Excerpt:

A mysterious shower of microscopic chemicals near a Fresno
shopping center could be the first evidence of a broad,
undetected assault on the lungs of San Joaquin Valley residents.
If confirmed in other Valley cities, it means many thousands of
people are daily breathing these cocktails of chemicals — known
as ultra-fine particles — that corrode and damage lungs.
The plume in Fresno probably spreads over many square miles, not
just the Fashion Fair area where they were discovered, said UC
Davis atmospheric scientist Anthony Wexler, who detected the
pollution.