This website uses a scoring system to help identify environmental releases of toxic chemicals that are likely to pose the greatest risk to human health. This system adjusts the amount of a chemical that is released (in pounds) using a weighting factor (a chemical's "toxic equivalency potential"), so that chemical releases can be compared on a common scale that takes into account differences in toxicity and exposure potential.

Carcinogenic chemicals vary widely in both their toxicity (the added cancer risk associated with exposure to a unit dose) and exposure potential (the total human dose associated with a one pound release). A chemical like benzoic trichloride is extremely toxic in terms of its carcinogenic potency, but it quickly degrades when released into water, limiting human exposure opportunities. In contrast, a chemical like hexachloroethane has a relatively low potency, but a very high exposure potential when released to water. A one pound release of each of these chemicals poses significantly different human health risks. If all chemicals are treated the same (a pound of one is no better or worse than a pound of another), we will miss important opportunities for risk reduction. Toxic equivalency potentials (TEPs) are designed to address this problem. If the TEP of chemical A is 10 times the TEP of chemical B, the emissions of 1 pound of chemical A is considered to be as harmful to human health as that of 10 pounds of chemical B.

This website converts reported releases of carcinogens into pounds of benzene-equivalents. Benzene-equivalents provide a common denominator for comparing carcinogenic releases, taking into account variations in toxicity and exposure potential across chemicals. The units indicate the number of pounds of benzene that would have to be released into the air to pose the same approximate level of health risk as the reported release of chemical X.

Benzene-equivalents are calculated by multiplying the reported releases of chemical X to air or water by its media-specific toxic equivalency potential. Because chemicals undergo different environmental fates if they are released to air or water (with subsequent differences in human exposure opportunities), this scoring system assigns them different air and water toxic equivalency potentials. To obtain a single common denominator of benzene-equivalents, water TEPs are normalized to air TEPs.

Benzene was selected as a reference chemical for cancer TEPs because it has a potency value in the middle of the observed range of carcinogenic chemicals and it is a familiar chemical name to the general public. Benzene-equivalents are also used as a common denominator for assessing the health risks of air toxics in a toxicity-weighting system developed by the international chemical manufacturer ICI.

On Risk Scores and TEPs.

TEPs are a tool for screening the potential human health impacts of environmental releases. TEPs are based on risk assessment values and environmental fate and exposure modeling that incorporate a number of assumptions that must be made to deal with scientific uncertainties. Scoring systems based on other assumptions (or focused on other environmental health concerns like acute toxicity to humans or ecotoxicity) would produce different rankings.

TEP-weighted releases do not characterize the estimated increase in health risk associated with a chemical exposure, and they cannot be combined with information about an exposed population to predict the incidence of adverse effects.

Note that this risk scoring method does not take into account qualitative differences in the types of cancer that chemicals may cause, or in the weight of evidence supporting the identification of a chemical as a carcinogen. Benzene is a known human carcinogen that causes leukemia. Expressing the potential health risk of other carcinogens in benzene-equivalents does not indicate they are known human carcinogens, or that they will cause leukemia; it indicates only the relative magnitude of added cancer risks associated with a one pound release.

The scoring system does not cover all carcinogens reported released to the environment, so some potentially high hazard chemicals will not be spotlighted.

The scoring system currently does not generate toxic equivalency potentials for land releases, so this category of environmental releases is not included in health impact rankings.