Last week, EPA posted its draft Integrated Science Assessment for Particulate Matter.  It’s the foundational document for EPA’s periodic review of its National Ambient Air Quality Standard for PM.  The current standard for PM_2.5, promulgated in 2012, is 12 ug/m³.

Section 109 of the Clean Air Act requires the Administrator to set the NAAQS “requisite to protect the public health” with “an adequate margin of safety.”

The new ISA states that:

Evidence from U.S. studies examining short-term PM2.5 exposure and mortality indicate a linear relationship at concentrations as low as 5 µg/m³ with cut-point analyses providing no evidence of a threshold.

There is some uncertainty in the discussion in the ISA, but it seems pretty clear to me that the current standard of 12 ug/m³ does not protect the public health with an adequate margin of safety.

Before Scott Pruitt left EPA, he instructed the Clean Air Scientific Advisory Committee to consider adverse economic and energy impacts resulting from changes to NAAQS.  I have a hard time seeing how that direction comports with the statutory language.  I suspect that the current Supreme Court may explain that to me at some point.