On Friday, EPA announced that it was reconsidering its 2020 decision to leave the National Ambient Air Quality Standards for ozone unchanged. 
The reconsideration will be based on the existing record. The notice does not identify any specific perceived flaws in the 2020 decision. However, EPA stated that it:
will reconsider the decision to retain the ozone NAAQS in a manner that adheres to rigorous standards of scientific integrity.… More
Yesterday, EPA formalized its decision to leave the ozone NAAQS unchanged, at 70 ppb. I don’t think that this decision is in the same category of egregiousness as EPA’s recent decision not to reduce the PM2.5 NAAQS. After all, only one decision can be the single worst environmental policy of an entire administration.
I’m not that close to the science on the ozone NAAQS,… More
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 PM2.5, 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.… More