In fact, you may unknowingly be wearing one that does more harm than good when it comes to preventing the spread of COVID-19.

It said, in part, “the ones with the valves or openings on the front are NOT safe, and may actually propel your germs further.”

It was a reminder that the city’s original public health order requiring face coverings said that any mask with a one-way valve was not to be used.

“Valves of that type permit droplet release from the mask, putting others at risk,” the order stated.

“The virus can be transmitted through the valves, which offer no filtration at all,” said Dr. Ali Raja, executive vice chair of the department of emergency medicine at Massachusetts General Hospital and an associate professor at Harvard Medical School.

“Any mask with a one-way valve is only going to protect the person wearing it. It won’t protect anyone around that person from potential exposure to virus particles they exhale,” he told Healthline. “It may give the people surrounding them a false sense of security.”

“In the spirit of ‘your mask protects me and my mask protects you,’ a mask with a valve virtually makes that impossible,” said Jessica Malaty Rivera, MS, a microbiologist and contributor at the COVID Tracking Project.

“If you don’t have anything filtering what’s going out, it’s kind of like your mouth is an open air exhaust. And that’s the danger,” she told Healthline.

The San Francisco Fire Department also posted a companion tweet last week with a video showing the type of mask they are seeing more often and how firefighters are having to cover the valves with a second mask to protect themselves.

“We put together this video because our firefighters, EMTs, and paramedics were coming on scene with COVID-positive patients who were wearing these masks,” said Adrienne Sims, the director of San Francisco Firefighters Local #798.

“We wanted to show our first responders how to approach a patient who is wearing such a mask and how to protect themselves without touching the mask and contaminating their own personal protective equipment (PPE),” Sims told Healthline.

As an example of what they’re seeing, the firefighters showed an 8511 Respirator with a Cool Flow Valve made by 3M.

However, on its website, 3M says the mask is recommended for relief from “dust and certain particles encountered during sanding projects… grinding, sawing, sweeping, installation of fiberglass insulation.”

“These masks are designed for situations in which the wearers need to protect themselves but aren’t at risk of transmitting infection to others,” Raja said. “The valves make breathing more comfortable and the masks less hot and humid for the wearer.”

“We have found that many people have these masks in their possession because they were left over from our wildland fire season in California,” said Sims.

Experts say similar masks with a valve are also readily available online. Consumers know that N95 masks are the gold standard for healthcare workers and are difficult to get.

“The ones for sale online are often super cool, sporty, with stretchy material. I think a lot of people thought they caught a white whale when they get their hands on N95 masks,” Rivera said.

“The average person looking at the mask with the valve won’t know that the valve is a huge compromise,” she added. “It diminishes the value of the public good you’re trying to do.”

The Centers for Disease Control and Prevention (CDC) did an about-face last month and began recommending people wear a facial covering in public places.

The agency said it could be a homemade cloth facial covering or mask. They asked the general public to stay away from scarce medical-grade N95 masks needed for healthcare workers.

The CDC said the masks were an additional step to take to protect others as you could have COVID-19 and be without symptoms.

However, agency officials said, the facial coverings were not a substitution for frequent handwashing and physical distancing to prevent the spread of the virus.

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