The Challenges of Air Pollution and Residual Risk Assessment

The Clean Air Act Amendments of 1990 authorized the U.S. Environmental Protection Agency (EPA) to promulgate and implement technology-based standards on emissions of 188 hazardous air pollutants. These emission limits were to reflect the maximum achievable control technology (MACT) for categories of source. Although the MACT standards have resulted in a significant reduction of toxic air emissions, an assessment of the residual risk of toxic air emissions from MACT-regulated sources is needed to determine whether further controls are needed. This article discusses the problems associated with air pollution, and describes steps taken by the EPA to develop a risk assessment methodology that evaluates the residual health risks and ecological effects associated with exposure to toxic air pollutants from different source categories. The method, called total risk integrated methodology, depends upon a dispersion model to assess human exposure to toxic compounds.