Toxins and Poisons

Toxins and poisons are chemical substances that destroy life or impair the function of living tissue and organs. Although used interchangeably, a toxin is usually considered to be any poison produced by an organism. A poison, however, is more generally any type of substance that harms an organism by its chemical action.

As a chemical harmful to living things, a poison can be almost anything. Nearly every prescribed drug can have negative effects if taken in the wrong dosage. In certain situations, whether or not something is harmful depends entirely on the dosage or amount taken. Other substances can be beneficial to one living form (penicillin cures a sick person) but deadly to another (it kills bacteria). Although in common usage, the words toxic and poisonous are often used interchangeably, as are the words toxin and poison, it is safe to say that poison is the broader term.

Poisons are so diverse in their origin, chemistry, and toxic action that it is nearly impossible to classify them simply. Many have natural origins, such as those that are produced by living things. Some of the most potent toxins known are produced by simple bacteria. For example, bacteria are responsible for the diseases botulism, diphtheria, and tetanus.

Certain mushrooms are highly toxic, while a fungus is responsible for the usually deadly disease called ergotism. More complex plants contain a large variety of poisonous substances that are part of the alkaloid group. Many of the better-known drugs, such as quinine, curare, morphine, and nicotine, come from this family of plants. These drugs can be poisonous depending on the dosage. Many animals also produce toxins, including certain snakes, frogs, and insects. A mammal, like the shrew, can produce a poison in its salivary glands.

Poisons are also found in the nonliving part of the natural world. Some organic elements like mercury, arsenic, and lead, all called heavy metals, are poisonous when absorbed into the systems of living things. Many strong acids and bases are damaging to soft tissues. A number of gases, like chlorine and ammonia, are toxic even at low doses. Many alcohols and solvents (a liquid that dissolves other substances) can do extreme damage to people's organs, while many plastics made from petroleum can be toxic. As a result, today's industrial society has come to present people with a major source of human-made poisons.

The action of these and other poisons is generally described in terms of what part of the body they affect or what biochemical changes they produce. While the actual chemical mechanism may be different for each poison, they can be grouped generally according to their effects. Many poisons block, or inhibit, something that should be happening in the body. Thus, the toxic gas carbon monoxide makes the body's cells unable to transport necessary oxygen. Other poisons prevent a certain critical enzyme (a protein that acts as a catalyst and speeds up chemical reactions in living things) from being released, while others block a crucial step in a person's metabolism (which are all of the chemical reactions and sequences that happen in living things).

The effects of toxins are classified according to time-based categories. That is, an acute effect happens immediately, as would the distress caused by a venomous snake bite. Acute effects often involve damage to organs. If the effects appear over days, weeks, or months, they are called subacute. For example, a tetanus infection takes some time to develop and show its worst symptoms. Effects that appear very gradually and last for an extended period of time are called chronic effects. A cancer caused by exposure to toxic waste is described as chronic. Often, an individual's susceptibility to toxins and poisons varies depending on his or her genetic background, age, weight, gender, overall health, and previous exposure to the toxin or poison.

[See alsoPollution ]