Can Air-Conditioning Aid in Spreading Coronavirus?

As the world counts COVID-19 deaths and tallies up confirmed cases, researchers are trying to fill in all the gaps in the information available on the novel coronavirus. The disease spreads through droplet transmission, but studies are ongoing on what can halt or aid this transmission despite social distancing and proper disinfection.

A recent Chinese study looks at the possibility of transmission aided by air-conditioning systems in enclosed spaces. The report published by the Center for Disease Control (CDC) takes into account 10 cases in three families.

The only common venue of probable contact of the three families (referred to as A, B and C by the researchers) before being diagnosed as positive for coronavirus was at a restaurant.

The case

One of the families (A) had just travelled from Wuhan, Hubei Province, China. Patient one, in this case from family A, had lunch with the family a day after their return at a restaurant. On the same day, families B and C were also diners at the same restaurant, overlapping in time with family A. The tables were further than 1 metre in distance from each other, effectively fulfilling the ‘physical distance’ rule.

The only positive case at that point was patient one in family A, who started showing symptoms later that day. The researchers found that this patient was the source of transmission for at least one member of families B and C. The rest of the cases in each family was determined to be within-family transmissions.

The restaurant, in this case, was closed 5-storey building with no windows. Each floor had its own air-con system. The families dined on the third floor. All other diners on the floor were monitored and quarantined but no one else tested positive for COVID-19.


The research report said: “We conclude that in this outbreak, droplet transmission was prompted by air-conditioned ventilation. The key factor for infection was the direction of the airflow.”

The study found that though virus droplets of small sizes can linger in the air and travel distances further than 1 metre, this particular case was not due to aerosol transmission and was not airborne. This was determined as no one else in the restaurant was infected and samples from the air conditioner tested negative.

The authors believe that while large virus-laden droplets cannot travel further than 1 metre, the strong airflow could have helped to propogate the droplets to the tables where families B and C dined.

The authors recommend “strengthening temperature-monitoring surveillance, increasing the distance between tables, and improving ventilation.”

WHO also mentions in its advisory for airborne diseases: “In health facilities where there is a high concentration of infectious patients, evidence shows that poorly ventilated buildings have higher risks of infectious disease transmission for patients, workers, and visitors.”

Opening a window helps

Experts recommended that buildings with air delivery systems should take care not to recirculate indoor air which “could potentially increase the transmission potential.”

The experts add that the simple act of opening a window can also help, while being cautious about sudden temperature changes. They said, “the easiest way to deliver outside air directly across the building envelope is to open a window.”

A California study also found that natural daylight could mitigate some risks. They determined this in a simulation where daylight exposure reduced the average half-life of exposed influenza virus from over 31 minutes to just over 2 minutes.


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