About one week after the scientists made their case, the WHO modified its stance on aerosol transmission and recognized these tiny droplets may be a bigger issue than previously thought.

“It’s nothing to be feared,” Joseph Allen, assistant professor of exposure assessment science at the Harvard T.H. Chan School of Public Health, says about the latest understandings of aerosol transmission. “It just means that we have to recognize it’s happening so we can apply the appropriate control strategies” — including how we circulate air in buildings.

Different air conditioning systems distribute air in different ways — from single window units that cool smaller spaces to complex HVAC systems that regulate temperatures in large buildings. One thing they have in common, however, is that “a lot of the air is recirculated,” albeit at varying percentages, explains Linsey Marr, the Charles P. Lunsford professor of civil and environmental engineering at Virginia Tech and an expert in aerosol science. “And when you recirculate that air, that means that the virus is still sticking around in that building, rather than being replaced by more outdoor air, which presumably is virus-free.”

There haven’t been any confirmed cases of the virus spreading by way of air conditioning units yet, Marr says, but the virus has been found in air-handling ducts in a hospital. (Whether it was enough virus to infect somebody is unclear; the study hasn’t yet been peer-reviewed.) An analysis published by the Centers for Disease Control and Prevention (CDC) also examines the role an air conditioning unit may have played in the spread of the virus in a small, inadequately ventilated restaurant in Guangzhou, China.

“There’s nothing inherently wrong about air conditioning; it’s just when it’s done poorly that it’s a problem,” Allen says. Which is why, as schools and businesses determine the safest course of action for reopening, much of the attention is on making sure buildings and their ventilation systems are as healthy as possible.

Your air conditioning at home

Running your air conditioning at home shouldn’t be a problem, especially if you are with the same group of people you’ve been exposed to this whole time. One situation where it may be a cause for concern is if you have a big party in your home, Marr says, “which you shouldn’t be doing anyway.” The CDC recommends older adults limit interactions with other people as much as possible to reduce their risk of infection.

Plus, in the summer months, the dangers of heat stroke and other heat-related illnesses can’t be overlooked, particularly among older adults, says Richard L. Corsi, dean of Portland (Ore.) State University’s Maseeh College of Engineering & Computer Science and an expert on indoor air quality. According to the CDC, air conditioning is the number one protective factor against heat-related illness and death.

However, if you are concerned about the quality of air circulating in your home, there are a few things you can do. First: Replace the filters in your air conditioning system, especially if it’s been a while. Both Corsi and Marr recommend upgrading to a MERV 13 filter, which removes a high percentage of particles from the air, including droplets and aerosols. If your system won’t handle a MERV 13, opt for the highest-quality filter it can handle and be sure to follow installation instructions.

If you don’t have air conditioning at home, and it’s comfortable to do so, open your windows to bring the outdoor air inside. This will help dilute and clear out the trapped air. You can also purchase a portable HEPA (high-efficiency particulate air) air cleaner to keep in your home. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends placing it in the area where most people in the household spend their time.

Finally: If you’re riding in a car with people from outside your household, roll the windows down, Marr says. “It makes a huge difference. Cars will usually recirculate the air for air conditioning.” The same goes for public buses, which often have windows riders can open.

What to do if someone at home is sick

If someone in your home has tested positive for a coronavirus infection, it’s best to keep them in a separate room, away from others in the household, the CDC says. If that room has windows, open them and consider bringing in a box or exhaust fan, and adjusting it so that the fan blows air out of the window. This keeps air that may contain virus particles from leaking out of the room to the rest of the house, explains William Bahnfleth, a professor of architectural engineering at Penn State University.

If there’s a bathroom attached to the designated isolation space, run its exhaust fan as often as possible. ASHRAE also recommends sealing off air vents and door gaps to keep airborne virus particles from potentially spreading to other areas of the house through the air system. (The ability to do this will depend on the temperature in the room. It’s important to make sure the person who is sick is comfortable and doesn’t overheat.)

Masks, distancing remain important in public buildings

Large spaces such as schools, office buildings and nursing homes typically have much more complex air systems that pull in outdoor air to mix with indoor air, and filter what’s being circulated with high-efficiency filters.

If you frequent a building such as this, and are concerned about ventilation, the best course of action is to ask someone in charge about how the building is adjusting its air operations in light of the pandemic. You can inquire about upgrading filters, for example, or “see if there are any adjustments they can do to the system, to the dampers, to bring in more outdoor air and use less recirculated air,” Marr says. If you work in a smaller structure with an older air system, opening windows may be the best option.