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. The Toxicity 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 available.