From the California Department of Public Health:
The California Department of Public Health today released updated PDF guidance that requires Californians to wear a face covering in high-risk settings. A growing body of scientific research has shown that people with no or few symptoms of COVID-19 can still spread the disease and that the use of face coverings, combined with physical distancing and frequent hand washing, will reduce the spread of COVID-19.
"Science shows that face coverings and masks work," said Governor Gavin Newsom. "They are critical to keeping those who are around you safe, keeping businesses open and restarting our economy."