The momentum continues to build for a more stringent National
Ambient Air Quality Standard for PM2.5. In June,
EPA announced it would revisit the Trump
Administration's decision to keep the PM2.5 NAAQS at
12 ug/m3. In early October, EPA staff released a supplement to its
assessment of the PM2.5 NAAQS. That supplement
also supported a more stringent standard. 
Now, the Clean Air Science Advisory Committee is about to weigh in. According to Nexis Newsdesk (subscription required), a "large majority" of CASAC members expressed support for a standard comfortably below the current standard. If I were a gambler, I'd bet that EPA lands somewhere between 8 ug/m3 and 10 ug/m3.
Whatever EPA does, I'll make two further predictions. First, as long as EPA picks a number that is not below 8 ug/m3, any challenge to the new standard that argues that the new standard is too stringent will lose. Even before judges appointed by Trump.
Second, the next time EPA assesses the PM2.5 NAAQS, the question will only be should it be made even more stringent; we're not going to see a boatload of evidence that EPA's new standard is overly protective.
This thing is only headed in one direction. And I continue to hope that we succeed in moving towards an economy with no fossil emissions, because - and here's yet another prediction - the calculation of the co-benefits of eliminating fossil emissions is only going to grow over time.
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