There have been numerous studies that support a decrease in the current PM2.5 annual standard of 12 ug/m3. EPA has nonetheless proposed to retain the current standard on the basis that there is too much uncertainty regarding whether those studies provide a basis for concluding that PM2.5 concentrations below the standard cause increased mortality. As I have previously noted, the statutory provision requires that NAAQS be set with an “adequate margin of safety.” That would seem to require EPA to resolve such uncertainty in favor of a more stringent standard.… More