Your hot-weather guide to coronavirus, air conditioning and airflow

“The more outside air you have, the more you dilute the virus,” said scientist Jose-Luis Jimenez.

This photo from 2018, before the coronavirus pandemic, shows Nauset Neighbors volunteer Frank Bridges, 66, install an air conditioning unit for Ann Miller, 79. Tuesday, July 3, 2018. (Christine Hochkeppel for the Boston Globe)

Despite its critical role in our daily lives, air is not something most of us spend a great deal of time thinking about. It’s that easy to take for granted. Unlike water, we don’t need to fill up a cup to consume it. If some escapes from the room, more will find its way back in, whether we open the door or not.

“If you are comfortable, you ignore it,” said Wade Conlan, a mechanical engineer who evaluates ventilation systems on behalf of Hanson Professional Services.

But like so many little luxuries we once took for granted, our days of blissfully ignoring air may be numbered, because a growing number of scientists are convinced that a significant amount of coronavirus transmission occurs through the air in indoor spaces and that poor ventilation magnifies the risk.


Not everyone has the ability or resources to make the changes to a home or workplace to improve air circulation. But scientists and engineers say that it’s worth trying to understand the basics of how airflow works — in case there is a relatively easy tweak that could keep you a bit safer.

When in doubt, open the windows. And remember that outdoor air is good.

The precise way that viral particles flow through a room when an infected person talks, sings, exhales or eats is something that scientists are continuing to investigate. Previous case studies have shown it to be complicated. If there is one easy-to-understand principle that aerosol scientists and engineers have come to agree on, though, it’s this: The more outdoor air coming into a room, the better for dispersing that cloud of viral particles that might be lingering. And one of the most reliable and cost-effective ways to get outdoor air into a room is to open a window.

“If you don’t know if the place is well ventilated, but you have the ability to open a window, I would do it,” said Shelly Miller, a professor of mechanical engineering at the University of Colorado Boulder. That, she said, or get out quickly if you’re swinging by an indoor location with other people in it.


The outdoor air that comes in will eventually replace the indoor air, according to Jose-Luis Jimenez, an aerosol scientist at the University of Colorado Boulder.

“The more outside air you have, the more you dilute the virus,” said Jimenez, who was among the scientists and engineers who sent a letter that pushed the World Health Organization to acknowledge that airborne transmission of the novel coronavirus is a threat in indoor spaces.

If you want to speed up the flow of outdoor air into a room, you could also take a box fan, place it in a window and blast it outward, Jimenez said. When any amount of air leaves, that same amount of air returns — it’s a fixed volume. Therefore, the fan should help pull in the same amount of outdoor air.

Your type of air conditioner matters. Some pull in outdoor air. Others simply recirculate indoor air.

If you have air conditioning in your home, no one is saying that you need to give up on it entirely. When it’s sweltering out, air conditioning can be essential not only to help you function but also to avoid heatstroke.

But if you are going to spend time in a cooled space with other people, it may be worth understanding a bit more about the cool air you are breathing. Basically, all air conditioning falls into one of three categories.


— The unit cools both indoor and outdoor air.

— The unit cools and recirculates only indoor air.

— The unit relies entirely on pulling in outdoor air. (These are uncommon outside hospitals and labs.)

Centralized air systems, such as those common in office buildings, dorms and some large apartment buildings, often fall in category one. Jimenez and other building scientists involved in coronavirus prevention are currently advising owners of businesses and buildings with category one systems to adjust the ratio to pull in more outdoor air, an enterprise that can be costly. Take a casino in Las Vegas, which is kept cool enough to keep people gambling inside while it feels like 120 degrees Fahrenheit outside. Cooling that hot outdoor air will be more expensive than recirculating the already cool inside air. But given that keeping customers healthy is also a priority, more are willing to revisit their approach, Jimenez said.

Few of us have the ability to adjust our air conditioning in this way. Most window units sitting with their rears facing the outdoors, for example, fall into category two. Instead of pulling in outdoor air, they are dumping heat from the room outdoors, said William Bahnfleth, a professor of architectural engineering at Penn State’s Institutes of Energy and the Environment.
If you live alone or with people you’re sure aren’t infectious, those units are fine. But if you give in to throwing that birthday dinner for your parents, or if your teenager has been less than strict about staying home, it’s worth remembering that “any virus that’s present will be mixed in” to the recirculating indoor air, Jimenez said.


So if you have to have people over, it may be preferable to revert to rule one: When in doubt, open the windows. Or better yet, go outside.

Not all filters are equal. But a good filter can be just as effective as pulling in outside air.

So, what do you do if you’re stuck with a unit that primarily recirculates indoor air, and it’s unrealistic to open the window? This is where filters come in. The right filter is just as effective as pulling in outside air, said Dr. Edward Nardell, a professor at Harvard Medical School who has written about the role that air conditioning plays in spreading airborne diseases.

Along with removing dust, pollen, cooking odors, tobacco smoke and chemicals, filters can take viral particles from the air. Some filters go directly in air-conditioning units and central air systems. Others are designed to stand alone. MERV and HEPA are two widely trusted, certified types.

MERV filters are rated on how efficiently they remove particles in a specific size range from the air. ASHRAE, a professional society of air conditioning, heating and refrigerating engineers, recommends MERV 13 and above for filtering out the coronavirus, said Bahnfleth, who leads the group’s epidemic task force. It is what Bahnfleth has in his own house. Any HEPA filter is even more efficient than the highest-rated MERV filter, he added, so either should effectively capture coronavirus particles.

Many central air systems are designed to incorporate specialized filters. But not all can handle the most advanced filters. Lower-rated filters still could be helpful, Conlan said: It’s not that they won’t ever catch smaller particles; they just won’t do it as frequently. Window units are typically designed for comfort, not health, and have even more filter limitations.


For those who can afford them — or push their employers or landlords to buy them — a stand-alone HEPA filter is a good option, Bahnfleth said. Some are designed for bigger spaces than others. The key, Jimenez added, is picking one that will filter all the air in the room at least twice an hour.

Be aware that if an air-filtration system sounds too good to be true, your instincts may be right. Some of them appear to rely on questionable marketing and science, Jimenez said.

There’s no ‘good spot’ in a room. Instead, keep your distance, wear a mask and get out quickly if you can.

Now that you’re an air expert, it may be tempting to think that you know how to pick the safest position in a restaurant or other indoor space you might find you have a reason to be in.

But even experts cannot easily eyeball the lowest-risk location, said Andrew Persily, who oversaw the development of an online tool to estimate exposure to infectious aerosols in rooms and buildings as chief of the Energy & Environment Division at the National Institute of Standards and Technology.

“Depending on the airflow pattern and where the aerosols are released, there may be regions in the room that result in higher exposure than others,” he said. “It’s tough to predict.”

It’s also hard to gauge how many is too many people in a given space. After all, it only takes one infected person to get other people sick. If you have a carbon dioxide detector, you could try a technique previously used to manage the spread of tuberculosis and use that to tip you off, Miller suggests. If carbon dioxide levels are above 1,000 parts per million, you’d be wise to decrease the number of people in the indoor space, increase the amount of outdoor air or both, she says.


An alternate approach is to look around. Do you see other people? If so, leave.

Get's browser alerts:

Enable breaking news notifications straight to your internet browser.


This discussion has ended. Please join elsewhere on