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INTRODUCTION TO TOXICOLOGY
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Toxicology is the study of the adverse effects of chemical, biological, or physical agents on living organisms and the environment. These toxic substances include naturally occurring harmful chemicals, or toxins, as well as foreign substances called xenobiotics. Toxins are poisons that originate from plants and microbial organisms and also include venoms released by animals in order to injure predators. Aflatoxin is an example of a toxin; it is produced and released from the fungus Aspergillus that grows on foods such as corn and nuts. Exposure to aflatoxin is associated with an increased risk of liver cancer. By comparison, xenobiotics include a variety of synthetic chemicals with different intended purposes. Pharmaceuticals are xenobiotics developed to treat disease, whereas pesticides are used to deter pests. In addition, a large number of chemicals are used in manufacturing and industrial processes. For example, the chemical “dioxin” (2,3,7,8-tetrachlorodibenzo-p-dioxin [TCDD]) is generated during the production and/or combustion of certain chlorinated organic chemicals. A unique skin toxicity, called chloracne, has been observed in individuals exposed to dioxin. Some toxic substances can be produced by both natural and anthropogenic activities. For example, polyaromatic hydrocarbons are produced by the combustion of organic matter through ordinary processes (e.g., forest fires) and human activities (e.g., combustion of coal for energy production and cigarette smoking). Arsenic, a toxic metalloid, largely appears in groundwater as a natural contaminant, but also enters groundwater from other sources as well. Generally, such toxic chemicals are referred to as toxicants, rather than toxins, because, although they may be naturally produced, they are not produced by biological systems.
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Toxic chemicals may also be classified in terms of their physical state (gas, dust, liquid, size; e.g., nanoparticles); their chemical stability or reactivity (explosive, flammable, corrosive); general chemical structure (aromatic amine, halogenated hydrocarbon, etc.); or ability to cause significant toxicity (extremely toxic, very toxic, slightly toxic, etc.). Classification of toxic chemicals on the basis of their biochemical mechanisms of action (e.g., alkylating agent, cholinesterase inhibitor, and endocrine disruptor) is usually more informative than classification by general terms such as irritants and oxidizers. However, more descriptive categories such as air pollutants, occupation-related exposures, and acute and chronic poisons may be useful to associate toxic chemicals that result in similar adverse events or are encountered under particular conditions. There is no single classification that is applicable to the entire spectrum of toxic chemicals. Instead, a combination of classification systems is generally needed to best characterize toxic substances. In this textbook, for example, toxic chemicals are discussed in terms of their target organs (liver, kidney, hematopoietic system, etc.), use (pesticide, solvent, food additive, etc.), source (animal and plant toxins), and adverse effects (cancer, mutation, etc.).
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Virtually every known chemical has the potential to produce injury or death if it is present in a sufficient quantity. Paracelsus (1493 to 1541), a Swiss/German/Austrian physician, scientist, and philosopher, phrased this well when he noted, “What is there ...