Hazardous air pollutants are regulated under Title III of the 1990 Clean Air Act Amendments. The Amendments replace the risk-based approach mandated in the 1977 Amendments with a prescriptive, technology-based approach requiring that maximum achievable control technology (MACT) be applied to all major industrial sources of 189 hazardous air pollutants. The change reflects political, rather than scientific consensus that the public health benefits justify the costs. The choice is put into perspective by looking at the interface between science and policy that occurs as part of regular decisionmaking. Particular emphasis is given to examining the interrelationships among facts (science), judgments (science policy), and policy (values) in the context of the risk assessment paradigm. Science and policy are discussed in relation to Title III, contrasting the political consensus for action with the scientific uncertainty about risks and benefits. It is argued that a balanced research program is needed to get the facts right about hazardous air pollutants, including research to meet statutory requirements, to reduce uncertainties in risk assessment, and to address strategic issues.
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