The report shows San Bernardino County is the worst county in the nation for smog, followed by Riverside and Los Angeles counties.
Orange County also received a failing grade.
The report comes as the region is experiencing unusually clean air because of recent rain and stay-at-home orders due to the COVID-19 pandemic.
RELATED: Coronavirus: Air quality significantly improves in Los Angeles amid COVID-19 pandemic
However, it's not clear yet if recently improved air quality is a result of reduced activity from coronavirus-related restrictions.
Wayne Nastri - the head of the South Coast Air Quality Management District, the agency that's responsible for monitoring air quality and enforcing environmental rules - said there are other factors which can be contributing to the improvement in the short term.
"I think there's no doubt the air quality is actually better," Nastri told Eyewitness News. "The question is, is it a result of COVID-19 or is it a result of other factors? For us the biggest factor we see is weather conditions."
For more on Nastri's take on Southern California's air quality and how his agency is responding to the coronavirus crisis, watch the video below.
WATCH: SoCal's air quality improving - but not necessarily because we're all staying home