Across the country, as COVID-19 deaths begin to slow and cities and states start opening up businesses, people are forced to try to answer questions about how likely transmission will be in places such as restaurants, barbershops, and offices. And now that Memorial Day and the unofficial start of summer is behind us, the question comes with an added complication: Will being in a space with the AC blasting put you more at risk for contracting the virus?
There are multiple factors to consider, such as how AC could enable the circulation of viral particles in the air and increase the speed and distance they travel. There are also different types of AC systems, which circulate air in different ways. Still, medical professionals and HVAC experts alike agree that, while it’s possible, there’s little evidence that AC is a large contributor to infection. In fact, some types of air-conditioning can, inversely, be an effective tool to mitigate the spread.
We’ve come to know that close-contact droplet transmission is the primary method of coronavirus infection. Those big, heavy viral droplets (microscopically speaking) can go from person to person—but if they don’t, they fall to the ground very soon after being expelled. In the summer, the humidity pulls them down even faster.
This aerosol transmission is the concern when considering the effect of AC. “If an infectious dose of the virus can hover in the air long enough to be recirculated by air-conditioning, it could cause transmission in a remote place,” says William Schaffner, professor of preventive medicine and infectious diseases at the Vanderbilt University Medical Center. But so far there’s little evidence of AC-spurred contamination. Though that may be because health professionals have been swamped with the crisis and taking care of sick people—maybe, as Schaffner says, “public health departments haven’t been looking, as epidemiologists usually do, for distinctive common denominators among cases.”
There is one South Korean CDC study that’s been cited as suggestive of a correlation. It investigates an air-conditioned restaurant in Guangzhou, China, where people dined far apart from each other on January 24, and nine people got sick by February 5, all from a single sick diner who had traveled from Wuhan. The researchers concluded that “in this outbreak, droplet transmission was prompted by air-conditioned ventilation.” The theory is that the force of the strong airflow of the air conditioner in the restaurant propelled large respiratory droplets faster and beyond the range normally associated with the coronavirus. The AC facilitated close-contact transmission, even though the diners were socially distant.
The study admits that it doesn’t consider the aerosol route. A follow-up study (which has not yet been peer-reviewed) was “more thorough,” according to Bill Bahnfleth, a professor of engineering at Penn State University and a fellow at the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), for which he’s chairing a coronavirus task force. That study suggests that transmission probably happened through the airborne route, due to poor ventilation.
Bahnfleth explains that the poor ventilation means that the aerosol contaminant that lingers in the air is able to become more concentrated into a dose large enough to be a threat. The risks are much higher if there is no chance for virus particles in the air to escape through a window or exhaust fan or to get diluted via a high-quality AC system (or air currents outdoors). The AC that was running in the restaurant, Banfleth says, would not pass U.S. standards for commercial spaces, because it didn’t bring in any fresh air from the outside—a key component of code-compliant HVAC systems.
Conversely, high-quality AC could actually have been useful to decrease the chance of infection. “There’s nothing inherently wrong with the HVAC system,” says Joseph Allen, assistant professor of exposure assessment science at the Harvard School of Public Health. In fact, HVAC should be leveraged to minimize the risk of airborne transmission, especially if it’s fitted with high-efficiency filters designed to strain pollutant particles out of the air. It’s also potentially better than industrial fans, in workplaces such as meatpacking buildings and factories, which could increase the velocity of large droplets—much higher than in an AC system—carrying them farther in fast jets of air.
That said, your trusty AC window unit at home is different. That does not bring in fresh air from the outside to dilute and replenish the internal air; it merely cools and constantly recirculates the air currently in the room. That’s where it could become a problem, Allen says, “because then you’re potentially moving contaminated air around the space.” That’s a concern in public places that use only a window unit, such as restaurants, and that have a higher density of people. That risk can be mitigated if the restaurant follows limited-capacity guidelines once it reopens. “If you’re then ventilating at the same rate,” Bahnfleth says, “you’re getting a lot more dilution of whatever infectious aerosol there is in the space.”
This guidance could change if epidemiologists find clusters of sick people that originate from a ventilated area, at which point they could be tasked with working back to find the source. That might spur the funding of controlled, scientific lab experiments, which were not done in the China study, to simulate an air-conditioned environment to test if animal subjects catch the virus from others.
But, at home, even if your AC window unit or a fan is recirculating the same air, there’s a lower density of people likely to be carrying infectious particles. And the chances are that if you’ve been self-quarantining safely with family or roommates, that air shouldn’t be infected, anyway.
There, at least, can you crank up the AC at home during the upcoming restless, stuffy nights? “Sure,” Schaffner says.