To your Health! Let’s clear the air … well-ventilated spaces reduce COVID-19 transmission

When did you first hear that you should stay 6 feet away from other people in order to keep from getting COVID-19? At this point, I feel as if I learned it decades ago, maybe around the same time I learned to look both ways before crossing the street. These days it’s a safety rule as basic as “Don’t stick a hairpin in an electrical outlet.”*

The problem is, when it comes to preventing coronavirus transmission, the reality isn’t quite that simple. The 6-foot rule is based on the idea that the coronavirus is transmitted primarily by what scientists call “large droplets”: heavier bits of moisture that fall to the ground right away and don’t tend to travel farther than 6 feet. But over the last year it’s become clear that large droplets aren’t the only way that COVID-19 is transmitted.

What are aerosols, and why do they matter?

Unlike large droplets, which fall quickly, aerosols are extremely small, light, micro-droplets that can float in the air for a long time. We know the coronavirus can be transmitted by aerosols because of evidence from superspreader events such as the famous Skagit Valley Chorale rehearsal in March 2020.

If COVID-19 could be transmitted only by droplets, we would expect that only people sitting within 6 feet of the infected singer would have gotten the virus. But people throughout the room got sick, even those who never got close to that unfortunate person and never touched anything they touched. As Shelly Miller from the University of Colorado Boulder demonstrated in her study of the event, those folks got sick because they breathed in aerosols carrying SARS-CoV-2, the virus that causes COVID-19. (Remember, this was in the Stone Age of the pandemic, so none of the singers was wearing a mask.)

Ventilation dilutes aerosols

The idea that aerosols can carry SARS-CoV-2 farther than 6 feet sounds scary, but there’s a nifty flip side to that knowledge. You aren’t going to get sick from breathing in just one or two aerosol particles; it takes a lot. And it’s not hard to decrease the amount of aerosolized virus in a space: Just pull some infected air out and push some clean air in. In other words, ventilate the room.

The ventilation doesn’t need to be high-tech. In one study of schools in New York City, the likelihood of COVID-19 transmission was lower in older school buildings—some of them over 100 years old—than it was in newer ones, simply because the elderly buildings were drafty. However, if your building happens to have a great HVAC system with MERV 13 filters that can remove virus particles, great! That will help. If you can purchase portable HEPA air filtration units for each room of your office, that will help, too. But if you don’t have access to that kind of technology, don’t despair. Just keep the windows open. According to Ian Colbeck of the University of Essex, that change alone can “reduce the risk of infection from particles by more than 70%.”

In fact, even if you do have all the tech in the world, if you’re dealing with a space that has to hold a group of people for a long time, keep the windows open as much as possible. That’s what teachers in Harvard’s schools have been doing all year. The bottom line is this: The more ventilation, the better.

Don’t forget your trusty mask, though. Yes, masks fog your glasses and make your face sweaty and are generally annoying, but they will help protect you even against aerosols. As I reported in a recent To Your Health column, CDC research has shown that if two people are wearing knotted and tucked surgical masks, transmission of aerosols from one to the other is reduced by about 96%. You can get that same benefit by wearing a surgical mask under a three-layer cloth mask. And if they work that well against aerosols, you can bet they work against large droplets. Masks help a lot.

So put on your masks and open the windows. Upgrade the air filters in your building if you can. Grab a HEPA filter for your room. And breathe a little easier knowing that you’ve decreased your risk of catching COVID-19.

*True story: My uncle told my mother to do this when he was 10 and she was five. She remembers it to this day.
  

Liz Ruark is the founder of covidsafeschools.org and co-chair of COVID-Safe Schools, Harvard Public Schools’ community COVID-19 screening program. She holds a DVM degree from Tufts Cummings School of Veterinary Medicine.