EDF 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.
Chemicals that cause noncancer health effects vary widely in both their toxicity (the amount of chemical it takes to cause damage) and exposure potential (the total human dose associated with a one pound release). A chemical like formaldehyde is relatively toxic, but it quickly degrades when released to air, limiting human exposure opportunities. In contrast, a chemical like 1-chlorobutane has relatively low toxicity, but a very high exposure potential when released to air. 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. EDF's 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.
WHAT ARE TOLUENE-EQUIVALENTS?
Scorecard converts reported releases of chemicals that cause noncancer health effects into pounds of toluene-equivalents. Toluene-equivalents provide a common denominator for comparing noncancer releases, taking into account variations in toxicity and exposure potential across chemicals. The units indicate the number of pounds of toluene 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.
Toluene-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), EDF's scoring system assigns them different air and water toxic equivalency potentials. To obtain a single common denominator of toluene-equivalents, water TEPs are normalized to air TEPs.
EDF selected toluene as a reference chemical for cancer TEPs because it has noncancer risk assessment values in the middle of the observed range of chemicals and it is a familiar chemical name to the general public.
On EDF's Risk Scores and TEPs.
LIMITS OF EDF'S RISK SCORING SYSTEM
TEPs are a tool for screening the potential human health impacts of environmental releases reported by manufacturing plants covered by the
Toxics Release Inventory. 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 EDF's method does not take into account qualitative differences in the types of noncancer health effects that chemicals may cause, or in the weight of evidence supporting the identification of a chemical as a health hazard. Toluene is a recognized developmental toxicant. Expressing the potential health risk of other chemicals in
toluene-equivalents does not indicate they are developmental toxicants; it indicates only the relative magnitude of noncancer hazard indices associated with a one pound release.
The scoring system does not cover all chemicals that manufacturing facilities release, 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 TRI releases is not included in Scorecard's health impact rankings.