Jennifer McPartland, Ph.D., is a Senior Scientist, and Lariah Edwards, Ph.D., is an EDF-George Washington University Postdoctoral Fellow
A recent article in Inside EPA ($) indicated that the US EPA’s Integrated Risk Information System (IRIS) will begin piloting a “unified approach” to the evaluation of chemicals for cancer and non-cancer endpoints. Specifically, it appears that the program intends to develop analyses demonstrating how a unified approach could work as supplements to some of its chemical assessments. This represents a major step forward in advancing the science of chemical assessment at EPA and is responsive to recommendations the scientific community and stakeholders have been making for years.
By way of background, chemical regulatory assessments generally assume that if a chemical is a genotoxic carcinogen, there is no exposure threshold for the effect. This means that across a diverse population, some level of risk for developing cancer exists at any level of exposure. Traditionally for all other toxicity endpoints, EPA and other regulatory agencies typically have assumed that there is a bright-line exposure threshold below which no adverse health effect will be seen.
This bifurcated approach to characterizing chemical hazards and risks is not scientifically supported. The assumption that there are “safe” exposure thresholds for all non-cancer endpoints ignores real-world variability in exposure and susceptibility across the human population. This variability influences whether any particular person or group will experience an adverse effect, and includes such factors as: co-exposures to other chemical and non-chemical stressors and differences in susceptibility that may arise from things like genetic differences or underlying health conditions.