In some states and major cities, it’s now mandatory to wear a mask in public to prevent the spread of COVID-19. That mask could be anything from a handkerchief to a designer cotton mask to a full-blown N95 respirator, so long as you have something between your mouth and nose and the world around you.

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But in California’s Bay Area, there’s a new stipulation: You can wear any style of mask you want, so long as it doesn’t have a valve in it. What? Why? Because a mask with a valve may protect you from some pathogens in the air, but it doesn’t protect the people around you from your own breath. Let us explain. What is a mask valve? Some masks and respirators have a little plastic piece embedded in the fabric. It’s a one-way valve. That means it closes when you breathe in, so pathogens can’t get in through it. But when you breathe out, this valve opens, creating a doorway for your exhalation to leave the mask. Why do public safety officials care? When you wear a mask with a valve, a significant portion of your exhalations are entirely unfiltered. A normal cotton mask is far from perfect at capturing virus particles that you might inhale or exhale. (Surgical masks were originally invented to protect patients from coughing and sneezing, not pathogens from the air!) But when you add the valve, you aren’t even filtering your breath with cotton. Your mouth is essentially an open exhaust. Why do any masks have valves in the first place? Surgical masks have been around for over 100 years, and they weren’t designed with valves. The N95 respirator is a relatively new invention, introduced in the 1970s, and it was brought to market largely in response to a need in industrial America rather than medicine. So an N95 mask comes in two flavors, with and without valves.

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Early respirators were designed to be used in factories and coal mines. They filtered particulate matter, aka dust, from the air that would be harmful to breathe in. But wearing a respirator can be a miserable experience. It traps heat and vapor from your mouth. That’s why these industrial respirators had valves, so that it kept the wearer cooler and more comfortable while working a long shift, Nikki McCullough, an occupational health and safety leader at 3M, explained to me last month. If people were to breathe out flu germs in a coal mine, the coal would be just fine! To this day, industrial N95 respirators have valves. Hardware stores sell N95s with valves (or they did when they were in stock) to the public for home improvement projects. And so a lot of the respirators out there have valves. (At the same time, many mask companies have added valves to their designs in response to COVID-19, presumably to distinguish their products.) When hospitals adopted N95 respirators, did those versions have valves? No. N95s were historically used in factories, but in the 1990s, with the rise of AIDS and a related outbreak of tuberculosis, doctors were looking for a new level of airborne protection, since tuberculosis is highly contagious through the air. That’s when the medical community bought into N95s and created its own standards for these respirators. The big difference? The medical community mandated no valves, so healthcare workers didn’t get the people around them sick. To this day, the CDC recommends hospitals not use N95s with valves. “Respirators with exhalation valves should not be used in situations where a sterile field must be maintained (e.g., during an invasive procedure in an operating or procedure room) because the exhalation valve allows unfiltered exhaled air to escape into the sterile field,” it writes. So is it OK to wear my mask with a valve if it’s legal in my area? Not really! Yes, if you have a mask with a valve, you are protecting yourself as well as you would be if you wore a mask without a valve. There’s no difference in personal protection according to anything published in the CDC or elsewhere. But if you have a mask with a valve, you’re not really protecting those around you, which is at least half of the point of wearing a cotton mask in the first place. The reason we all need to wear masks is that we can all be infected with COVID-19 and not know it for weeks before showing symptoms, if at all. Doctors have also tracked occasions where people have tested positive, negative, and then positive again. There is clearly a whole lot we don’t know about COVID-19. But given that we’re reaching the consensus that this pandemic can spread through the air, your mask should treat filtering as a two-way street.