PM2.5 standards

The New York Times reported that the US Environmental Protection Agency (EPA) will reconsider reducing the National Ambient Air Quality Standard (NAAQS) from its current 12 μg/m3 as an annual average. What’s new, and what’s a tired retread? 
Recently we've experienced a trifecta regarding the issue of mortality associated with fine particles (PM2.5, i.e., particles with median diameters less than 2.5 millionths of a meter). Let's take a more in-depth look behind the headlines.