While the language is cautious, the new advice marks a reversal for state officials. The guidelines still discourage the public from using medical-grade masks needed by beleaguered health care workers, but they do say the public can wear cloth face coverings “made of a variety of materials, such as cotton, silk or linen”.

California Governor Gavin Newsom has warned, however, that face coverings were not a substitute for other safety measures.

“They are not a substitute for physical distancing. They are not a substitute for the stay-at-home order. They are not a call for folks to buy N95 masks and surgical masks, pulling them away from … our first responders,” he said on Wednesday.

Vitally, masks do not prevent the wearer from breathing in infectious particles, but they do help curb transmission when worn by infected people. According to the California health department: “Their primary role is to reduce the release of infectious particles into the air when someone speaks, coughs or sneezes, including someone who has Covid-19 but feels well.”

In light of findings by the US Centres for Disease Control and Prevention (CDC) that as many as 25 per cent of those infected may not show symptoms of any kind, more widespread mask wearing among the general public could make a big difference in slowing the virus’ spread.

The official tune is changing not just in California but in Washington. CNN reported on Wednesday that most members of President Donald Trump’s coronavirus task force now agree that Americans should wear face coverings in public, and that formal guidance on the masks was likely forthcoming.

The Washington Post said the CDC was also considering changing its guidelines and will encourage everyone to wear face coverings in public.

On a local level, the official line on masks has also been changing. On Tuesday, Southern California’s Riverside County rolled out new guidelines “recommending residents cover their nose and mouth when leaving home for essential travel to doctor appointments, grocery shopping or pharmacy visits.”

Sew your own face mask: YouTube lesson goes viral

Since the beginning of the outbreak, American officials and the CDC have urged private citizens not to use masks, saying they should only be worn if a person is symptomatic.

As recently as two weeks ago, US Surgeon General Jerome Adams wrote on Twitter: “Seriously people – STOP BUYING MASKS! They are NOT effective in preventing [the] general public from catching coronavirus, but if health care providers can’t get them to care for sick patients, it puts them and our communities at risk!”

But on Wednesday, Adams went on NBC’s Today show to say he was asking the CDC to investigate whether it should rethink its recommendation on public mask wearing.

Dr Sonia Angell, director of the California Department of Public Health, warned this week that masks would give people a false sense of security, leading them to flout social distancing restrictions. Still, she admitted that they could be an effective measure in preventing infection. “In short,” she said, “there might be some benefit to using these, but only when they’re used well.”

“There is some evidence that using face coverings may reduce asymptomatic infections,” said Angell, “simply decreasing the amount of infectious particles that go into the air when we cough, when we sneeze or particularly when we speak closely to other people.”

Trump to urge Americans to wear masks when outside

The World Health Organisation has continued to say that healthy people do not need to wear masks, although that view is at odds with government actions in many Asian countries.

On Wednesday, Prime Minister Shinzo Abe said his government would send cloth masks to every household in Japan. In major Chinese cities like Beijing and Shanghai, mask wearing was made compulsory during the outbreak, and many businesses continue to refuse service to anyone not wearing facial protection.

The use of masks for private citizens has emerged as a point of contention not just between countries but also between different communities in the United States.

Among the East Asian population, wearing a mask when unwell is a cultural norm and considered a part of each person’s collective responsibility for the common good – and doing so has been commonplace since the very beginning of coronavirus. Among non-Asians, many still view mask wearing with suspicion or as unnecessary.

“We had a conference with my Hong Kong University colleagues on Monday and they were all wearing masks while giving the talk,” says Peter Chin-Hong, an infectious disease expert at the University of California San Francisco. “And that was startling to the American docs here at UCSF because it’s not culturally normal here.”

How face masks became part of the Hong Kong identity

Even as official response has lagged, Americans have been coming around to the idea of mask wearing, and some are also making their own masks. What began as a way to help overwhelmed health care workers has emerged as a cottage industry, with people comparing their favourite patterns online and masks becoming a popular gift, customised for friends and family members.

Sue Hogan is a retired doctor and also a quilter, and lives in northern California. She started making masks for friends working in undersupplied emergency rooms, then for the staff at her retirement home, and now for her loved ones.

“Many quilters started making them when several hospitals sent out emails to quilt guilds asking them to make cloth masks,” Hogan said. “Quilters always forward to other quilters, and many nurses are quilters. About 10 ladies who live here are making them now.”

She said she learned how to do it from YouTube. “They are very easy to make,” she said. “The first one took me about 20 minutes, now I’m down to about 15.”

Chin-Hong believes that masks have a place among the general population.

“Social distancing for the community is great but not everyone can control their environment – on the subway, the bus, at the cashier at supermarket,” he said, “so in those cases masks or other protection is crucial.”

What if you don’t want to deprive of your local hospital of a mask but you don’t know how to sew?

Chin-Hong says there are still options: “Masks don’t have to be all high grade out in the community. Home-made masks, bandanas, scarves are examples that work.”

Angell, the California public health director, reiterated that masks are just one safety element.

“The most important thing is physical distancing,” she said, “and when that’s done with additional face covering, you may indeed get additional protection.”