|Historically, the southern part of California's central valley has the highest levels of PM2.5|
Apologies for the different scales. The people who published these graphics chose color scales that "spanned" the data with the image of the continental United States being deep red at 20 micrograms/m^3 of very small particulate matter PM2.5
In this image of Communist China, every region that is not one of the two most saturated shades of green would be deep RED if the graphic used the same scale as the one used for the image of the United States.
Furthermore, the most intense shade of red on the image for Communist China is seven-and-a-half times more polluted than the same shade on the US image.
Wuhan's PM2.5 rating is 65 micrograms/m^3
The PM2.5 particles are biologically important because
Airborne fine particulate matter (PM2.5) attracts more and more attention due to its environmental effects. The immune system appears to be a most sensitive target organ for the environmental pollutants. Inhaled PM2.5 can deposit in different compartments in the respiratory tract and interact with epithelial cells and resident immune cells. Exposed to PM2.5 can induce local or systematic inflammatory responses. Source