There have been numerous studies that support a decrease in the current PM_2.5 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 PM_2.5 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.

Putting aside EPA’s interpretation of the statutory requirement, evidence continues to roll in.  Last week, Science Advances published a study which uses causal inference approaches to conclude that lowering the PM_2.5 standard to 10 ug/m³ would save more than 143,000 lives over ten years.

Will EPA nonetheless still conclude that the current standard provides an adequate margin of safety?  If I were a betting man, I’d certainly put my money on EPA holding firm.  I will only add that the idea that EPA will not lower the standard just blows my mind.