solids or liquids range in size from those that are visible as soot and smoke, to those that are so small they can only be seen through a microscope. Aerosols can remain suspended in the air for long periods and can be carried over great distances by the wind. Most particulates are produced by burning fossil fuels in power plants and other stationary sources. Controlling particulates usually involves washing, centrifugal separation, or electrostatic precipitation.

Particulates are harmful for a number of reasons. Some particles contribute to acid rain. Toxic materials such as lead or mercury can appear as particulates. However, the greatest health risk comes from breathing particulates. Some particles can lodge deep in the lungs and cause inflammation or chronic lung disease.

Carbon monoxide.

Carbon monoxide is a colorless, odorless, flammable, poisonous gas. The incomplete burning of carbon fuels produces carbon monoxide. Carbon monoxide comes largely from motor vehicles, with lesser amounts from other internal combustion engines such as those on lawnmowers and leafblowers, and from open fires and industrial processes. Carbon monoxide emissions can be controlled by more efficient burners or improved combustion chambers. Modern computer-controlled engines with catalytic converters successfully remove most carbon monoxide from automobile exhaust.

Sulfur oxides.

Sulfur oxides are the major contributor to acid rain in the United States. Sulfur oxides include sulfur dioxide, sulfuric acid, and various sulfate compounds. Sulfur oxides are produced when fuel that contains sulfur is burned, or when metal ores are processed. Sulfur oxide emissions in the United States come primarily from plants that use fuels containing sulfur to generate electricity. The best way to reduce sulfur oxide emissions is to use fuels that naturally contain less than 1 percent sulfur, but these fuels are more expensive. Other techniques include removing sulfur from fuels and sulfur oxides from the combustion gases. Removing sulfur from stack gases after fuel is burned is difficult and expensive. However, the byproduct, sulfuric acid, can be sold to recover some of the cost.

Nitrogen oxides.

Nitrogen oxides are also contributors to acid rain and are a principal component of photochemical smog. Nitrogen oxides primarily result from the high-temperature combustion of gasoline or diesel in internal combustion engines. During combustion, nitrogen in the air chemically combines with oxygen to produce nitric oxide. Much of the nitric oxide is converted to nitrogen dioxide in a chemical reaction promoted by sunlight. Computer-controlled combustion and optimally designed combustion chambers can partially reduce the formation of nitrogen oxides. Special catalytic converters can combine nitric oxide with carbon monoxide and unburned hydrocarbons to produce nitrogen, carbon dioxide, and water.

Unburned hydrocarbons.

Unburned hydrocarbons in air also represent wasted fuel. Gaseous hydrocarbons are not toxic at concentrations normally found in the atmosphere, but unburned hydrocarbons are a major contributor to the formation of ozone and smog. Catalytic converters on automobile engines have substantially reduced the emission of unburned hydrocarbons.

Ozone.

Ozone is a form of oxygen in which the molecule contains three atoms instead of two. Ozone is beneficial when it is high in the atmosphere, but near the surface, ozone can damage rubber and paint as well as damage lung tissue. Ozone is a constituent of smog, and is produced from the reaction of nitrogen oxides with gaseous hydrocarbons in the presence of sunlight. A small amount of ozone is also produced by lightning storms. The control of ozone and other photochemical oxidants depends on the effective control of both nitrogen oxides and gaseous hydrocarbons.

Water Pollution

Water pollution is caused by any chemical, physical, or biological substance that affects the natural condition of water or its intended use. We rarely stop to think about how important a reliable and safe water supply is until it is restricted or damaged. Water pollutants are produced primarily by the activities of humans. Our fresh water supply is under worldwide threat from pollution.

Water in lakes and rivers (surface water) throughout the world must satisfy a wide variety of different needs. Some of these needs partially conflict with others:

• The public wants lakes and rivers preserved in their natural state.
• Lakes and rivers must support a healthy population of fish and wildlife.
• Surface water must be safe for recreational uses such as swimming.
• Many localities depend on surface water for a safe drinking water supply.
• Surface water must be safe for agricultural use.
• Surface water must accommodate a variety of industrial purposes
• Surface water is used to generate power or cool power plants.
• Surface water is counted upon to dilute and transport human and industrial waste.

Because of the complex factors involved, there is no precise definition of water pollution. Instead, the intended use of the water must be considered. Once the intended use of the water is specified, pollutants can be grouped as not permissible, as undesirable and objectionable, as permissible but not necessarily desirable, or as desirable. For example, if water is to be used for wildlife support and enhancement, toxic compounds are not permissible, but oxygen is desirable. If the water is to be converted to steam in a power plant, some toxic materials might be desirable (because they reduce zebra mussel infestations), while excess oxygen that could corrode equipment would be undesirable.

Another method of classifying water pollutants is to distinguish between pollutants that are not altered by the biological processes occurring in natural waters, and those that will eventually break down into other, perhaps less objectionable, compounds. Inorganic chemicals are diluted by water but do not chemically change. Industrial waste often contains this sort of pollutant (for example, mercury). On the other hand, domestic sewage can be converted into inorganic materials, such as bicarbonates, sulfates, and phosphates, by the action of bacteria and other microorganisms in the water. If the water is not too heavily laden with waste, bacteria can break down the waste to safe levels.

Until early in the twentieth century, efforts to control water pollution were directed toward eliminating potential disease-causing organisms, such as typhoid. This led to treatment plants to provide safe drinking water and measures to enhance the natural biological activity of streams and rivers in order to assimilate and break down waste. By the middle of the twentieth century, the focus had shifted to the treatment of chemical pollutants not removed by conventional water-treatment methods.

By the middle of the twentieth century, the situation was changing. Rapidly growing urban areas generated large quantities of waste that had to be processed. Increased manufacturing capacity greatly increased the amount and variety of industrial waste. Commercial fertilizers and pesticides created many new pollution problems. Sewer systems were often unable to keep pace with rapid urban growth. Today, virtually every body of water on Earth has some degree of water pollution. Even the oceans, which were once thought to be able to absorb an unlimited amount of waste, are now showing significant stress due to pollution.

Major Water Pollutants

Water is considered polluted if it contains an amount of any substance that renders the water unsuitable for a particular purpose. The list of substances that may pollute water is very long, and only a few major pollutants can be discussed here.

Organic waste.

Organic waste comes from domestic sewage, agricultural runoff, feedlot operations, and industrial waste of animal and plant origin, such as from a paper mill. Domestic sewage is the largest and most widespread source of organic waste. Industrial organic waste tends to occur in larger quantities at fewer locations. Industries that make food and paper (and wood pulp) produce the largest amounts of industrial organic waste.

Bacteria can efficiently break down organic waste. However, bacterial action also removes oxygen from the water. Because fishes and other forms of aquatic life depend on dissolved oxygen, the bacterial action necessary to break down the waste damages the aquatic environment. If organic waste consumes oxygen at a rate greater than it can be replenished, then anaerobic bacteria dominate the decay process. Anaerobic decomposition by bacteria is smelly and aesthetically unpleasant.

Plant nutrients.

Nitrogen and phosphorus are the two main plant nutrients acting as polluting agents. If plant nutrients get into water, they stimulate the growth of algae and other water plants. When these plants die and decay, they consume oxygen, just like any other organic waste. The excess plant growth caused by fertilization and subsequent build up of dead plant matter is called eutrophication. If the oxygen level drops even a small amount, desirable species of fishes, such as trout and bass, will be replaced by less desirable species such as carp and catfish. If the oxygen level drops low enough, all species of fishes, crayfishes, shrimp, and other organisms may die.

Synthetic organic chemicals.

The water pollution problem causing the greatest concern is the ever-increasing variety of new chemical compounds. Often new compounds are developed and old ones abandoned before their environmental impact is known. Some of these compounds will remain in water for decades, or longer. The presence of these synthetic chemicals adversely affect fishes and other aquatic life. Many researchers think that some synthetic chemicals mimic natural hormones, disrupting growth and reproductive cycles in affected populations.

Inorganic chemicals.

Inorganic chemicals such as mercury, nitrates, phosphates, and other compounds may also enter surface water. Many of these chemicals destroy fish and aquatic life, cause excessive hardness of water supply, and corrode machinery. This adds to the cost of water treatment.

Mercury pollution has been recognized as a serious, chronic, and widespread danger in many waterways. Even very small amounts of mercury can cause serious physiological effects or even death. Because mercury is not normally found in food or water, no organisms have developed the ability to process and excrete mercury. So it collects in tissues until toxic levels are reached. Mercury also undergoes biomagnification . Organisms at higher trophic levels consume a large number of organisms at lower trophic levels. The concentration of mercury becomes progressively higher at higher trophic levels. Predators at the highest trophic levels can accumulate dangerously high levels of mercury in this way.

Radioactive materials.

Radioactive materials are a recent addition to the list of potential water pollutants. Radioactive waste comes from the mining and processing of radioactive ores, from the refining of radioactive materials, from the industrial, medical, and research uses of radioactive materials, and from nuclear-powered reactors. Some radioactive waste still remains from the atmospheric testing of nuclear weapons in the 1940s and 1950s. The two most common radioactive materials found in water are strontium-90 and radium-226.

Oil.

Oil pollution can enter water through bilge flushing, from accidental or deliberate discharge from ships, or from accidental spills of crude oil during transport. Some experts estimate that 1.5 million tons of oil are spilled into the oceans each year. Water polluted by oil greatly damages aquatic life and other wildlife such as birds that depend on the water for food and nesting areas. Waterfowl alighting on oil-covered waters usually become so oil soaked that they are unable to fly. One speck of oil on a bird's feathers can poison the bird if it ingests the oil while preening. Oil destroys much of the aquatic life of oceans. It is particularly damaging to shellfish and other filter feeders . It also damages the small shrimp and other organisms that serve as food for larger fish.

Thermal pollution.

Thermal pollution is caused by the release of heat into the water or air. Electric power plants are a major source of thermal pollution, since they convert only about one-third of fuel energy into electricity. The remaining heat is discharged to the local environment as heated water or air. This can alter the ecological balance of a large area. For example, if warm water is discharged into a lake, the warm water will not be able to dissolve as much oxygen. This may result in more desirable species of fishes being replaced by less desirable species.

Land and Soil Pollution

One of the miracles of technology in the late twentieth century has been the extraordinary ability of agriculture to increase the productivity of croplands to previously unheard of levels. However, this increased productivity requires the heavy use of pesticides and fertilizer. Most pesticides today are designed to decompose very quickly into harmless compounds. Thousands of pesticides are currently in use, and in most cases their agricultural value balances their risks. However, many scientists think that we may be in an unbreakable cycle of having to continually develop new and more potent pesticides to overcome pests that are resistant to older pesticides.

Noise Pollution

Noise pollution is a recently identified source of environmental degradation. The hearing apparatus of living things is sensitive to certain frequency ranges and sound intensities. Sound intensities are measured in decibels. A sound at or above the 120 decibel level is painful and can injure the ear. Noise pollution is present even in the open ocean. Researchers have shown that whales communicate over great distances using low frequency sound waves. Unfortunately, the noise generated by engines and screws of ships falls in the same frequency range and can interfere with the whale's communication.

Light Pollution

Professional and amateur astronomers have recently identified a problem that did not exist a generation ago: light pollution. This form of pollution has spread so widely in the last few decades that many people are only able to see a few of the brightest stars. From many of our big cities, no stars at all are visible. Light pollution is not an inevitable consequence of making our streets and neighborhoods safer. Most light pollution comes from wasted light. At least 75 percent of the sky glow in most cities comes from poorly designed or improperly installed light fixtures. According to a study conducted by the International Dark-Sky Association in 1997, about $1.5 billion per year is spent on wasted light that does nothing to improve security or safety.

Efforts to Control Pollution

Because pollution does not recognize national borders, the solution to many pollution problems requires cooperation at regional, national, and international levels. For example, smokestacks of coal-burning power plants in the United States cause some of the acid rain that falls in Canada.

In the United States, the Environmental Protection Agency (EPA) is charged with enforcing the many and complex laws, rules, executive orders, and agency regulations regarding the environment. The EPA came into being in 1969 with the passage of the National Environmental Policy Act (NEPA). This act also required the filing of environmental impact statements. Almost all government agencies and many businesses are required by law to file these statements, which state the potential harmful environmental effects of such activities as opening new factories, building dams, and drilling new oil wells.

Additional laws to protect the environment were passed in the 1970s and 1980s, including the Clean Air Act, the Safe Drinking Water Act, and the Comprehensive Environmental Response, Compensation, and Liability Act, known as Superfund. Then-president George Bush signed the Clean Air Act of 1990. This new law called for substantial reductions in emissions of all types. The act also added to the list of potentially toxic chemicals that must be monitored by the EPA. The pace of new environmental legislation has waned since 1990, but congress enacted the Food Quality Protection Act in 1996 and the Chemical Safety Information, Site Security and Fuels Regulatory Relief Act in 1999.

see also Habitat Loss; Trophic Level.

Elliot Richmond

Bibliography

Art, Henry W., ed. The Dictionary of Ecology and Environmental Science. New York: Holt, 1993.

Bowler, Peter J. Norton History of Environmental Sciences. New York: Norton, 1993.

Caplan, Ruth. Our Earth, Ourselves: The Action-Oriented Guide to Help You Protect and Preserve Our Planet. Toronto, Canada: Bantam, 1990.

Carson, Rachel. Silent Spring. Boston: Houghton Mifflin, 1994.

Chiras, Daniel D. Environmental Science: Action for a Sustainable Future, 4th ed. Menlo Park, CA: Benjamin-Cummings Publishing Company, 1993.

Franck, Irene M., and David Brownstone. The Green Encyclopedia. New York: Prentice Hall General Reference, 1992.

Harms, Valerie. The National Audubon Society Almanac of the Environment. New York: Putnam Publishing, 1994.

Luoma, Jon R. The Air Around Us: An Air Pollution Primer. Raleigh, NC: Acid Rain Foundation, 1989.

Miller Jr., G. Tyler. Living in the Environment, 6th ed. Belmont, CA: Wadsworth Publishing Company, 1990.

Mott, Lawrie, and Karen Snyder. Pesticide Alert: A Guide to Pesticides in Fruits and Vegetables. San Francisco: Sierra Club Books, 1997.

Newton, David. Taking a Stand Against Environmental Pollution. Danbury, CA: Franklin Watts Incorporated, 1990.

Rubin, Charles T. The Green Crusade: Rethinking the Roots of Environmentalism. New York: Free Press, 1994.

In the United States, concern for the natural condition of water was first expressed in the 1899 Rivers and Harbors Appropriation Act. The measure made it illegal to dump waste into waters used by any kind of vessels, except by special permission.

The 1990 Pollution Prevention Act created a new office in the Environmental Protection Agency, with the mission to help industries limit pollutants.

POLLUTION

The term "pollution," which carries with it a sense of an impurity, can be defined as a chemical or physical agent in an inappropriate location or concentration. The sources of pollution are varied. Natural sources include those that are not directly under human control, such as volcanoes, which spew forth sulfur oxides and particles; and those people could avoid, such as groundwater with naturally high levels of arsenic, which has caused poisoning in Bangladesh and Taiwan. All human activities have the possibility of polluting the environment by contaminating air, water, food, or soil, The earliest human pollution-control efforts dealt with avoidance of diseases caused by contamination of water and food by human excreta and with the control of smoke from fires used for cooking and heating. Sanitary engineering to manage human wastes remains a central public health need. Indoor air pollution due to the use of wood and fossil fuels in poorly ventilated residences also remains a major source of exposure to pollutants and a cause of respiratory disease in much of the world.

TYPES OF POLLUTION

Pollution production can be considered under the heading of the four major human activity sectors: industry, energy, transportation, and agriculture. With the marked increase in human population and the industrialization of much of the globe has come a whole new set of pollutants. Scientific advances based upon understanding the chemical and physical forces underlying nature have led to new processes and new products that have transformed society and have had a major positive impact on human health. But these industrial activities also result in air and water emissions and contamination of the soil and of food as by-products of the processes involved in manufacture. The products themselves may be the means by which pollutants are distributed to the general population, such as lead poisoning through the use of lead in house paints. In the United States and other more wealthy countries, there recently has been a marked decline in industrial pollution emissions per unit produced. This has come about through regulatory control of emissions and, in part, through the recognition by industry that emissions represent a loss of raw materials or product that is economically advantageous to retain. As developed countries move into the information era, much of the production of textiles and durable goods has shifted to developing countries, not always with the same level of pollution control or protection of the work force. In developing countries, industrial production often occurs in smaller units, such as backyard smelters, which have significant local effects and are more difficult to control.

The energy sector continues to grow rapidly worldwide. There are basically three types of energy sources: the burning of fossil fuels and biomass; nuclear power; and energy derived from natural processes such as the sun, wind, and the flow of water. Energy from fossil fuels results from the conversion of carbon to carbon dioxide, with the least efficient and most polluting fossil fuels reflecting the extent of components other than carbon and hydrogen in the fuel source. The most plentiful fossil fuel is coal, which is also among the most polluting. Coal contains mineral ashes, nitrogen, and sulfur, which produce particulates, nitrogen oxides and sulfur oxides, when coal is burned. The use of high-sulfur coal for electric power generation and for home heating was a dominant cause of major air pollution episodes in London in 1952, Donora, Pennsylvania, in 1948, and the Meuse Valley in Belgium in 1930. Much of the U.S. electric grid is powered by low-sulfur oil. Natural gas, which is a relatively pure hydrocarbon, is increasing in use and is particularly effective as a source of peak electric power during periods of high demand. The combustion of all fossil fuels produces nitrogen oxides, which are a major precursor of ozone and particulates. One form of nitrogen oxide, nitrogen dioxide, is itself a pollutant of concern. Carbon dioxide, the end product of efficient fossil fuel energy production, is a major contributor to global climate change. Reduction in carbon dioxide emissions requires more efficient production, transmission, and use of fossil fuel-derived energy. A switch to other energy sources will also help to reduce emissions.

Nuclear power has the advantage of not producing carbon dioxide or any of the sulfur oxides, nitrogen oxides, or particulates that are associated with fossil fuels. Its major disadvantages are the release of low-level radiation, the need for major water resources for cooling (with attendant ecological challenges), and, most importantly, the small but not absent risk of an uncontrolled nuclear reaction. The worst such example, and the only one in which there were substantial short-term health impacts from the civilian use of nuclear power, occurred in Chernobyl in the former Soviet Union in 1986. The extent of long-term effects due to the radiation that spread widely over Europe and globally is still being evaluated.

Wind and solar energy are expected to increase in use as the costs of fossil fuels increase and as new technology is developed. These are, in essence, free of pollution emissions. Hydroelectric power is a mainstay in some parts of the world, but dams have significant ecological implications and there is a growing movement against them. The most effective means of decreasing energy use is by lessening demand.

The transportation sector worldwide is increasingly dominated by automobile and truck emissions. In the United States there has been a marked decrease in pollutant emissions per mile driven that has been almost counterbalanced by an increase in the number of miles driven. Pollutants from gasoline-powered automobiles include the evaporation of volatile organic compounds and tailpipe emissions such as carbon monoxide, nitrogen oxides, benzene, and polycyclic aromatic hydrocarbons (PAHs). Increased engine efficiency and catalytic converters have been effective in decreasing all but nitrogen oxide emissions. Diesel engines, which in the United States are primarily used on trucks, emit high levels of particulates and PAHs. Two-cycle engines on mopeds and other smaller vehicles are relatively inefficient, with much of the fuel evaporating. This is particularly a problem in developing countries. All internal combustion engines lead to the production of carbon dioxide. Future growth in the use of personal automobiles will be a major threat to global carbon dioxide production unless new engines and power sources are developed. Control of automotive emissions is as much a function of effective planning of transportation systems, including mass transit, as it is of technology. There have been relatively few studies of airport-related pollutant emissions, a segment of transportation that is increasing rapidly.

Agriculture is also a major source of pollution. World population growth has been accompanied by increased crop yields, which have been made possible by heavy use of fertilizers and pesticides. Nitrogenous fertilizers, an important part of the increased yield, result in nitrite contamination of drinking water, to which infants are particularly vulnerable. Nitrogenous fertilizers contribute to oxygen problems in water bodies and to greenhouse gas emissions. Phosphate fertilizers are of concern because of trace amounts of cadmium and other heavy metals that sometimes are part of natural phosphates. Cadmium can be taken up into certain crops, can cause renal toxicity, and is a potential carcinogen.

There are a wide range of pesticides and herbicides that are central to modern agriculture. Each of these is chosen because of its ability to have a biological effect on a plant or insect, and there is always a possibility that the biological effect will extend to humans or to other species. Major problems have been caused by pesticides that persist in the environment, such as heptachlor. This has led to bans on persistent organic pollutants and to testing protocols to avoid developing new ones.

OTHER POLLUTION CATEGORIES

Categorizing pollution in terms of the four sectors of industry, energy, transportation, and agriculture obscures the fact that some of the most important sources of pollution are intersectorial. As just one example, the Aswan High Dam provides Egypt with an important hydroelectric source and is effective in controlling flooding and providing irrigation for agriculture. But by retaining silt it decreases the nutrient load to the Nile Delta, which leads to a much heavier requirement for chemical fertilizers for agriculture as well as loss of sardine and salmon fisheries. The lack of the flushing effect of Nile floods has led to increased salinization of the land and has optimized breeding conditions for snails that carry schistosomiasis, an ancient scourge of this area. Similarly, the use of wood for local energy in developing countries is more than just a potential source of indoor and outdoor air pollution. Loss of forests can lead to soil erosion, flooding, and desertification, and have a negative impact on global climate.

Activities that lead to human development within and across each of these major sectors have the potential for producing a pollution impact that outweighs any benefit. There is, unfortunately, one common human activity that has an enormous environmental impact with no redeeming developmental consequences: war.

Pollutants can also be characterized by chemical or physical class; by use; by industrial source; by whether they are likely to be present in air, water, food, or other media; by the organs they attack or the effects they have; by the laws that control their use; and by whether they present a local, regional, or global problem. All of these categorization schemes are valuable, but none are without its faults. Chemicals have multiple properties and uses, and are able to move across environmental boundaries. Pollution episodes have often come about through an inappropriate focus on only one aspect of a chemical. For example, the 1990 U.S. Clean Air Act required the use of oxygenated fuels, which have chemical characteristics that were thought to be beneficial in decreasing automotive emissions in polluted areas. Yet another chemical characteristics of the most commonly used oxygenate compound, methyl tertiary-butyl ether (MTBE), caused it to be a major groundwater contaminant, a problem that was not foreseen because of an inappropriately narrow focus.

A more holistic approach to environmental pollution is particularly important during the current transition period. Pollution control techniques have been largely successful in dealing with end-of-pipe emissions. Through regulatory command and control of major pollution sources there has been a steady diminution of measured emissions to air and water in developed countries, and an improvement in air and water quality. Yet major problems remain, and in some instances they are getting worse. Two interrelated categories of particular concern are global climate change and pollutants from nonpoint sources.

Our planet maintains itself through a series of feedback loops involving interconnected biological, geological, and physical processes. The science that has enhanced our understanding of these processes has also demonstrated their vulnerability to the increasing dominance of human activities, including the effect of pollutants. One example is the diminution of the stratospheric ozone layer that protects humans against the harmful effects of short-range ultraviolet light. A major source of this diminution is chlorofluorocarbons (CFCs). These compounds were seemingly ideal for refrigeration and a variety of other industrial purposes, in part because they are inert and cause little or no direct biological effects. But this lack of reactivity allows CFCs to persist and rise into the stratosphere where they enter into a reaction that decomposes ozone. An international treaty, the Montreal Protocol, has led to a decrease in this particular threat to the ozone layer. The feedback loops involved in global climate change, including the greenhouse effect which is now warming the earth, are far more complex and less well understood. Further, competitive economic and nationalistic interests have made it more difficult to deal with carbon dioxide and nitrogenous greenhouse gases.

Nonpoint source emissions refer to pollution for which there is no readily obvious target, or source. An example is damage to the Chesapeake Bay due to runoff of nitrogenous fertilizer compounds from farms along the Susquehanna River, including a heavy contribution from farms using natural fertilizing techniques. Agricultural practices and energy and transportation decisions contribute heavily to regional air and water pollution and to global warming.

UNDERSTANDING POLLUTION EFFECTS

A transition is also occurring in our understanding of the health effects of pollutants. It is now recognized that there are subtle health effects of environmental pollutants, such as endocrine disruption and neurobehavioral changes, for which newer toxicological paradigms are being developed. The unraveling of the human genome may provide a better understanding of the role of genetic susceptibility factors in response to pollution.

Understanding the effects of pollutants requires understanding how pollutants change following their release from a source, and how they can have effects many miles from their sources. For example, there are no significant direct emitters of air pollutant ozone. Rather, this major component of oxidant smog is formed in the air through the action of sunlight on a mixture of nitrogen oxides and hydrocarbons coming from many different sources, primarily automobiles. The precursors may have been emitted hundreds of miles upwind of where the ozone is eventually formed. For the northeastern United States, this means that statewide control strategies, which are the major enforcement focus of the U.S. Clean Air Act, are an inadequate approach to a regional issue. Similarly, acid rain and other forms of particulate air pollution can be derived from atmospheric reactions of gaseous sulfur dioxide and nitrogen oxides precursors occurring many hundreds of miles downwind. Agents released into water can also undergo significant changes. For example, methyl mercury, which is far more toxic than elemental mercury, is formed in water through the action of bacteria and makes its way into the food chain. The dumping of inorganic mercury from a single chloralkali plant in Minimata Bay, Japan, led to contamination of fish with methyl mercury and to over a hundred deaths and thousands of people being affected by what is known as Minimata disease. There is also a global air circulation of metals, such as mercury, and of persistent organic pollutants, such as PCBs, which tends to carry these agents toward the arctic where they often bioaccumulate.

Understanding the effects of pollutants on human health requires not only an understanding of the intrinsic hazard of the chemical or physical agent, but also the extent of human exposure. Exposure is often determined by local pathways within a community, such as whether drinking water comes from wells or from surface sources or whether individuals consume vegetables grown in their backyards or brought to market from far away. Individual activities can also alter pollutant intake; exercise, for example, increases respiratory uptake of air pollutants. Health effects due to pollutants are heavily dependent upon susceptibility factors, including age, gender, and genetic predisposition.

MANAGING POLLUTION

A variety of approaches have been developed to manage existing pollution. These include punishment of polluters through regulation, taxation, fines, toxic tort suits, and other disincentives; encouragement of nonpolluting approaches through tax and other incentives; and education of the public. The increased awareness of the potential harmful effects of pollution has had a major impact on industries and on individuals, particularly the young, who have led the way in activities such as recycling. Risk assessment has developed as a useful technique to estimate the risks of environmental pollutants and to establish priorities for environmental control and remediation efforts. These efforts to manage existing pollution are largely a form of secondary prevention in that the pollution already exists and the focus is on lessening the extent or the effects.

Primary prevention of pollution has occurred through approaches that, like any form of primary prevention, are both highly effective and difficult to quantify. The United States National Environmental Policy Act of 1969 was the first major action arising out of the new environmental movement aimed at avoiding unwanted environmental consequences. It contained the requirement that significant newly proposed federal activities have an environmental impact statement prepared in advance, the goal being the incorporation of environmental concerns into all planning processes and the avoidance of those activities that would have an adverse impact. Advances in science have had a significant primary preventive effect, in part through providing assessment tools of use in preventing the development of new harmful products by the chemical industry. As examples, a basic understanding of the role of mutation in cancer and recognition of the structural aspects resulting in the environmental persistence of chemicals have led the chemical industry to detect and quickly drop out of its development programs those new chemicals that are mutagens or are likely to persist in the environment. The Precautionary Principle is basic to public health practice, but is also now being advocated as a form of primary prevention of environmental pollution.

Control of the more challenging insidious pollutant effects related to the health of the planetary biosphere and to nonpoint sources cannot depend solely upon standard command and control regulatory approaches. Central to avoiding significant long-term consequences to health and the environment is the development of innovative pollution prevention and control strategies, including emissions trading, taxation of consumption and international compacts; better targeting of controls through improved scientific understanding of the processes involved; and a more informed public.

Bernard D. Goldstein

(see also: Acid Rain; Airborne Particles; Ambient Air Quality [Air Pollution]; Ambient Water Quality; Arsenic; Automotive Emissions; Benzene; Carcinogen; Chlorofluorocarbons; Clean Air Act; Clean Water Act; Climate Change and Human Health; Ecosystems; Emissions Trading; Endocrine Disruptors; Environmental Impact Statement; Exposure Assessment; Groundwater; Human Genome Project; Lead; Mercury; National Environmental Policy Act of 1969; Nuclear Power; PCBs; Persistent Organic Pollutants [POPs]; Pesticides; Precautionary Principle; Radiation, Ionizing; Risk Assessment, Risk Management; Sulfur-Containing Air Pollutants [Particulates]; War )

Bibliography

United Nations Environment Programme (1999). Global Environment Outlook 2000—UNEP's Millennium Report on the Environment. London, UK: Earthscan Publications Ltd.

World Health Organization (1992). Report of the WHO Commission on Health and Environment. Geneva: Author.

Pollution

Pollution refers to situations in which some material or some form of energy occurs in larger quantity than can be tolerated by humans, plants, or animals without suffering some kind of harm. Probably the best-known forms of pollution are air and water pollution, which are discussed below. But other forms of pollution also exist. For example, the term noise pollution has been used to describe the loud noise level of airplane takeoffs and landings, construction, highway traffic, boom boxes, and other modern-day machines. Similarly, the expression visual pollution has been used to describe areas so clogged with signs, billboards, and other objects that the beauty of the natural environment is diminished.

Natural and anthropogenic pollution

Pollution can be caused both by natural sources and humans. Volcanic eruptions are an example of natural sources of pollution. When a volcano explodes, it releases sulfur dioxide, carbon monoxide, solid particles, and other materials into the air at a much greater rate than is normally the case. Plants, animals, and humans may be killed or injured by these materials.

A concrete example of natural pollution can be found at an area known as the Smoking Hills, located in a remote wilderness in the Canadian Arctic. The local environment around Smoking Hills is virtually uninfluenced by humans. However, naturally occurring low-grade coal deposits found in the area have spontaneously ignited from time to time, causing the release of clouds of sulfur dioxide over the nearby tundra.

As this gas is carried to Earth's surface, soil and freshwater become acidified. At some level, this acidification causes metals to become soluble (able to be dissolved). The toxicity (poisons) associated with sulfur dioxide, acidity, and soluble metals at the Smoking Hills has caused great damage to the structure and function of the local ecosystem.

Words to Know

Acid: Substances that when dissolved in water are capable of reacting with a base to form salts and release hydrogen ions.

Acid rain: A form of precipitation that is significantly more acidic than neutral water, often produced as the result of industrial processes.

Anthropogenic: Any effect caused by humans.

Fossil fuel: Fuels formed by decaying plants and animals on the ocean floors that were covered by layers of sand and mud. Over millions of years, the layers of sediment created pressure and heat that helped bacteria change the decaying organic material into oil and gas.

Greenhouse effect: The warming of Earth's atmosphere as the result of the capture of heat by carbon dioxide molecules in the air.

Oxide: An inorganic compound whose only negative part is the element oxygen.

Oxygen-demanding agent: Any substance that reacts with oxygen dissolved in water.

Particulate: Solid matter in the form of tiny particles in the atmosphere.

Primary pollutant: Any pollutant released directly from a source to the atmosphere.

Radiation: Energy transmitted in the form of electromagnetic waves or subatomic particles.

Secondary pollutant: Any pollutant formed in the atmosphere from compounds released from some source.

Smog: A form of air pollution characterized by hazy skies and a tendency to cause respiratory problems among humans.

Thermal inversion: A condition in which there is an atmospheric zone in which temperature increases with altitude, instead of the usual decrease with increasing altitude.

Volatile organic compound: Any organic liquid that changes easily (volatilizes) to a gas.

History of anthropogenic pollution

Natural forms of pollution have existed since the dawn of time, and there is not much humans can do to control such events. On the other hand, the vast majority of pollution affecting human societies today originates from human activities and is therefore susceptible to human control.

Human-caused pollution is sometimes referred to as anthropogenic pollution. Anthropogenic pollution has existed for centuries. People living in London, England, in the late eighteenth century, for example, were exposed to huge quantities of noxious gases in the air and dangerous levels of harmful materials in their water supplies. However, most people of the time probably accepted such risks as part of being a city dweller.

Modern concerns about pollution began to increase in the 1960s largely as the result of two factors. First, population growth in many urban areas meant that more people and more industries were releasing a higher concentration of pollutants to the environment than ever before. Second, modern science had developed a number of new materials and new procedures that resulted in the release of many new and often dangerous chemicals to the environment.

As people became more and more conscious of pollution problems, they began calling for government efforts to control the release of pollutants and to clean up a dirty environment. Some results of this effort included the Clean Air Acts of 1965, 1970, and 1977; the Safe Drinking Water Act of 1974; the Clean Water Act of 1977; and the Toxic Substances Control Act of 1976.

Air pollution

A complete list of air pollutants would include nearly two dozen solids, liquids, and gases. It would include well-known pollutants such as sulfur oxides and carbon monoxide and less-familiar materials such as pesticides and fluorides. In terms of the quantities of pollutants released in a year, the five materials that cause the most damage are sulfur oxides, oxides of nitrogen, carbon monoxide, particulate matter, and volatile organic compounds.

Sulfur oxides, oxides of nitrogen, and carbon monoxide are chemical compounds. Particulate matter and volatile organic compounds are groups of related pollutants. The term particulate refers to tiny specks of solid matter in the atmosphere, including smoke, haze, aerosols, and tiny particles of carbon. Volatile organic compounds are organic liquids, such as benzene, toluene, the xylenes, and trichloromethane, that change easily (volatilize) to a gas.

Effects of air pollutants. By definition, all forms of air pollution have some harmful effect on humans, other animals, plants, or other materials in the environment. For example, carbon monoxide is a well-known toxic gas that reduces the blood's ability to transport oxygen. Prolonged exposure to carbon monoxide can cause heart and respiratory disorders; headaches, nausea, and fatigue; and, at high enough concentrations, coma and death. The oxides of both sulfur and nitrogen attack the human respiratory system, leading to irritated eyes and throat and impaired breathing (at low concentrations), and to emphysema, bronchitis, and lung cancer (at higher concentrations).

The effects of particulate matter are wide, from preventing photosynthesis (food production) in plants to clogging the breathing passages in lungs (leading to respiratory disorders). Particulate matter also soils buildings, statutes, and other objects, leading to their decay and deterioration.

Sources of air pollution. The major single source of air pollutants is the combustion (burning) of fossil fuels (coal, oil, and gas) to run industrial machines and generate electricity. All anthropogenic pollutants can be traced to some extent to this source. A second major source of pollutants is the incomplete combustion of fuel in cars, trucks, railroad trains, airplanes, and other forms of transportation. Smaller amounts of pollutants are released during the incineration of solid wastes and by a variety of industrial processes.

In some cases, pollutants are released by these sources directly to the air and are known, therefore, as primary pollutants. Sulfur dioxide, oxides of nitrogen, and carbon monoxide are all primary pollutants. In other cases, materials released by a source undergo a chemical reaction in the atmosphere and are converted to a secondary pollutant. Examples of secondary pollutants are ozone and peroxyacyl nitrates (PANs), major components of the form of air pollution known as smog.

Specialized forms of air pollution. Under specialized conditions, certain forms of air pollution have developed that are so dramatic or so serious that they have been given special names. These conditions include smog, acid rain, the greenhouse effect, and ozone depletion.

The term smog actually applies to two quite different atmospheric conditions. The term itself comes from a combination of the words smoke and fog. One form of smog, known as industrial smog, is produced when sulfur dioxide, particulates, and other pollutants released by industrial and household burning of fossil fuels is trapped by a thermal inversion. A thermal inversion is an atmospheric condition in which a layer of cold air is trapped by a layer of warm air above it. Some of the most dramatic photographs of urban areas covered by air pollution are those that show a city smothered in a cloud of smog.

A second form of smog is photochemical smog, produced when oxides of nitrogen, produced largely by internal-combustion engines (those most often used in automobiles, for example), react with oxygen in the air to form a complex mixture of pollutants that includes ozone, PANs, and other organic compounds. Photochemical smog often has a similar appearance and similar effects to those of industrial smog. Indeed, in most cities, the two forms of smog occur in combination with each other.

Acid rain. Acid rain, or more generally acid precipitation, is the name given to any form of precipitation (rain, snow, sleet, hail, fog) that is more acidic than normal. The high acidity of acid precipitation results from the formation of acids in the atmosphere from chemicals released by human sources.

Most powerplants that produce electricity release as by-products large quantities of nitrogen and sulfur oxides. Once in the atmosphere, these oxides react with moisture in the air to produce nitric and sulfuric acid. When precipitation occurs, these acids are carried to Earth's surface, where they attack plants, animals, and nonliving materials. Some experts believe that large regions of forests on the east coast of the United States and Canada have been badly damaged by acid precipitation resulting from gases released by industrial plants in the midwestern United States. Acid precipitation has also been blamed for the death of fish and other aquatic organisms and for damage to stone buildings and sculptures.

Greenhouse effect. Carbon dioxide is normally not considered to be a pollutant since it has no harmful effects on plants or animals. It does have one other effect, however, that may affect life on Earth. Solar energy that reaches Earth's atmosphere experiences a variety of fates. Some of that energy is reflected back into space, while some passes through the atmosphere and reaches Earth. Of the solar energy that reaches Earth's surface, some is absorbed and some is reflected back into the atmosphere. A large fraction of the reflected energy is captured by carbon dioxide molecules in the air and retained as heat. This effect has been compared to the way in which the glass in a greenhouse may capture heat and is called, therefore, the greenhouse effect. Experts believe that, without the greenhouse effect, Earth's temperature would be about 8°C (18°F) cooler than it actually is, making it impossible for most forms of life to survive.

Since the turn of the twentieth century, however, the rate at which carbon dioxide is being added to Earth's atmosphere has increased dramatically. The combustion of fossil fuels for heating, industrial operations, transportation, and other uses is primarily responsible for this trend. With the increase in carbon dioxide in the atmosphere comes an increased greenhouse effect and, some experts believe, a general warming of the planet's annual average temperature. In a report released in early 2001, scientists concluded that if greenhouse emissions are not curtailed, the average global surface temperature could rise by nearly 11°F (6°C) over the next 100 years. The scientists also stated that man-made pollution has "contributed substantially" to global warming and that Earth is likely to get a lot hotter than previously predicted. Such a warming could cause a massive melting of the polar ice caps, resulting in the flooding of many coastal areas.

Ozone depletion. Ozone is a form of oxygen whose molecules contain three atoms (O₃) rather than two atoms (O₂). Ozone occurs in very small concentrations in upper regions of Earth's atmosphere, where it has a function vital to life on Earth. Ozone molecules have the ability to capture infrared radiation that enters Earth's atmosphere as part of sunlight. Infrared radiation is known to have a number of undesirable effects on plants and animals, from damage to leaves and fruits of plants to skin cancer and eye problems in humans.

Scientists have learned that certain synthetic (human-made) chemicals known as the chlorofluorocarbons (CFCs) have the ability to attack and destroy ozone molecules in the atmosphere. CFC molecules are broken apart by solar energy with the release of chlorine atoms. These chlorine atoms then attack ozone molecules and convert them to ordinary oxygen.

The loss of ozone molecules as the result of attacks by CFCs means that the ozone "shield" in the atmosphere is decreasing in concentration. With this decrease, a larger percentage of ultraviolet radiation is likely to reach Earth's surface, resulting in an increase in complications from exposure to ultraviolet radiation.

Air pollution controls. A number of approaches are possible for the reduction of air pollutants. For example, it would be desirable simply to reduce the use of various processes that release pollutants into the air. Getting people to ride bicycles or walk to work instead of driving a car is a simple way of reducing the emission of nitrogen oxides and carbon monoxide.

Another approach is to convert harmful pollutants to harmless forms before they are released to the atmosphere. The catalytic convertor that is now standard equipment on passenger vehicles does just that. It converts harmful oxides of nitrogen and hydrocarbons to harmless nitrogen, oxygen, carbon dioxide, and water vapor.

Efforts can be made to trap pollutants as they are released from a source, thus preventing them from reaching the atmosphere. Devices known as scrubbers on smokestacks are an example. Polluting gases, such as oxides of sulfur and nitrogen, are captured in scrubbers, where they react with chemicals that convert them to harmless (and sometimes useful) by-products.

Water pollution

A broad variety of materials can be classified as water pollutants, including synthetic organic compounds, human and animal wastes, radioactive materials, heat, acids, sediments, and disease-causing microorganisms. The following discussion describes the major water pollutants, their sources, and possible means of control.

Oxygen-demanding agents. An oxygen-demanding agent is some substance which, when placed into water, reacts with oxygen dissolved in the water. As oxygen is removed from the water, other organisms that also depend on that oxygen (such as fish and other forms of aquatic life) may die or migrate away from the polluted waters. Sources of food and recreation may be destroyed by the presence of such agents.

The obvious way to prevent pollution by oxygen-demanding wastes is to prevent sewage and other solid wastes from entering water supplies or to treat those wastes before they are released to lakes and rivers.

Synthetic organic chemicals. The term synthetic organic chemical applies to a wide variety of products invented by modern chemistry to serve various human needs. These products include plastics, pesticides and herbicides, detergents, toxic by-products of industrial operations, and oils. Many of these products are directly toxic to fish, aquatic life, and even to humans. Others may not present a serious health effect to organisms, but can cause the unsightly accumulation of trash, destroying the recreational value of a waterway.

The release of synthetic organic chemicals to waterways can be controlled by establishing and enforcing methods of producing, storing, shipping, and disposing of such materials. For example, many people are accustomed simply to dumping used motor oil into city sewers. This practice guarantees that rivers and lakes will become polluted with such oils. A better practice is for cities to provide special collection procedures for used motor oils.

Industrial chemicals. A number of inorganic chemicals are released from industrial operations as the by-products of certain processes. For example, the element mercury is used in the production of light switches, air conditioners, fluorescent lights, floor waxes, medicines, plastics, paper, clothing, and photographic film, to name but a few of its applications. Each time one of these products is made, some small amount of mercury metal is likely to escape into the environment and, eventually, into lakes and rivers. This is problematic because mercury is highly toxic to humans and other organisms. It causes damage to the nervous system, kidneys, liver, and brain.

One way to limit the release of industrial chemicals to water supplies is to find alternative chemicals to use in manufacturing operations. Another approach is to pass and enforce legislation that requires appropriate methods of storage and disposal of such chemicals.

Sediments. Sediments washed from Earth's surface also pollute water. Any time it rains, a certain amount of sand, clay, silt, and other forms of earthy material are washed away. This sediment has a number of consequences, such as the silting of harbors and reservoirs, damage to shellfish and fish, reduction in the clarity of water, and the loss of water's ability to integrate (blend) oxygen-demanding wastes.

The loss of sediments during rainstorms is a natural event and cannot, therefore, be totally eliminated. However, careless building and forestry practices may contribute enormously to the loss of sediments. Tree roots, for example, are important for holding soil in place. When trees are removed from an area, the soil is much more easily washed away.

Heat. Warm water is not able to dissolve as much oxygen as is cool water. If the water in a river becomes warmer, it holds less oxygen. Organisms that depend on oxygen for their survival, then, will either die or migrate to other areas. Many industrial and energy-generating plants use water in their operation. They take water from a river or lake, use it, and return it to the same body of water, but at a higher temperature. This practice, sometimes known as thermal pollution, poses a serious hazard to organisms living in the water.

The most common method for avoiding thermal pollution is to cool water discharged from a plant before returning it to a body of water. Many plants now have large cooling towers or artificial lakes through which waste waters must pass before they are returned to a lake or river.

[See also Acid rain; Agrochemical; Atmosphere, composition and structure; Carbon dioxide; Carbon monoxide; DDT (dichlorodiphenyltrichloroethane); Greenhouse effect; Oil spills; Ozone ]