Estimation of adverse health e®ects associated with ambient exposure to Particulate Matter (PM) constitutes one of the most interesting, recent case studies on the use of epidemiological evidence in public policy (Samet, 2000; Greenbaum et al., 2001). Under the Clean Air Act (Environmental Protection Agency, 1970), the US Environmental Protection Agency (EPA), is required: 1) to set National Ambient Air Quality Standards (NAAQS) for six \criteria" air pollutants at a level that protects the public's health (Environmental Protection Agency, 1996, 2001), and 2) to periodically review these standards in light of the accumulated scienti¯c evidence
