Alameda County public health officer Dr. Erica Pan issued an order on Friday requiring everyone in the county to wear face coverings when they're around people from outside of their households.
Pan said her strict order becomes effective at 11:59 p.m. on Friday but enforcement won't begin until 8 a.m. on Wednesday so people have time to comply by making or buying a face covering.
"A face covering may reduce the risk of transmitting the virus that causes COVID-19 when maintaining at least 6 feet of distance isn't possible," Pan said in a statement.
Pan said, "Everyone should wear a face covering when engaged in essential activities and work, but there is no substitute for physical distancing and staying at home."
She said, "Consistent and broad use of face covering may help to slow the spread of COVID-19 in Alameda County because people may have COVID-19 but not show symptoms or their symptoms are so mild they don't think they have become infected."
Pan said face coverings must be worn in three main settings: at essential businesses, when seeking health care, and when using public transportation or other types of shared transportation.
She said the face coverings can be simple and homemade, made of cloth, fabric or other soft or permeable materials without holes, that cover the nose and mouth.
A similar order also was issued by Contra Costa County health officer Dr. Chris Farnitano on Friday.