Now that cars burn cleaner, cows are the #1 cause of air pollution in
the Central Valley of California.

To quote you:
"That's the craziest thing I've read in a long time"

CA Central Valley has a remarkable variety of pollution sources, from
dust to ag burning to cows to cars. Each produces a different mixture
of emissions - some more particulates, some more ozone precursors. The
Central Valley has such bad air quality in summer (due to emissions and
climatological factors) that *any* reduction in *any* pollutant is
desirable.

Bruce Jensen