• Sebastian Leck

FACT CHECK: Can COVID-19 spread via air conditioning?

INSUFFICIENT EVIDENCE. Although there is some evidence that air conditioning in a poorly-ventilated space could spread droplets, the science remains unclear.

For the past few months, there have been a number of news reports suggesting that air conditioning could spread the COVID-19 virus. In April, the National Post reported that a group of engineering researchers were designing a new HVAC system set-up to curb potential infections, while the CBC reported at the end of May that some Toronto businesses, like boxing gyms, are improving air flow in the hopes of preventing COVID-19 from spreading.

As the summer months get warmer and businesses begin to slowly reopen, it’ll be important to know if air conditioning can play a role in spreading the virus—and if so, what we can do about it.

Studies so far are not conclusive

Whenever this claim comes up, it usually refers to a study that investigated a series of COVID-19 cases linked to a restaurant in the city of Guangzhou, China. It found that nine people sitting near an asymptomatic person were infected with COVID-19, including five people who were sitting at two neighbouring tables. One of them was more than four metres away from the asymptomatic patient.

Respiratory droplets, which are the main way that COVID-19 is thought to spread, generally stay in the air for a few seconds before dropping to the ground and are believed to spread only one or two metres. The researchers in the restaurant study, however, suspected that strong air conditioning had carried these droplets to the two other tables.

A follow-up study of the restaurant showed that poor ventilation combined with air conditioning was likely responsible. Researchers found that the establishment had no outdoor source of air, which meant the air conditioning system was circulating the same contaminated air over and over again across the three tables.

Air conditioning blowing air across three tables in a restaurant.

This image from "Evidence for probable aerosol transmission of SARS-CoV-2 in a poorly ventilated restaurant" by Yuguo Li, et al. shows the simulated spread of droplets across the restaurant's tables. See the full paper here.

However, there is still ongoing debate in the scientific community about whether COVID-19 can spread through the air. The World Health Organization reported at the end of March that a study of 75,465 COVID-19 cases in China did not find any cases of airborne transmission, but warned that further research was needed. At the time, the organization also did not find the existing studies on airborne transmission conclusive one way or the other.

A study of two Wuhan hospitals detected the COVID-19 virus in the air in areas such as patient bathrooms. However, the researchers didn’t establish whether the virus could actually be transmitted from smaller airborne droplets.

Some academics are convinced that airborne transmission is possible. Researchers Lidia Morawska and Junji Cao have argued that because SARS spread through the air, COVID-19 likely spreads that way too.

Poor ventilation is probably a bigger problem than air conditioning

Have you ever walked into a room where the air felt stale and muggy? That’s poor ventilation.

Ventilation refers to the introduction of fresh, outside air into a building. If an enclosed space is poorly ventilated and an air conditioning system doesn’t have a filter, there could be a risk that air conditioning will circulate contaminated droplets throughout the entire space.

A study out of University of California, Davis and the University of Oregon found that simply opening a window may help to dilute contaminants in the air. Installing a filter in an air conditioning system in restaurants or public spaces could also help mitigate the risk of contaminants in the air recirculating.

But based on what we know so far, there is no evidence that the COVID-19 virus can travel long distances within a building’s climate control systems.

Abdu Sharkawy, an infectious disease specialist at Toronto Western Hospital, told CTV News in May that the virus likely doesn’t travel long distances through ventilation, like from the top floor of a building to the bottom floor.

Sharkawy also emphasized the need for adequate air flow indoors. “It’s always a good idea to open windows to optimize air flow and reduce the risk of a virus being confined to an indoor space,” he told CTV.

Until scientists conduct more research, airborne transmission will remain a mystery. For now, there is some evidence that air conditioning could spread contaminated droplets in places with poor air flow, but there’s no evidence that it spreads through air conditioning vents or across large distances.