The American Lung Association has released its annual State of the Air report.
It grades the air quality of California counties on harmful ozone and particle pollution.
San Luis Obispo County received an F for ozone and a C for particle pollution.
The SLO metro area is 18th for worst levels of particle pollution in the nation.
Santa Barbara made the list for cleanest counties for short-term particle pollution.
Elsewhere, the most polluted cities for ozone include Long Beach in the Los Angeles area at number one, Bakersfield at number two and rounding out the top three are Visalia, Porterville and Hanford in the San Joaquin Valley.