As a result of the Clean Air Act Amendments of 1990, technology-based MACT standards to control hazardous air pollutant (HAP) emissions from approximately 150 source categories will be promulgated. These include 14 source categories for ferrous and nonferrous metal processing. Within eight years following the promulgation of a MACT standard, EPA must assess residual HAP emissions and their associated risks, in order to determine whether any unacceptable health risks remain after MACT implementation. This paper addresses some of the key technical issues facing the metals industry in residual risk assessments and summarizes the tiered residual risk assessment scheme recently recommended to EPA by the Presidential/Congressional Commission on Risk Assessment and Risk Management These recommendations support the application of comprehensive multipollutant, multimedia, and multipathway health (i.e., carcinogenic and non-carcinogenic) and ecological risk assessments. This paper reviews the current EPA guidance for conducting these multipathway assessments, as well as the limitations of these techniques. The paper will also present the methodology used in recently completed air pathway residual risk assessments for coke plants, in which a number of refinements to EPA risk evaluation techniques were applied, including: 1) the use of an innovative air dispersion modeling technique to more accurately characterize the buoyancy-induced plume rise due to convective heat transfer from coke battery surfaces and hot fugitive emissions, and 2) the incorporation of updated inhalation exposure factors.
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