CHEMICAL PROFILES|Toxicity and Exposure Potential Scores

EDF's risk scoring system is based on consideration of a chemical's toxicity and exposure potential. The likelihood that a chemical will harm human health or wildlife depends on the dose that organisms receive, so rankings systems that use environmental modeling to estimate doses to target organisms provide the most reliable indicator about whether adverse impacts will occur. To describe human exposure potential, EDF estimates the concentration of a chemical in different media (e.g., water) that result from a chemical's release to the environment and then predicts the dose people will receive given their intake rates (e.g., amount of water ingested daily). EDF Risk Scores then combine these dose estimates with toxicity data to characterize cancer or noncancer health impacts. Separate Risk Scores are developed for chemical releases to air and water.

EDF Risk Scores can be combined with environmental release data to weight and rank chemicals by toxicity and exposure potential. Since the exposure modeling is based on releases to ambient air or water, the scores are not appropriate for ranking chemicals released to land (like a landfill) or to an indoor environment (like a workplace).

WHY RANK CHEMICALS BASED ON TOXICITY AND EXPOSURE POTENTIAL?
Health risks are a function of the total dose received by exposed populations from multiple pathways and media. Setting priorities based on chemical toxicity alone will not optimize risk reduction opportunities because substantial differences in the exposure potential of chemicals of similar toxicity are ignored. Substances that are rapidly degraded upon release to the environment are scored the same as substances with long half-lives, even though releases of persistent or bioaccumulative substances result in higher ambient concentrations and total human doses.

Indicators of exposure potential can be developed by using measures that determine environmental concentrations of a chemical in different media (such as environmental persistence or bioaccumulation factors), or by using models that estimate the total dose humans acquire as a result of contact with different contaminated media. The best measure of dose would be derived from a comprehensive exposure assessment that addresses the specific facts regarding a chemical release (height of smoke stacks, local meteorological conditions, etc.) and population characteristics that determine exposure (activity patterns, intake rates, etc.) However, acquiring such detailed data for environmental releases from important pollution sources is clearly not feasible.

A toxicity and exposure potential weighting system can provide a screening-level indicator of health risks associated with different release scenarios, without requiring extensive, case-by-case exposure assessment. The results of such a risk ranking are likely to be more reliable for priority-setting because the system accounts for the three major factors that determine whether a chemical poses a serious health risk: inherent toxicity, environmental fate, and the extent of human exposure.