Marin is newly requiring its residents to wear masks in a variety of settings, a policy change reflecting evidence that a significant number of individuals are asymptomatic or contagious before showing symptoms of Covid-19. 

With continued low case numbers in the county, health officials say there is still widespread vulnerability. Mask wearing is the latest aspect of the social distancing requirements that are key to the regional shelter order, which will be renewed with guidance on re-opening next week. On the coast, mask makers are already hard at work. 

Everyone over age 12 now must wear a mask while inside or while waiting to enter an essential business or when seeking health care. For public transit, those driving and riding must wear masks. For those working for both essential businesses and those continuing minimal operations, masks are required when interacting with the public, when packaging or preparing food, or when working indoors with anyone outside one own’s household. 

The order is enforceable, but public health officials remain positive about the goodwill from residents. “Our work is to try to design the best strategies possible, balancing all the factors, but it is the community, the behavior of everyone, that determines the success of that policy,” Matt Willis, Marin’s public health officer, said during a Monday briefing. “We see through demonstration that people really did take the shelter-in-place seriously, and we see a corresponding decrease in disease and death: that’s the equation that is going to lead us through the rest of this.”

The numbers remain encouraging. Marin has tested 2,800 people, 7 percent of whom were positive. Out of 200 confirmed cases, 140 patients are on the road to recovery. Out of 37 hospitalizations, only six patients are still in the hospital today. There have been 10 deaths, all people over the age of 65, and none since April 6. 

Out of the 17 million people in the six Bay Area counties that acted first in the nation to impose sheltering orders, the University of California, San Francisco estimates around 44,000 lives were saved. 

Yet Dr. Willis, who is drafting the new shelter order with regional health officials before the current order expires on May 3, said it was far too early for any celebration. 

“From the point of view of our level of immunity [as a community], we remain largely susceptible,” Dr. Willis said. “When you prevent infection, you prevent immunity, and so we would estimate, right now, between 95 and 97 percent of our community is still susceptible.”

The latest mask rules, which went into effect yesterday, state that “wearing a face covering, when combined with physical distancing of at least six feet and frequent hand washing, may reduce the risk of transmitting coronavirus when in public and engaged in essential activities by reducing the spread of respiratory droplets. And because it is not always possible to maintain at least six feet of distance, members of the public and workers should wear face coverings while engaged in most essential activities and other activities when others are nearby.” 

That’s a far cry from initial guidance around the virus issued in February, which recommended masks only for those showing symptoms. Though it is not a statewide requirement to wear a mask, several other counties and cities have already instituted the policy. Marin first made the recommendation on April 3. 

Under the newest instructions, masks are not required during outdoor activities, but they are recommended. At the least, bring one along—perhaps already around your neck—for circumstances that arise in which it may be difficult to maintain social distancing, the order states. 

Because running or bicycling causes people to more forcefully expel airborne particles, the order also says that six feet is an inadequate distance between exercising individuals. It recommends taking other measures: crossing the street, slowing down and moving aside on sidewalks, and avoiding running or cycling directly in front of or behind other runners or cyclists not sharing the same household. The order says never to spit. 

For as long as medical-grade and surgical masks are in short supply for medical professionals, the order asks people not to purchase them. It defines an acceptable mask as a face covering over the nose and mouth, made from cloth or some other soft permeable material without holes. It emphasizes that masks can be made from household materials. 

Seamstresses in West Marin have been hard at work for weeks.  

In Bolinas, a group of around 10 women who like to call themselves “masketeers” have been at their sewing machines since the week of the shelter order. The masks, which follow various patterns pulled from the web, are advertised through social media and donated. 

Sage Storm, who closed a sewing store last year and had leftover fabric, has been hanging the masks she makes—which now total around 600—outside the Bolinas Store by clothespins. 

In Point Reyes, the Dance Palace Community Center is organizing another group of volunteers. Since they started several weeks ago, seamstresses have distributed around 170 masks, and many more have been requested, said Elizabeth Zarlengo, the board chair of the Dance Palace. She encouraged residents to purchase masks through local shops—such as at Epicenter, where owner Dana Davidson is making designer masks—if they could. But the Dance Palace will not turn anyone away, she added. The masks are free. 

“I was very hesitant to do it,” said Mara Nelson, who owns Cover Girls Upholstery and is donating masks to the Dance Palace. “I have friends that are nurses, and there is a protocol for putting it on and taking it off. People could depend on it too much and get too close to each other. But then again, this is all you can do, to try to protect each other.”

Ms. Nelson is using scraps from her shop and taking a number of precautions to keep the material clean, washing all her material at high heat. 

All mask-makers are recommending that people wash the masks once they have them. The C.D.C. also recommends hand washing before putting on and taking off any face coverings, and only touching the straps or strings to do so.

Ms. Nelson said she is using cloth ties instead of elastic, which is hard to come by these days. “In my fabrics, I’m looking for 100 percent cotton, and that they are a tight-enough weave not to let you see through, loose enough to breathe through, comfortable enough to breathe through,” she said. 

She also designs them so that they have a spot to insert a filter, which could be made from a T-shirt or even a coffee filter, to add another layer of protection. 

Ms. Nelson, who has researched mask effectiveness, said a well-made homemade mask can be 70 percent effective at stopping droplets. At the same time, some medical-grade masks have a valve on the front allowing air out, making them ineffective protection against the virus. 

“The idea is to protect other people from you,” she said. “You want to protect other people. We all have an element of wanting to protect ourselves of course—that’s just how our minds work.” 

Many local businesses have been ahead of the curve in requiring employees to wear masks. 

Britany Hartwell, the store director for the Palace Market, said staff were rounding their third week in masks. This week she said she will start ensuring that customers also keep their noses and mouths covered. 

“It’s hard to communicate nonverbally,” she acknowledged. “We are smiling at our regular customers, and realize that they can’t see your face. We have to adjust accordingly. If people see us, know that we are smiling at them, even if they can’t tell.”

James Finch, a co-owner of the Bolinas Hardware Store, said that immediately after the first order, the store enacted policies that to him seemed common sense, including limiting hours and the number of customers allowed in at a time, and installing a hand-washing station at the door. 

“When people have to do things that they’ve never done before, you just have to make rules, to take away the opportunity for people to make mistakes,” he said. 

As far as his own mask-wearing, Mr. Finch said, “I’ve been wearing my bandanna so much that now I find myself accidentally wearing it even when I don’t need to.”