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Coronavirus FAQ: Got any tips on improving indoor air flow to reduce infection risks?

Cracking a window can help reduce the risk of indoor COVID transmission.
Cracking a window can help reduce the risk of indoor COVID transmission.

We regularly answer frequently asked questions about life during the coronavirus crisis. If you have a question you'd like us to consider for a future post, email us at goatsandsoda@npr.org with the subject line: "Weekly Coronavirus Questions." See an archive of our FAQs here.

Over the past two years, we've all had a crash course in understanding how to prevent respiratory infections.

And we've learned that clean air – via ventilation (i.e. fresh air flow via open windows and doors) and filtration (removing particles from the air with a filtering device) — is really important for preventing COVID and other respiratory illnesses. It's something many experts knew all along. Now the public is catching on.

"Most of the air that we breathe in our lifetimes, we breathe indoors," says Richard Corsi, dean of the University of California Davis College of Engineering. And virus particles can linger in the air of unventilated places, increasing chances of getting sick.

Of course, those particles are not visible. "If people could see COVID in the air, it would make a lot more sense that what you need to do is clean the air in your house, exchange the air out, get fresh air in, improve ventilation so that you don't have a lot of air hanging around where other people can breathe it in and get infected," says Abraar Karan, an infectious disease physician at Stanford University.

So what can you do, as an individual — at home, work, school, the gym — to make the air cleaner and safer?

That's what we asked three experts.

What's the most basic way I can improve ventilation?

"Just getting more air flow into the house itself" helps, Karan says.

Open windows if you can, says Linsey Marr, professor of civil and environmental engineering at Virginia Tech. "Especially if you can open them on opposite sides of the room, so you get some cross-ventilation — air coming in one and going out the other."

Even if you can't open all of the windows or can't open them all of the way, cracking windows a little bit is still very helpful. "They don't have to be wide open," Marr says.

Opening doors in shops and gyms also helps.

Marr worked with the owner of her local gym to improve ventilation early in the pandemic. The gym didn't have central air conditioning, so it couldn't rely on filtration. The easiest option was to open the doors. "I estimated how much benefit we would get from opening the doors and it was a ton, so we kept them open all winter," Marr says.

And there was no known transmission in the gym, says Marr, who advised the facility and kept track of COVID developments. Even when staff picked up the virus from other places, they don't seem to have passed it on at work, she says.

How can I improve my home filtration system?

If you have an HVAC system – forced air heating/cooling/ventilation — you can do two main things: run the fan more and upgrade the filter in order to catch more viral particles. Every HVAC system has replaceable filters that trap allergens and dust in the air – and viral particles, too.

HVAC systems typically don't circulate air 24/7, only running part of the time — when indoor temperatures drop or rise.

To make sure the air is getting filtered through the HVAC system, "if you can, run the fan continuously," Corsi says.

When the windows are open, you can also turn on other fans, like the bathroom and kitchen exhaust fans, to "help pull in that clean air from the outside and flush out the virus," Marr says. True, they can be noisy, but they can create some air flow by pulling the air up toward the ceiling and out of the room.

But when the windows are closed, most home HVAC systems simply recirculate the same indoor air, and the standard filters you use may not be effective at catching the tiny virus particles. So you can also look into replacing those filters with higher-quality options, like a filter with a minimum efficiency reporting value (MERV) of 13.

"If you can put a MERV-13 filter in your HVAC system and if the fan is recirculating continuously, then you're going to remove a lot of the aerosol particles," Corsi says.

The majority of school ventilation systems can also be upgraded to MERV-13 filters, he says.

But, he cautions, not all systems can handle the more effective filters, so it's a good idea to have an HVAC specialist inspect yours. You don't want your whole system to break down because it is too strained trying to pull air through incompatible filters.

HEPA is another kind of filter that is even more effective at removing viruses from the air, but most home HVACs don't work with HEPA filters. However, you can get a portable HEPA air purifier.

Can portable purifiers help?

"If it says HEPA, then it's going to filter out over 99% of the air that passes through it," Marr says. "In general, price goes with size in terms of how much air it moves through it." That means the bigger the unit, the more it costs.

It's important to find the right-sized unit to purify the air of a room in an hour or two. "There's a big difference between like a $50 one that can clean the air in a closet in a reasonable amount of time and a much bigger one that probably costs $200 or $300 and can clean the air in a bedroom in a reasonable amount of time," Marr says.

But, she says, there are cheaper options: for example, a do-it-yourself system involving a simple box fan – a portable model that is typically square, has a propeller blade and can often fit in a window — and MERV-13 filters. That "is actually more effective in many cases" than expensive HEPA units, says Marr.

Known as the Corsi-Rosenthal box after its creators, this DIY filter is easy and inexpensive to make: you create a square shape with four filters making up four sides and the fan, pointing up toward the ceiling, duct-taped on top. A piece of cardboard is taped to the bottom, and the homemade air purifier can be put anywhere a more expensive purifier might go.

Karan agreed. "The Corsi-Rosenthal box is basically a very low-cost way to get better ventilation."

Corsi also agreed – and only partly because he was one of the inventors. When he first sketched out the idea early in the pandemic, he thought it might be a more cost-effective way to offer some air filtration. And it is – the materials cost between $80 and $100.

"But I also didn't realize how incredibly effective it would be," Corsi says.

The boxes can be up to 2.5 times more effective than a $300 HEPA filtration unit, according to a study by other researchers.

No matter what portable filtration system you go with, make sure you position it carefully. Don't put the device in a corner, where it might just recirculate the same air. Keep your purifier a foot or so away from the wall for the same reason.

For larger rooms, two or more units can be a good idea, says Corsi, and you can put them across the room from each other so all the air in the room gets filtered well.

What about those little travel-size air filters? Any benefit?

You might have seen little HEPA purifiers about the size of a water bottle that you can stash in a purse or backpack. But do these small devices actually help?

"They should, and you'd want to direct the airflow," Marr says. "That can clean a small area of a small amount of air."

Just make sure you position the stream of clean air as close to your face as possible, setting it on the table or desk in front of you.

"If you're using them so that the air is right in front of you working at your desk and it's blowing the air up toward your face, it can reduce the concentrations of aerosols as you're breathing," Corsi says. "I wouldn't overemphasize their benefit, but there can be some benefit."

Such purifiers, which cost $30 to $50, can add another layer of protection while you're traveling, eating at restaurants or going to work or school.

Should I buy a CO2 monitor to check how good the ventilation is in different places I go?

"If you are a very cautious person, then that can be a good tool to help you gauge the risk in different spaces," Marr says.

CO2 monitors measure how much carbon dioxide is in the air. They can't tell how much COVID is in the air, but if there is a lot of CO2 in a space, then it's not well-ventilated.

"If the CO2 readout is under 1,000, that's pretty good. Anything over that is a warning sign that a space is poorly ventilated. At high rates, you're breathing in a lot of "other people's exhaled breath — like drinking backwash," Marr says — a nice way of saying drinking other people's spit while sharing a drink.

But these are loose rules that depend on how big the space is, how many people are there, whether they are masked, and how many cases are circulating in the community.

"I'm not a big fan of using CO2 monitors for very specific analysis," Corsi says. "They're not exact." But, he says, they are very useful for telling you when air quality is very good or very poor.

Does improving ventilation mean I can skip wearing a mask?

Improving air quality means you're less likely to get sick, because there's less virus in the air. But it's not 100% effective.

The experts all agreed that wearing a mask is one of the most effective ways to reduce your chances of getting sick even further, or – if you're sick – passing the virus on.

That means you should continue to mask up in indoor public places when cases are high, as they are in the U.S. right now.

And if you're sick or someone in your household is, wearing a mask can keep the virus from transmitting even at home.

"If you have someone who is sick, then you want to try, if it's reasonable, to have them wear a mask because that will reduce the amount [of virus] that gets into the air in the first place," Marr says. And it's a good idea to mask up when you are around them to protect yourself.

If a family member gets sick, will good ventilation keep me from catching COVID?

Yes! It may take a little work, but it is possible to stop transmission in the home.

As an infectious disease doctor, Karan has seen many patients who got sick at home from other family members. "That's the one thing that we have a really hard time with right now," he said.

But "I know that there are ways you can make the home safer — I've done it myself," Karan says.

All of the advice the experts offer here is especially important to keep cases from spreading at home: open windows, have a fan blowing air out the windows, wear masks as source control, improve air filtration.

In the sick person's room, try to keep the door closed and face a fan to blow out the window, so "what they're emitting into their room doesn't get back into the rest of the house underneath the door and that kind of thing," Corsi says. "That'll actually go a long way to helping others in the house not get infected."

Should I press for better air quality in public spaces?

Sometimes you will be in places where you can't control air quality, like at work, school, restaurants or businesses.

But it doesn't hurt to ask what improvements such places have made to air quality. If you're worried about your kid returning to school in the fall, for instance, you can talk to the teacher about opening windows or using a portable air filter.

"We need to be holding businesses and then the government responsible," Karan says, to make sure they upgrade ventilation and establish new indoor air quality standards.

"It's not just about COVID," he says. Cleaning the air can reduce other respiratory viruses, like the flu and RSV, as well as mold and allergens.

It's easy to want to give up on COVID precautions, thinking this is the best we can do. But "people need to hear that there is a way to solve this problem," Karan says.

"We're not going to eliminate COVID. But what we can do is we can reduce COVID transmission significantly."

His verdict: "Ventilation is the way forward."

Melody Schreiber (@m_scribe) is a journalist and the editor of What We Didn't Expect: Personal Stories About Premature Birth.

Copyright 2022 NPR. To see more, visit https://www.npr.org.

Melody Schreiber