Carcinogen

Carcinogens and cancer

Indutrial carcinogens

Other carcinogens

Resources

A carcinogen is a substance that causes a normal cell to change into a cancerous cell (a cell that grows and divides uncontrollably). The explosive growth of cells that have been triggered by a carcinogen can deprive other cells of nutrients and crowd out the cell from the available space. These changes can be lethal.

The word “carcinogen” is derived from Greek and means in English, cancer-causing. Carcinogens fall into two broad categories; naturally occurring substances that are found in food or soil, or artificial substances created by chemists for various industrial purposes. Although the way carcinogens cause cancer is still not completely understood, cancer researchers believe that humans and other animals must be exposed to a carcinogen for a certain period of time and at a high enough concentration for cancer to occur.

Cancer is a group of diseases in which cells grow abnormally. Like all living things, normal cells grow, reproduce, and die. These processes are controlled by chemicals and reactions within the cell, which are in turn controlled by the cell’s genetic material within its nucleus. In a cancerous cell, the genetic material is altered, and the genes which encode and direct the chemical reactions within the cell are mutated, or changed. Cancerous cells grow uncontrollably, forming a large mass of cells called a tumor, which invades tissues and kills non-cancerous cells. Sometimes cancerous cells “break off” from a tumor and enter the bloodstream, traveling to other parts of the body and infecting other organs and tissues in a process known as metastasis. In this way, a cancer can spread from an isolated tumor to the entire body.

Agents such as viruses, medication, and synthetic carcinogens can cause mutations within a cell’s genetic material. Some kinds of cancers are caused by viruses. For example, a special kind of virus called a retrovirus causes a rare form of leukemia (cancer of the white blood cells). Radiation from naturally-occurring radioactive substances (such as uranium) can disrupt a cell’s genetic material and bring about cancer. Synthetic carcinogens are found in processed foods and industrial chemicals.

Carcinogens and cancer

Available research evidence is consistent with the suggestion that a carcinogen is apt to cause cancer if a person’s exposure to the chemical is present in a sufficiently high enough concentration for a suitable period of time. The conditions of time and concetnration vary depending upon the carcinogen. Repeated exposure to a carcinogen over an extended period (such as 20 years) increases the likelihood that a normal cell’s genetic material will mutate and initiate cancer. Cigarette smoke contains potent carcinogens; it can take many years and many repeated exposure to the carcinogens in smoke for smoking to cause cancer. Smoking-related lung cancers typically develop between 10 and 20 years of continuous smoking. In Hiroshima and Nagasaki in Japan, where atomic bombs were dropped in 1945, the leukemia rates in the surviving population increased dramatically some five years after the bombs were detonated. The intense radiation that was emitted in each explosion caused damage to genetic material that almost immediately mutated cells.

Some carcinogens are more powerful cancer-causing agents than others. Powerful carcinogens are called tumor promoters; these can cause genetic mutations directly within cells. Less powerful carcinogens are called tumor initiators; they can cause latent changes in the cell’s genetic material. These changes are not enough to actually cause cancer, but sensitize the tissue for later exposures to tumor promoters. If a tumor initiator has already damaged the cell’s genetic material, the likelihood that a tumor promotor will cause the cell to become cancerous is increased.

The development of a tumor is typically not a one-step process, but involves multiple events. Thus, it is rare that exposure to a carcinogen is the sole cause of most cancers. Exposure to a carcinogen must usually be combined with other environmental factors for a cancer to develop. Some environmental risk factors are difficult to identify, while others such as prolonged, heavy cigarette smoking have been easy to identify. Heredity, such as whether close relatives have developed cancer, is another risk factor that is not as easily characterized.

Some carcinogens such as cigarette smoke can be avoided. Other factors, such as a diet, can be modified. Additional risk factors such as gender, immune status, metabolic rate, levels of certain enzymes, and age can neither be avoided nor modified.

Indutrial carcinogens

The idea that chemicals could cause cancer was first promoted in 1775 by English physician Percivall Pott, who noted that young chimney sweeps had a high incidence of scrotal cancer. Because most sweeps began their careers very early in life and seldom washed or changed clothes, they were exposed to soot repeatedly and for long periods of time, leading to scrotal cancer in young adulthood. Not until 150 years later were the actual carcinogenic substances in soot identified, but Dr. Pott made his case for more humane treatment and better working conditions for the chimney sweeps by noting the connections between cancer and this profession.

With the advent of industrial development in the nineteenth century, other connections between certain cancers and chemicals were noticed. Shale oil and coal and tar workers had a high incidence of skin cancer. Dyestuff workers developed bladder cancer. And a chemical called vinyl chloride, used in the manufacture of leather goods, caused a rare liver tumor.

In response to growing concern about cancer in industrial workers, the International Agency for Research on Cancer and the National Toxicology Program formulated a system to classify chemicals according to their cancer-causing risk. A chemical could be classified either as a probable carcinogen, or as a non-carcinogen.

A problem with this and other classification systems is that much of the research is based on experiments on animals. If cancer can be induced in experimental animals with high levels of a chemical, it is sometimes assumed that humans could also be at risk even with lower exposure levels over a long period of time. The science of risk assessment investigates the possibility of developing cancer from very low levels of exposure to chemicals that cause cancer at very high levels.

Other carcinogens

Some foods contain naturally occurring carcinogens. Safrole, found in sassafras root; estragole, found in the herb tarragon; allyl isothiocynate, found in mustard seed; and benzene, found in eggs, fruits, vegetables, cooked meats, and fish are all carcinogens. However, these substances must be consumed in large amounts, over a long period, to initiate cancer.

Processed foods such as bacon, sausages, and canned meats contain the preservative nitrite. Frying the cured bacon can convert some non-carcinogenic substances in the nitrites into potent carcinogens. Browning meats such as hamburger can also cause carcinogenic chemicals to be produced. However, in both cases, the amount of carcinogen is extremely small. Interestingly, microwave cooking does not release the carcinogens in beef.

Foods that involve fermentation in their production, such as beer, wine, bread, and yogurt, all contain mildly carcinogenic substances. Again, these foods must be eaten in large amounts over decades to possibly cause the genetic mutations related to cancer.

A type of fat known as trans-fat, which has been popularly used in the deep-frying of foods and in other foods including cookies, has been demonstrated to be a carcinogen. Countries such as Canada have banned the use of trans-fats in prepared foods. However, as of 2006, foods prepared at fast food outlets are not included in the legislation.

Radioactive substances found in rocks and soil are also considered potentially carcinogenic. While most people do not come in contact with radioactive chemicals on a day-to-day basis, these substances can emit radioactive particles that can be dangerous. Radon, a radioactive substance emitted by uranium, can seep from rocks into buildings. In some areas of the country, radon can be emitted in relatively large amounts into buildings which should be tested for radon. If the levels are found to be high, changes should be made to the building’s ventilation system to reduce the amount of radon in the building.

KEY TERMS

Cancer— A disease in which cells grow abnormally.

Carcinogen— Any substance capable of causing cancer by mutating the cell’s DNA. Mutation—A change in the genetic material of a cell.

Risk assessment— The study of the risk of exposure to certain levels of an agent that may lead to the development of a disease, such as cancer.

Risk factor— Any habit, condition, or external force that renders an individual more susceptible to disease. Cigarette smoking, for example, is a significant risk factor for lung cancer and heart disease.

It is recommended that people eat a varied diet high in fresh fruits and vegetable and avoid excess consumption of foods high in nitrites. While it is not possible to completely eliminate one’s exposure to carcinogens, it is possible to avoid the concomitant risk factors that may lead to cancer. Avoiding smoking; eating a varied, balanced diet that includes fiber; and limiting alcohol consumption are all associated with a lowered cancer risk.

See also Mutagen; Mutation.

Resources

BOOKS

Bryenton, Betty B. Playing With Fire: Wisdon for Women Who Smoke. Crewe, UK: Trafford Publishing, 2006.

Meister, Kathleen. America’s War on “Carcinogens”: Reassessing the Use of Animal Tests to Predict Human Cancer Risk. Washington: American Council on Science, 2005.

Pohanish, Richard P. Sittig’s Handbook of Toxic and Hazardous Chemicals and Carcinogens. Bracknell, UK: Noyes Publications, 2002.

Kathleen Scogna

Carcinogen

A carcinogen is any substance or agent that produces or induces the development of cancer . Carcinogens are known to affect and initiate metabolic processes at the level of cellular DNA.

Cancer accounts for slightly over 20% of all deaths each year. It is estimated that one out of every four Americans will develop cancer eventually and that six out of 10 in this group will die from the disease itself or complications arising from the disease. Half of all cancer deaths occur before the age of 65. Among women between 30 and 40 and children between three and 14, cancer is the leading cause of death next to accidents. It is the most frequent cause of death among Americans under 35 years of age.

The testing of chemicals as cancer-producing agents began with the observation of Sir Percival Pott in 1775 that scrotal cancer in young chimney sweeps resulted from the lodgement of soot in the folds of their scrotums. Pott was the first to link an environmental agent, coal tar, to cancer growth. In 1918 scientists began to test chemical derivatives for their cancer-causing efficacy. These first experiments looked at polycyclic hydrocarbons , specifically benzo(a)pyrene found in coal tar, and they demonstrated that a certain degree of exposure to coal tar produced cancer in laboratory rats. In 1908, Vilhelm Ellerman and Oluf Bang of Denmark reported that an infectious agent could cause cancer, after they found that a leukemia-like blood disease was transmitted among domestic fowl via a virus . In 1911, Peyton Rous established a viral cause for a cancer called sarcoma in domestic fowl, and he was awarded a Nobel Prize for this discovery some 55 years later. In 1932, Lacassagne reported that estrogen injections caused mammary cancer in mice. This opened up investigation into the role hormones played in the development of various types of cancers.

In 1896, Wilhelm Roentgen discovered the x ray, a radioactive emission capable of penetrating many solid materials including the human body. x rays quickly found use as a diagnostic tool in medicine; but operators of x ray devices, unaware of their harmful effects, determined the proper intensity of the beams by repeatedly exposing their hands to the rays. Many operators of x ray equipment began to suffer from cancer of the hand, and Roentgen himself died of cancer. The most dramatic environmental link to cancer induced by radioactivity was observed after the bombing of Hiroshima, Japan when there was a radical increase in leukemia type cancers among people exposed to the atomic blast.

Environmental agents such as toxic chemicals and radiation are considered responsible for about 85% of all cancer cases. A great many environmental agents such as synthetic chemicals, sunlight (exposure to UV and UVB rays), air pollutants, heavy metals , x rays, high-fat diet, chemical pesticides, and cigarette smoking are known to be carcinogenic. Surveys carried out on the geographic incidence of cancer indicate that certain types of cancer are far more common in heavily industrialized areas. New Jersey, the site of approximately 1,200 chemical plants and related industries, has the highest overall cancer rate in the United States.

Tobacco use, particularly cigarette smoking, is now recognized as the leading contributor to cancer mortality in the United States. Currently one third of all cancer deaths are due to lung cancer, and of the 130,000 new lung cancer victims diagnosed each year, 80% are cigarette smokers. Several years ago, the primary cause of cancer among women was breast cancer—but by the late 1980s, lung cancer had surpassed breast cancer as the leading cause of death among women. Current controversy rages over the role of secondary smoke as a contributing cause of cancer among nonsmokers exposed to cigarette smoke .

Dietary factors have been extensively investigated, and experiments have implicated everything from coffee to charcoal broiled meat to peanut butter as possible carcinogens. A major concern among meat producers was the use of diethylstilbestrol (DES) as a source for beefing up cattle. DES is a synthetic hormone that increases the rate of growth in cattle. In the 1960s, DES was fed to about three fourths of all the cattle raised in the United States. It was also used to prevent miscarriages in women until 1966, when it was shown to be carcinogenic in mice. DES is now linked to vaginal and cervical cancers in women born between 1950 and 1966 whose mothers took DES during their pregnancies.

In 1971, DES was banned for use in cattle by the Food and Drug Administration (FDA), but the federal courts reversed the ban, contending that DES posed no danger since it was not directly added to foods but was administered only to cattle. When the FDA subsequently showed that measurable quantities remained present in slaughtered cattle, the courts reinstated the ban. But the issue of using growth additives in meat production remains unresolved today. Environmentalists are still concerned that known carcinogenic chemicals used to "beef up" cattle are being consumed by humans in various meat products, though no direct links have yet been established. In addition, various food additives , such as coal tar dyes used for artificial coloring and food preservatives, have produced cancer in laboratory animals. As yet there is no evidence indicating that human cancer rates are rising because of these substances in food.

Air pollution has been extensively investigated as a possible carcinogen and it is known that people living in cities larger than 50,000 run a 33% higher risk of developing lung cancer than people who live in other areas. The reasons behind this phenomenon, referred to as the "urban factor," have never been conclusively determined. Areas with populations exceeding 50,000 tend to have more industry, and air pollutants can have a profound effect in regions such as New Jersey where they are highly concentrated.

Occupational exposure to carcinogenic substances accounts for an estimated 2–8% of diagnosed cancers in the United States. Until passage of the Toxic Substances Control Act in 1976, which gave the federal government the power to require testing of potentially hazardous substances before they go on the market, hundreds of new chemicals with unknown side effects came into industrial use each year. Substances such as asbestos are estimated to cause 30–40% of all deaths among workers who have been exposed to it. Vinyl chloride , a basic ingredient in the production of plastics , was found in 1974 to induce a rare form of liver cancer among exposed workers. Anaesthetic gases used in operating rooms have been traced as the reason nurse anesthetists develop leukemia and lymphoma at three times the normal rate with an associated higher rate of miscarriage and birth defects among their children. Benzene , an industrial chemical long known as a bone-marrow poison, has been shown to induce leukemia as well. A major step forward in the regulation of these potential cancer causing agents is the implementation by the Occupational Safety and Health Administrations (OSHA) of the Hazard Communication Standard in 1983, intended to provide employees in manufacturing industries access to information concerning hazardous chemicals encountered in the workplace.

With the erosion of the ozone layer of our atmosphere , increased concern about over-exposure to ultraviolet radiation and its subsequent effect on the formation of skin cancer has developed. The EPA estimates that a five% ozone depletion in the stratosphere would result in a substantial increase in a variety of skin cancers. This would include an average of two million extra cases of basal-cell and squamous-cell skin cancers a year and an additional 30,000 cases of the often fatal melanoma skin cancer, which currently kills 9,000 Americans per year.

See also Hazardous waste siting; Love Canal, New York; Ozone layer depletion; Radiation exposure; Radiation sickness; Radon; Toxic substance

[Brian R. Barthel ]

RESOURCES

BOOKS

Aldrich, T., and J. Griffith. Environmental Epidemiology. New York: Van Nostrand Reinhold, 1993.

McCance, K. L. Pathophysiology: the Biological Basis for Disease in Adults and Children. St. Louis: Mosby, 1990.

National Academy of Sciences. Ozone Depletion, Greenhouse Gases and Climate Change. Washington, DC: U. S. Environmental Protection Agency, 1989.

U. S. Environmental Protection Agency. The Potential Effects of Global Climate Change on the United States. Washington, DC: U. S. Government Printing Office, 1988.

Carcinogen

A carcinogen is a substance that causes a normal cell to change into a cancerous cell. The word "car cinogen" is derived from Greek and means in English, cancer-causing. Carcinogens fall into two broad categories, naturally occurring substances that are found in food or soil , or artificial substances created by chemists for various industrial purposes. Although the way carcinogens cause cancer is still not completely understood, cancer researchers believe that humans and other animals must be exposed to a carcinogen for a certain period of time and at a high enough concentration for cancer to occur.

Cancer is a group of diseases in which cells grow abnormally. Like all living things, normal cells grow, reproduce, and die. These processes are controlled by chemicals and reactions within the cell, which are in turn controlled by the cell's genetic material within its nucleus. In a cancerous cell, the genetic material is altered, and the genes which encode and direct the chemical reactions within the cell are mutated, or changed. Cancerous cells grow uncontrollably, forming a large mass of cells called a tumor , which invades tissues and kills non-cancerous cells. Sometimes cancerous cells "break off" from a tumor and enter the bloodstream, traveling to other parts of the body and infecting other organs and tissues in a process known as metastasis. In this way, a cancer can spread from an isolated tumor to the entire body.

Several agents, such as viruses, medication, and synthetic carcinogens can cause mutations within a cell's genetic material. Some kinds of cancers are caused by viruses. For example, a special kind of virus called a retrovirus causes a rare form of leukemia (cancer of the white blood cells). Radiation from naturally-occurring radioactive substances (such as uranium ) can disrupt a cell's genetic material and bring about cancer. Synthetic carcinogens are found in processed foods and industrial chemicals.

Carcinogens and cancer

For a carcinogen to cause cancer a person must be exposed for a certain length of time to the carcinogen and at a high enough concentration. Repeated exposure to a carcinogen over an extended period (such as 20 years) increases the likelihood that a normal cell's genetic material will mutate and initiate cancer. Cigarette smoke contains potent carcinogens; it can take many years and many repeated exposure to the carcinogens in smoke for smoking to cause cancer. Smoking-related lung cancers typically develop between 10 and 20 years of continuous smoking. In Hiroshima and Nagasaki in Japan, where atomic bombs were dropped in 1945, the leukemia rates in the surviving population increased dramatically some five years after the bombs were detonated.

Some carcinogens are more powerful, cancer-causing agents, than others. Powerful carcinogens are called tumor promoters, and can cause genetic mutations directly within cells. Less powerful carcinogens are called tumor initiators and can cause latent changes in the cell's genetic material. These changes are not enough to actually cause cancer, but sensitize the tissue for later exposures to tumor promoters. If a tumor initiator has already wrought some damage to the cell's genetic material, the likelihood that a tumor promotor will cause the cell to become cancerous is increased.

Cancerous tumors develop over many stages, and it is rare that exposure to a carcinogen is the sole cause of most cancers. Exposure to a carcinogen must usually be combined with other environmental factors for a cancer to develop. Some environmental risk factors are difficult to identify, while others such as prolonged, heavy cigarette smoking have been easy to identify. Heredity, such as whether close relatives have developed cancer, is another risk factor that is not as easily characterized.

Some carcinogens such as cigarette smoke can be avoided. Other factors, such as a diet, can be modified. Additional risk factors such as gender, immune status, metabolic rate , levels of certain enzymes, and age can neither be avoided nor modified.

Carcinogens used in industry

The idea that chemicals could cause cancer was first promoted in 1775 by Percivall Pott, a London physician. Dr. Pott noted that young chimney sweeps had a high incidence of scrotal cancer. Because most sweeps began their careers very early in life and seldom washed or changed clothes, the sweeps were exposed to soot repeatedly and for long periods of time, leading to scrotal cancer in young adulthood. Not until 150 years later were the actual carcinogenic substances in soot identified, but Dr. Pott made his case for more humane treatment and better working conditions for the chimney sweeps by noting the connections between cancer and this profession.

With the advent of industrial development in the nineteenth century, other connections between certain cancers and chemicals were noticed. Shale oil and coal and tar workers had a high incidence of skin cancer. Dyestuff workers developed bladder cancer. And a chemical called vinyl chloride, used in the manufacture of leather goods, caused a rare liver tumor.

In response to growing concern about cancer in industrial workers, the International Agency for Research on Cancer and the National Toxicology Program formulated a system to classify chemicals according to their cancer-causing risk. A chemical could be classified either as a probable carcinogen, or as a non-carcinogen.

A problem with this and other classification systems is that much of the research is based on experiments on animals. If cancer can be induced in experimental animals with high levels of a chemical, it is sometimes assumed that humans could also be at risk even with lower exposure levels over a long period of time. The science of risk assessment investigates the possibility of developing cancer from very low levels of exposure to chemicals that cause cancer at very high levels.

Dioxin is a case in point. In the early 1980s, dioxin was sprayed on the roads of Times Beach, Missouri, to seal the pavement. Dioxin had been classified by the Environmental Protection Agency (EPA) as a probable carcinogen. When the townspeople discovered that dioxin had been sprayed on their roads, the town was abandoned and lawsuits against the road contractor were initiated which were challenged by the defendants.

Carcinogens in food

Some foods contain naturally-occurring carcinogens. Safrole, found in sassafras root; estragole, found in the herb tarragon; allyl isothiocynate, found in mustard seed; and benzene , found in eggs, fruits , vegetables , cooked meats, and fish are all carcinogens. However, these substances must be consumed in large amounts, over a long period to initiate cancer.

Processed foods such as bacon, sausages, and canned meats contain the preservative nitrite. Frying the cured bacon can convert some non-carcinogenic substances in the nitrites into potent carcinogens. Browning meats such as hamburger can also cause carcinogenic chemicals to be produced. However, in both cases, the amount of carcinogen is extremely small. Interestingly, microwave cooking does not release the carcinogens in beef.

Foods that involve fermentation in their production, such as beer, wine, bread, and yogurt, all contain mildly carcinogenic substances. Again, these foods must be eaten in large amounts over decades to possibly cause the genetic mutations related to cancer.

Other carcinogens

Radioactive substances found in rocks and soil are also considered potentially carcinogenic. While most people do not come in contact with radioactive chemicals on a day-to-day basis, these substances can emit radioactive particles that can be dangerous. Radon , a radioactive substance emitted by uranium, can seep from rocks into buildings. In some areas of the country, radon can be emitted in relatively large amounts into buildings which should be tested for radon. If the levels are found to be high, changes should be made to the building's ventilation system to reduce the amount of radon in the building.

In 1993, the EPA designated second-hand smoke from cigarettes to be a known human carcinogen. It is estimated that 2,000 lung cancer deaths a year are caused by second-hand smoke, which has led to the designation of many public areas as smoke-free zones.

Avoiding carcinogens

It is recommended that people eat a varied diet high in fresh fruits and vegetable and avoid excess consumption of foods high in nitrites. While it is not possible to completely eliminate one's exposure to carcinogens, it is possible to avoid the concomitant risk factors that may lead to cancer. Avoiding smoking, eating a varied, balanced diet that includes fiber, and limiting alcohol consumption are all associated with a lowered cancer risk.

See also Mutagen; Mutation.

Resources

Periodicals

Ashby, John. "Change the Rules for Food Additives." Nature 368 (April 1994).

Begley, Susan. "Don't Drink the Dioxin." Newsweek 124 (September 1994): 57.

Boyle, Peter. "The Hazards of Passive-and Active-Smoking." New England Journal of Medicine 328 (June 1993): 1708.

Moolenaar, Robert J. "Overhauling Carcinogen Classification." Issues in Science and Technology 8 (Summer 1992): 70.

Nesnow, Stephen. "Breakthroughs in Cancer Risk Assessment." EPA Journal 19 (Jan/Mar 1993): 27.

"Second-hand Smoke Designated as a Known Human Carcinogen." EPA Journal 19 (2): 5. April/June 1993.

Yuspa, S. H. "Overview of Carcinogenesis: Past, Present and Future." Carcinogenesis 21 (2000): 341–344.

Kathleen Scogna

KEY TERMS

Cancer

—A disease in which cells grow abnormally.

Carcinogen

—Any substance capable of causing cancer by mutating the cell's DNA.

Mutation

—A change in the genetic material of a cell.

Risk assessment

—The study of the risk of exposure to certain levels of an agent that may lead to the development of a disease, such as cancer.

Risk factor

—Any habit, condition, or external force that renders an individual more susceptible to disease. Cigarette smoking, for example, is a significant risk factor for lung cancer and heart disease.

Carcinogen

A carcinogen is a substance that causes a normal cell to change into a cancerous cell, resulting in uncontrolled cell growth. Cancer cells can multiply unchecked, forming a mass of tissue called a tumor. Sometimes cancerous cells "break off" from a tumor, traveling through the body and multiplying in other organs and tissues.

How carcinogens cause cancer

Carcinogens cause cancer by producing changes (or mutations) in the genetic material, or DNA, of a cell. These mutations result in uncontrolled cell division. A cancer-causing substance can alter the DNA of a cell directly or it can react with other chemicals in the body to form substances that cause gene mutations. (Genes are sections of DNA that serve as units of hereditary information.) A transformed cell may continue to function normally and not begin to multiply and develop into a tumor until many months or years later. It is not unusual for cancer to appear 20 to 25 years after initial exposure to a carcinogen.

Types of carcinogens

Carcinogens include both naturally occurring and artificially produced chemicals, ultraviolet light, and radioactive substances such as radon (a radioactive gas that is present in rock).

Words to Know

Aflatoxin: A carcinogenic poison produced by a mold that grows on peanuts and grains.

Cancer: A disease of uncontrolled cell growth.

DNA (deoxyribonucleic acid): The genetic material in the nucleus of cells that contains information for an organism's development.

Mutation: A change in the genetic material of a cell.

Radiation: Energy that is sent out as waves or particles.

Tumor: A mass of tissue formed by abnormal cell growth.

About 23 chemicals have been identified as carcinogens in humans, with many more shown to cause cancer in laboratory animals. Tobacco smoke contains several carcinogenic substances and is the major cause of lung cancer. Some drugs used in the treatment of cancer are themselves cancer-causing. In addition, various chemicals used in industrial processes, such as vinyl chloride and certain dyes, are known human carcinogens. Nitrites—chemicals added to processed meats such as bacon, sausage, and bologna to prevent spoilage—react with substances in the digestive tract to form carcinogenic chemicals called nitrosamines. Even synthetic dyes added to food for coloring are potential mutagens (substances that cause mutations in the genetic material of cells).

Sunlight is a well-known carcinogen that can cause changes in skin cells that may lead to skin cancer. Radiation emitted from an atomic bomb or released in nuclear power accidents can result in cancer in people exposed to it. Repeated exposure to radiation from medical X rays or other sources also may increase a person's risk of developing genetic mutations.

Some foods, such as celery, black pepper, white mushrooms, and mustard contain naturally occurring carcinogens. Aflatoxin is a cancer-causing chemical produced by molds on peanuts. However, these foods must be consumed in large quantities over a long period to initiate cancer.

Ames Test

The Ames test is a quick method of determining if a substance is capable of producing mutations. A culture of a strain of Salmonella bacteria that lacks an enzyme needed for growth is exposed to possible carcinogens. If the substance added to the culture is carcinogenic, it will cause mutations in the bacteria that allow the bacteria to grow. The Ames test has positively identified many carcinogens. It is used by cosmetic companies, drug manufacturers, and other industries that must prove that their products will not cause cancer in humans.

[See also Cancer; Cigarette smoke; Mutation; Nucleic acid; Radiation exposure; Virus ]

CARCINOGEN

A carcinogen is an agent that can cause cancer. Carcinogens can be chemicals, viruses, hormone, ionizing radiation, or solid materials. Carcinogens produce cancer by changing the information that cells receive from their DNA, causing immature cells to accumulate in the body rather than differentiate into normal functional cells. Carcinogens may be genotoxic, meaning that they interact physically with DNA to damage or change its structure. Ionizing radiation is a genotoxic carcinogen. Other carcinogens may change how DNA expresses its information without changing its structure directly, or may create a situation in a cell or tissue that makes it more susceptible to DNA damage from other sources. These are known as nongenotoxic carcinogens, or promoters. Arsenic and estrogen are nongenotoxic carcinogens. Still other carcinogens, such as nickel, may interfere with cell division, changing the number or structure of chromosomes in new cells after a cell divides.

Several changes in a cell's DNA are usually needed to transform a normal cell into a cancer cell. Such changes can accumulate over time, and can sometimes be repaired. Cells can also die before enough changes occur to turn them cancerous. The places that become altered in the DNA of cancer cells are called oncogenes and tumor suppressor genes. Oncogenes and tumor suppressor genes are particular locations on DNA that control a cell's ability to perform its biological functions and to control its growth.

Susceptibility to the action of carcinogens is very complex and is affected by genetic heritage, behavior, physiology, nutrition, external exposures, and other factors. For example, some chemicals are carcinogenic in their original form (direct carcinogens), while some must be metabolized in the body to their active form (indirect carcinogens). In such cases, individual susceptibility to a chemical carcinogen is affected by the rate at which the chemical metabolizes in the body into a cancer-causing form or into a harmless form. This rate varies from person to person.

Some carcinogens have been identified from studies of people exposed to various substances over time. These include cancer in cigarette smokers and leukemia in people breathing benzene in the workplace. Carcinogens have also been identified using laboratory animals exposed over time, usually to high doses. Saccharin was found to be a carcinogen through experiments to produce bladder cancer in rats, and aflatoxin was found to produce liver cancer in rats. Some substances that are carcinogens in laboratory animals, like saccharin, are not carcinogens in people because of differences in how they are metabolized or differences in how they produce cancer. The International Agency for Research on Cancer (part of the World Health Organization) and the U.S. National Toxicology Program publish documents listing chemicals and other exposures that they believe are known to be carcinogenic to humans and those that are suspected or likely to be carcinogens to humans.

Gail Charnley

(see also: Ames Test; Cancer; Carcinogen Assessment Groups; Carcinogenesis; Toxicology )

Bibliography

Weinberg, R. A. (1996). Racing to the Beginning of the Road: The Search for the Origin of Cancer. New York: W. H. Freeman.

Zuddon, R. W. (1987). Cancer Biology. New York: Oxford University Press.