Poisons and Toxins

A chemical is said to be a poison if it causes some degree of metabolic disfunction in organisms. Strictly speaking, a toxin is a poisonous chemical of biological origin, being produced by a microorganism, plant, or animal. In common usage, however, the words poison and toxin are often used interchangeably, and in this essay they are also treated as synonyms.

It is important to understand that potentially, all chemicals are toxic. All that is required for a chemical to cause toxicity, is a dose (or exposure) that is large enough to affect the physiology of an organism. This fact was first recognized by a Swiss physician and alchemist known as Paracelsus (1493-1541), who is commonly acknowledged as the parent of the modern science of toxicology. Paracelsus wrote that: "Dosage alone determines poisoning." In other words, if an exposure to a chemical is to cause poisoning, it must result in a dose that exceeds a threshold of physiological tolerance. Smaller exposures to the same chemical do not cause poisoning, at least not on the short term. (The differences between short-term and longer-term toxicities are discussed in the next section.) Species of plants, animals, and microorganisms differ enormously in their tolerance of exposures to potentially toxic chemicals. Even within populations of the same species, there can be substantial differences in sensitivity to chemical exposures. Some individuals, for example, may be extremely sensitive to poisoning by particular chemicals, a phenomenon known as hypersensitivity.

Because chemicals are present everywhere, all organisms are continuously exposed to potentially toxic substances. In particular, the environments of modern humans involve especially complex mixtures of chemicals, many of which are synthesized through manufacturing and are then deliberately or accidentally released into the environment. People are routinely exposed to potentially toxic chemicals through their food, medicine, water, and the atmosphere.