Can Air Conditioning Transmit the Coronavirus?

In early April, the CDC posted a draft of a scientific paper titled “COVID-19 Outbreak Associated with Air Conditioning in Restaurant, Guangzhou, China, 2020,” which a number of news outlets (including The New York Times, Wirecutter’s parent company) quickly picked up, leading to the headline-grabbing and unsettling idea that an air conditioner may increase your chance of catching the coronavirus and developing COVID-19. All of that has prompted some follow-up research and a lot of (understandable) concern.

Much of the reporting on this topic refers to a study out of the University of California Davis and the University of Oregon (PDF) that found simply opening windows can effectively dilute the concentration of the coronavirus in enclosed spaces. If you’re concerned about contaminated air indoors, that is a great first step to take. But opening your home’s windows isn’t usually a practical option for the entire summer, and exposure to extreme temperature stress carries its own health risks. So we reached out to a few HVAC experts, including members of the American Society of Heating, Refrigeration, and Air Conditioning Engineers (ASHRAE) Epidemic Task Force, to get their perspectives on the potential risks of different types of air conditioning systems, as well as any strategies they might have for people concerned about staying both cool and healthy this summer.

One consistent detail we noticed across these interviews: Everyone was careful to acknowledge the limits of their own understanding of the virus’s transmission. Although people are eager for clear and definitive information, no one can provide an absolute, categorical answer to the question in this article’s headline (not yet, at least), and neither we nor the professionals who spoke with intend to speculate. But with concerns dating back to April that the virus may be transmitted through the air, followed in early July by a growing number of experts arguing to the World Health Organization that the virus is airborne (which the WHO has since acknowledged), it’s clear why this would be on people’s minds already. We thought it would be helpful to share some information on the broader topic of air quality, to aid with your own research and decision-making.

The first thing worth noting is that most air conditioners recirculate air within a space (meaning they don’t draw fresh air in from the outdoors, as an open window would). In a large system serving multiple rooms, Kathleen Owen of Owen Air Filtration Consulting told us, there is often a way to filter out at least some ambient particulates. “If somebody is really worried about COVID or susceptible to it, I personally think having a filter in their own HVAC system […] will help, assuming that we’re right about the size of the [coronavirus] particle and the transmission,” she said. Owen added, “I’m putting enough caveats in there so that no one tries to shoot me later on.”

After doing our own research into residential filters for central HVAC systems, we settled on a MERV-12 (PDF) level as a good medium between effective filtration and likely compatibility with your existing equipment. Owen recommended a similar level of filtration for shared building spaces, where air circulated between separate rooms or offices might be a greater concern than the air circulating within your own house: “If they’re using the MERV-13 system, for example, they should be removing at least 85 percent of [airborne particles] each time the air goes through. So you’re still taking the air from one place to another, but you’re at least mitigating it in some way.” MERV-13 (“or industry equivalent”) is the level of filtration New York state requires on air conditioning systems for shopping malls reopening in July 2020.

This kind of filtration is typically not possible on the window ACs and portable air conditioners Wirecutter recommends. Such units do come with their own built-in filters, but those filters are far too coarse to catch particles the size of the coronavirus; they’re meant to catch big stuff like pet hair and keep it from clogging the workings.

There’s also a limit to how much AC filtration alone can do against airborne viruses, whether in your home or in a shared space, said Lew Harriman, director of research & consulting at HVAC consulting company Mason-Grant, and a member of the ASHRAE Epidemic Task Force. “If you’re looking to reduce the risk of transmission in your house, or let’s say you’ve got someone who’s at risk and you want to protect them, what you want is a portable HEPA air filter,” he said, using a synonymous term for the types of portable air purifiers Wirecutter recommends. “Don’t let the air conditioner carry that load. Frankly, you should have an air purifier even if you’re not concerned.”

But even a great air purifier may not necessarily protect you. As of June 16, 2020, the position taken by the CDC is that the coronavirus is primarily transmitted by person-to-person contact and by contact with virus-laden droplets expelled through coughing and sneezing. This is where the definition of “airborne” gets tricky. It remains unclear whether any filtration devices effectively neutralize the virus prior to the point of infection, or, for that matter, what level of exposure to the virus causes an infection to begin with. As we report this in real time, we can go with only the most recent evidence, and it seems clear that community spread can occur even when taking a reasonable level of precaution—whether at churches or among Major League Baseball teams.

All that said, adequate filtration remains a consensus preference over the other option of disabling HVAC equipment completely. “If shutting off air-conditioning means that ventilation and filtration functions will be disabled, it is quite possible that risk of infection will increase, but if there is no ventilation or filtration, circulation of air in a space may contribute to risk,” writes William Bahnfleth, PhD, PE, director of the Indoor Environment Center in the Department of Architectural Engineering at Pennsylvania State University and the chairman of the ASHRAE Epidemic Task Force, in a post on LinkedIn. “This underscores the importance of ventilation, whether provided by the HVAC system or by opening windows, and filtration in reducing risk of infection.”

Both Owen and Harriman echoed this notion, further confirming the widely accepted advice to regularly open windows and use fans to bring in fresh, clean air from outside to dilute concentrations of potential contaminants. Whether you’re opening windows, running your AC, or doing some combination of both, a good HEPA filter or air purifier should also help you out.

Good air quality is just one factor in protecting yourself from the spread of the coronavirus, and you should make it a priority alongside following the rest of the CDC’s guidelines, such as social distancing, handwashing, wearing a mask, and treating frequently touched surfaces with disinfectants.