Determining Inhalation Risk Tools for Assessing Hazard.
The Clean Air Act focuses on reduction of the potential for specific air pollutants to cause adverse health effects. Implementation of standards to control release of the 188 hazardous air pollutants (HAPs) requires the Environmental Protection Agency (EPA) to evaluate the health risk remaining after controls are in place (residual risk). The Urban Air Toxics Strategy identified a group of 30 HAPs, which were considered to pose the greatest health and environmental risk which must be controlled to reduce adverse health effects and cancer incidence. To support the hazard evaluation of HAPs, EPA developed and maintains tools for assessing toxicity. The primary tool for evaluating inhalation toxicity from longer term exposures is the reference concentration (RfC) methodology , which derives exposure-response estimates through application of dosimetric adjustments that account for species specific relationships of exposure concentration to deposited or delivered doses. The methodology continues to evolve as new scientific information expands understanding of the respiratory system and how it handles gases and aerosols. For short-term exposures, EPA proposes a quantitative exposure-response assessment of acute inhalation effects to develop Acute Reference Exposures (ARE) values. To support these assessments, software is available for evaluating acute toxicity data using categorical regression, a potentially useful approach to evaluate exposure across time and species. To improve evaluation of non-cancer and cancer data, EPA also developed software to calculate the benchmark dose. This widely discussed alternative for examining dose-response relationships from toxicity studies provides statistical considerations of experimental data which calculate the dose, and its lower confidence limit, for a specified response level. The benchmark dose software (BMDS) includes models for evaluation of continuous and dichotomous data, and several specialized models. Proper application of these tools provides technical support for the assessments required under the Clean Air Act.