Many places have ended indoor mask mandates, and public health experts say it’s okay to forgo mask-wearing where vaccination rates are high and infections and hospitalizations are low. As maskless people begin venturing inside, they still may wonder whether it is safe.
When experts talk about the safety of indoor spaces, the conversation often turns to ventilation, meaning the introduction of fresh air.
Two routes of airborne transmission
Even where infections have plummeted, the danger of catching a virus is not zero. Being inside always is riskier than being outside when it comes to airborne viral transmission, of which there are two primary routes, according to research. Each requires a different approach. Large respiratory droplets — those bigger than 10 microns — rapidly fall onto surfaces, typically within about 6 feet from the source. Hand washing, social distancing and face masks offer the best protection against exposure to these relatively large particles that people can inhale or transmit by touching mucous membranes of the eyes, nose and mouth, Kipen says.
Smaller respiratory droplets ranging from a few microns to 0.1 microns — the size of a single particle of the coronavirus — can stay suspended in the air for hours. (“One micron is one one-hundredth of a human hair in diameter,” Kipen says.) Filters and increased ventilation are the best tools to disperse and reduce their concentration, and thus their infectiousness, he says.
“Wearing a mask, preferably an N95, is the best way to prevent exposure to particles that are in the air around you,” Kipen says, and it’s key “to minimize the amount of time you are in a potentially contaminated place.”
Questions to ask
There are several questions to ask to help make an informed decision about whether to stay inside without a mask:
How much fresh air flows into the facility?
“People should ask: How much air is outdoor air, and how much is recirculated?” says Linsey Marr, professor of environmental and water resources engineering at Virginia Tech’s school of engineering, who has studied airborne transmission of the virus that causes covid-19. “The best-case scenario is 100 percent outdoor air.”
How often does the air inside change, that is, completely turn over?
“What is the ventilation rate, that is, how many air changes per hour?” Marr says. “Hospitals have 6 [to] 12 changes per hour or more. Homes have maybe a half air change per hour. With a gym or a locker room, for example, you want it to be as high as possible, over 4 is good. Higher is better.”
She says that many places such as bathrooms, gyms and locker rooms also often have fans and blowers — which help push the air through the filters — and exhaust systems. Remember, however, that fans just blow the air around, they don’t dilute it with fresh air.
What kind of filter are you using? If you are using a MERV filter, what is its rating?
When it comes to MERV filters, the higher the number, the better. MERV, which stands for minimum efficiency reporting value, is rated from 1 to 16, and represents the filter’s ability to catch larger particles between 0.3 and 10 microns. The ratings were developed from a test method designed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. The most effective MERV filters are rated 11 or higher, Marr says.
The gold standard for filters is HEPA, which stands for high efficiency particulate air. This type of air filter can trap at least 99.97 percent of dust, pollen, mold and bacteria, including large and small airborne particles, according to the Environmental Protection Agency. “MERV ratings go from 1 to 16, but HEPA is even better than that,” Kipen says.
Many large air systems, however, cannot handle a HEPA filter, so you’re unlikely to find one in your facility. “Their fans that move the air are not strong enough to move air through the tighter filter,” Kipen says. “They would have to rebuild their whole HVAC systems.” You can, however, buy small units with HEPA filters and set them up in your home. “Portable air cleaners with HEPA filters can do the job one room at a time,” he says.
It’s important to remember that questions about ventilation apply to reducing the risk of transmission from a distance, experts say.
“If you are close to someone unmasked who is infected, the ventilation won’t be as helpful,” Marr says. “The more crowded it is, the more important ventilation is.”
I asked my periodontist about his practice since mask-wearing is impossible when someone is working inside your mouth. He said that he uses medical grade filters (HEPA) in each room and that the air turns over every 10 minutes. Both are good.
Not every facility, however, has the answers or will provide them. When I asked a fitness center about its locker room, where you cannot effectively mask in the shower, I learned (via email) they use a MERV filter of at least 8 and pump in 100 percent outdoor air. I then asked (also by email): How often does the air change? Would they upgrade the filter? I received no response.
I decided to take a chance, living in a highly vaccinated county with a low infection rate, although I remain a little uneasy.
“There are different degrees of risk,” Kipen says. “Everyone has to decide how much risk they are willing to tolerate.”