Some ranking systems are based only on a measure of chemical toxicity,
and do not take into account differences between chemicals that
affect their persistence in the environment or the likelihood that
humans or other organisms will be exposed to that substance.
Toxicity-based scores focus on a measure of inherent hazard (the potency
of a chemical in causing adverse effects), without any consideration
of factors that may modify the likelihood that a chemical's toxicity potential will be expressed. Chemicals of similar toxicity are scored the same, even if one is rapidly degraded in the environment and has little likelihood of human exposure, while another persists and bioconcentrates in the food chain with a high likelihood of human exposure.
Ranking systems vary in the number of toxicity measures they consider.
Weights (RSEI) developed by US EPA for its current human health Risk-Screening Environmental Indicators are based on a chemical's chronic risk assessment values: reference doses or cancer potency values are used to rank chemicals. In contrast, the UTN Human Health Effects Score (and the UTN Ecological Effects Score) are based on indicators of both acute and chronic toxicity.
WHY RANK CHEMICALS BASED ON TOXICITY ONLY?
Minimizing the inherent toxicity of chemicals used in industrial
processes and consumer products is an important goal of pollution
prevention programs. In many situations early in the design of
processes or products, it is difficult to predict possible patterns of
release or exposure, so it is a sensible precaution to avoid chemicals
with high inherent toxicity when lower toxicity substitutes are