In 1990, Congress established a health-based goal for the Clean Air Act: to reduce lifetime cancer risks from major sources of hazardous air pollutants to one in one million. The Act required that over time EPA regulations for major sources should "provide an ample margin of safety to protect public health" [Clean Air Act § 112(f)(2)]. For noncarcinogens, EPA generally assumes that an ample margin of safety requires that exposure to a toxic chemical be kept below its Reference Concentration, or safe level.
To evaluate the potential public health significance of exposure to specific HAPs, Scorecard compares predicted HAP concentrations to benchmark concentrations that meet the CAA goals. For carcinogens, the benchmark concentration is the pollutant level associated with a 1 in 1,000,000 added cancer risk. For noncarcinogens, the benchmark concentration is the exposure level defined by a pollutant's Reference Concentration. The Hazard Index for a HAP (the ratio of predicted exposure to Reference Concentration) should not exceed 1.
The one in one million added cancer risk goal has been utilized to evaluate HAP concentrations in recent reports by the U.S. House of Representatives Committee on Government Reform on air pollution in Los Angeles and the San Francisco Bay area. This cancer risk benchmark (and noncancer benchmarks based on Reference Concentrations) have also been used in publications by EPA staff on the public health significance of hazardous air pollutants.