Due to almond growers and CAFO's.  The San Joaquin Valley, of all places - - used to be pristene and beautiful, now it's a cesspool of air pollution.  No wonder everyone we know from that area are now using oxygen tanks.  Not all of them are "old" either.  I don't consider mid 50's to mid 60's old at all.  Yet they are having these types of issues.

CA is not going to do a thing to clean this up either, because corporate money is involved. 

As my gramma used to say, "people think money makes the world go 'round".  Guess that's true, sorry to say.

https://articles.mercola.com/sites/articles/archive/2019/01/01/cafo-california-air-pollution.aspx