1. Los Angeles-Long Beach-Riverside, CA
Ranked 4th for year round particle pollution and 3rd for short-term particle pollution.
Since ozone is a secondary pollutant (not formed directly by the burning of fossil fuels, but from nitrogen oxides produced by such combustion, and that then react in the presence of sunlight) it is the biggest problem in areas that are sunny and hot. In addition, LA is a low basin surrounded by mountains, with an enormous amount of automobile traffic.
Here are just a few steps you can take to improve air quality: