Toxic Substance

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Tolerance level refers to the maximum allowable amount of chemical residue, such as a pesticide, legally permitted in food. Tolerance levels are determined by government agencies such as the Environmental Protection Agency (EPA) and the Food and Drug Administration (FDA), and are based on the results of testing, primarily animal testing. This testing determines the dosage level at which few or no effects are observed. This dose is then adjusted to a human equivalency dose with a margin of error built in for added safety.

While pesticides play an important role in modern society, and are used for a variety of purposes, not until recent decades was it clear how dangerous and persistent many pesticides are. The EPA monitors only a small fraction of pesticides, and in many cases their probable effects on the environment are unknown. Pesticide tolerance levels are particularly important worldwide, since pesticides are found in the tissues of people living even in very remote areas of the world. For example, as a result of environmental contamination, the concentration of pesticides in human breast milk has, at certain times in some areas, exceeded the tolerance level for cow's milk.

To establish tolerance levels, scientists conduct a risk assessment, or an evaluation of the hazards to the environment, including human health, from exposure to the substance. Data on the toxicity of the substance are combined with data about exposure. The calculation assesses the theoretical maximum residue contribution, or the amount of a chemical that would be present in the average daily diet if all foods treated by that chemical had amounts at the tolerance level. The highest concentration allowable is called the maximum acceptable tolerance concentration. For drinking water, tolerance levels are called the National Primary Drinking Water Requirements. For air pollution standards, tolerance levels are called the permissible exposure level.

Environmentalists who criticize the government's tolerance levels charge that standards are not safe enough either for the general population or for workers exposed to toxins. Critics also claim that tolerance levels do not take into account either the cumulative effect of residues from a variety of sources in the environment, the duration of exposure, or the unanticipated effects of two or more chemicals combined. Tolerance levels also are criticized for not recognizing threats to at-risk populations such as children, the elderly, and pregnant women. Animal rights activists have criticized the use of animals to determine tolerance levels, and advocate the use of computer models or in vitro testing (testing on tissue cultures) to make determinations of toxicity or safety of chemicals.

Pesticide Residue; Toxic Substance

Harte, John, et al. Toxics A to Z. Berkeley: University of California Press, 1991.