By Zee Krstic

New research suggests that air conditioning may circulate infectious droplets containing SARS-CoV-2, the virus that causes the novel coronavirus.

Air conditioning's role on your risk may be greater in public than at home.

Experts explain why new research touts humidity as being influential in how infectious airborne droplets can be, even as it relates to summer AC units.

Federal officials are exploring the role of the upcoming summer season on the spread of the novel coronavirus in the United States, debating experts' arguments that humidity, temperatures, and direct sunshine won't have a direct impact on the virality of COVID-19. But a new piece of evidence published in the journal Emerging Infectious Diseases, shared by the Centers for Disease Control and Prevention, suggests that a more pressing concern might be the role that air conditioners will play in keeping communities safe throughout the warmer months.

The newly published study found that nine people were infected with COVID-19 simply by sitting near an air-conditioning vent in a restaurant in Wuhan, China, where the coronavirus was first identified. The study examined how one asymptomatic diner managed to impact diners around their vicinity as infectious droplets were circulated by the air conditioning system. The person in question sat at a table that was located in front of an AC unit. Four people at the person's table later tested positive for COVID-19, as well as five people at neighboring tables.

John Lednicky, PhD, a microbiology and virology research specialist within the University of Florida's College of Public Health and Health Professions, has identified different kinds of potential air-borne exposures related to COVID-19 by actually pulling the virus out of the air. "Normally, many people can produce larger sized droplets, which tend to fall near the person, or medium-sized droplets that can fall a little further, up to six feet away," Lednicky explains, stressing the importance of maintaining a six-foot berth around other people. "But there are also very small particles that can stay adrift in the air, causing inhalation exposure to aerosols… That's exactly what happened in this restaurant." The study also shared that "strong airflow from the air conditioner could have propagated droplets" from table to table in the immediate dining space.

Air conditioning works to keep spaces cool by removing humidity from the air, as water vapor can hold onto heat, Lednicky explains — but moisture also works to weigh down viral droplets and particles as they float through the air. If water vapor is removed to cool the air, which is what happens when you run air conditioning, it may also enable infectious droplets to linger longer than they would outside or in another space where humidity is higher.

But could air conditioning systems pose the same danger in a more intimate setting, like your home? Dr. Lednicky helps to interpret the latest research, including one report published in the journal mSystems that may have sparked the debate about the role of summer's heat on how viral SARS-CoV-2, the virus that causes COVID-19, will be later this year.

Should I be concerned about air conditioning in my home?

Despite the study's potential implications on social distancing procedures over the summer season, Lednicky says the largest risk continues to be close contact with other people. If you've practiced safe social distancing measures consistently or have been isolating yourself or your family, you should not be worried about running air conditioning in your own home. "The bottom line is, you know, as long as you're not hosting parties or have a lot of unknowingly sick people coming into your home, air conditioning isn't going to be a problem," Lednicky says. "Especially if you're actively practicing social distancing measures when you need to step out of the house, and otherwise quarantining yourself and your family at home."

What if you have a family member of a close friend visit your home this summer? Researchers at the University of Oregon and the University of California, Davis, published a report that suggests opening windows rather than using air conditioning systems could prevent the spread, citing existing data on SARS-CoV-2. Their concerns have to do with how a building may be ventilated in the first place — if there's a central cooling unit present, then the risk for circulating infectious droplets could be much higher, they argue. Some ventilation systems may also force air to travel across various rooms or entire units to be cooled, which could also pose a risk.

"In air conditioning, if you don't have good air exchanges, meaning that you don't have a lot of air current in your room that's being diffused by a vent, the chances of encountering virus particles in the air is higher," Lednicky explains. "So your risk would be higher, theoretically. Unless you're somehow living in an apartment complex that is very densely populated and you're sucking air directly into a vent that blows up against all of them... but this really doesn't happen. It's not how air conditioners work."

The concern is much more apparent for shared public spaces, Lednicky says, than homes.

Could air conditioning impact safety in a public space?

Believe it or not, air conditioning has always had the ability to influence viral output; virus particles from any respiratory virus can potentially be circulated by HVAC systems throughout the year. But the concern for SARS-CoV-2 is more extreme because there's no vaccine available for COVID-19, and immunity is largely at risk, even with everyone practicing social distancing measures as best as they can.

In the Wuhan study, there were 73 other people who dined in the five-story restaurant that day, but none ended up testing positive for COVID-19. The reason that more people weren't infected may have had to do with high ceilings, as Lednicky explains that particles tend to drift upwards towards the ceiling as they circulate. On the flip side, it also could have been potentially worse: Air flow and how many vents are in the room can exacerbate the risk of coming into contact with COVID-19 droplets. And the more people in the room, the more likely air conditioning can play a negative role.

"Anytime you have a crowd, you're at risk, because when you put people in an air-conditioned environment, the bigger the crowd, the worse it's going to be. Here in Florida, they're always big crowds in public spaces," Lednicky explains, mentioning Disney World, popular beach bars, and plenty of hotels. Air vents in these locations may collect higher concentrations of COVID-19 droplets if the crowd is large enough, and if there's little to no circulation in a public space, the lack of humidity may impact how likely you may be to breathe in infectious particles.

While Lednicky says more research needs to be conducted to fully understand the study's implications — after all, the study had a small sample size and it didn't replicate conditions in a lab — it seems that public spaces designed to hold large amounts of people may continue to be half full or less in the summer months. Offices, supermarkets, retail stores, movie theaters, restaurants, and gyms, among others, are already looking at new ways of conducting normal business during the COVID-19 pandemic.