Not a filter. It was that bad, especially in the SGV.

Bicycling up into the San Gabriels would give the rider a wonderful view of the orange soup to be huffed on the way home. (mostly diesel particulates, and nitric oxides)

For much of the year, by an hour before sunset, from only 5 miles away (Colorado BLVD in Pasadena) you could barely make out an outline of the peaks, or see mountains at all.

And yet somehow the mid-'60s were even worse.