When experts talk about the safety of indoor spaces, the conversation often turns to ventilation, that is, the introduction of fresh air.
Two routes of airborne transmission
Even where infections have dropped, the danger of catching a virus is not zero. Being indoors is always riskier than being outdoors when it comes to airborne viral transmission, of which there are two main routes, according to research. Each requires a different approach. Large respiratory droplets – those larger than 10 microns – fall quickly to surfaces, usually about 6 feet from the source. Handwashing, social distancing and face masks offer the best protection against exposure to these relatively large particles, which people can inhale or transmit by touching the mucous membranes of their eyes, nose and mouth, Kipen says.
Smaller respiratory droplets ranging from a few microns to 0.1 microns – the size of a single coronavirus particle – can remain airborne for hours. (“A micron is one-hundredth of a human hair in diameter,” Kipen says.) Filters and increased ventilation are the best tools for dispersing and reducing their concentration, and therefore their infectivity, he says.
“Wearing a mask, preferably an N95, is the best way to prevent exposure to particles that are in the air around you,” says Kipen, and it’s essential “to minimize the time you go to a potentially contaminated place”.
There are several questions to ask to help make an informed decision about whether to stay indoors without a mask:
How much fresh air enters the installation?
“People should ask: how much air is outside air and how much is recirculated?” says Linsey Marr, a 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% outside air.”
How often does the air inside change, that is, does it completely turn around?
“What is the ventilation rate, i.e. how many air changes per hour?” says Marr. “Hospitals have 6 [to] 12 changes per hour or more. Houses have maybe half an air change per hour. With a gym or a locker room for example, we want it to be as high as possible, more than 4 is good. The higher, the better.
She says many places such as bathrooms, gyms and locker rooms also often have fans and blowers — which help push air through filters — and exhaust systems. Remember, though, that fans only blow air, they don’t dilute it with cool air.
What type of filter do you use? If you use 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 Ratio Value, is rated from 1 to 16 and represents the filter’s ability to capture larger particles between 0.3 and 10 microns. The ratings were developed using a test method developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers. The most efficient MERV filters are rated 11 or higher, says Marr.
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% of dust, pollen, mold and bacteria, including both large and small airborne particles, according to the Environmental Protection Agency. “MERV ratings range from 1 to 16, but HEPA is even better than that,” says Kipen.
However, many large air systems cannot handle a HEPA filter, so you are unlikely to find one in your facility. “Their air-moving fans aren’t powerful enough to move the air through the tighter filter,” says Kipen. “They should rebuild all of their HVAC systems.” You can, however, purchase small units with HEPA filters and install them in your home. “Portable air purifiers 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 long-distance transmission, experts say.
“If you’re close to an unmasked person who’s infected, ventilation won’t be as helpful,” says Marr. “The more people there are, the greater the ventilation.”
I asked my periodontist about his practice because wearing a mask is impossible when someone is working in your mouth. He said he uses medical grade (HEPA) filters in every room and the air changes every 10 minutes. Both are good.
However, not all institutions have the answers or will provide them. When I asked a fitness center about their locker room, where you can’t mask effectively in the shower, I learned (via email) that they used a MERV filter of at least 8 and pumped 100% outside air. I then asked (also via email): how often does the air change? Would they like to improve the filter? I have not received any awnsers.
I decided to take a chance, living in a highly vaccinated county with a low infection rate, although I am still a little worried.
“There are different degrees of risk,” says Kipen. “Everyone has to decide what level of risk they are willing to tolerate.”