It's possible this NAAQS (national ambient air quality standard) value will not change as a result, but the discussion is illuminating: EPA failed to show how the value would protect human health, which is required by mandate. This is particularly (ahem) relevant to anyone who wants to claim that compliance with EPA regulation = protective of human health, in, say, some environmental review of a polluting project.
PM2.5 is fine particulate matter.
D.C. Circuit Court of Appeals Remands Annual Fine Particulate Matter Standards, Upholds EPA's Decision on Coarse Particles
I've read the opinion and I think the D.C. Circuit does a bang-up job of articulating the standard of review and applying it propertly. Very well-written. The pm2.5 section would be good for those wanting a quick intro to clean air law.