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Air conditioning risks?

MIT Medical answers your COVID-19 questions. Got a question about COVID-19? Send it to us at CovidQ@mit.edu, and we’ll do our best to provide an answer. 

I’m an MIT student living in a high-rise apartment building. As the days get warmer and warmer, I’m wondering if it is okay to turn on the air conditioning in my unit. Is it possible to get the coronavirus from the HVAC system, since it may pump air from other nearby units (where a person with the virus may live) into my unit? Do you think it is worthwhile to keep the AC off for that reason?

Illustration of an AC vent with question marks among the lines lines representing airflow from the vent

Based on the design and function of large HVAC systems and everything we know about the virus at this point, we would say that the risk of contracting COVID-19 through your building’s air-handling system is very close to zero. Theoretically, it’s possible; practically speaking, it’s not at all likely.

To date, all evidence shows that SARS CoV-2, the virus that causes COVID-19, spreads primarily during close person-to-person contact, transmitted from one individual to another through large respiratory droplets expelled when people cough, sing, breathe, or even talk. But scientists have also been concerned about the possibility of airborne or aerosol transmission. Aerosols are smaller, microscopic particles that can stay aloft and travel for longer distances. 

Evidence for the possibility of this type of airborne transmission is mixed. There is clear evidence that viral particles may linger in the air after medical procedures, like intubations, that are known to create and launch high-velocity aerosols into the air — but that’s not the sort of thing that would happen anywhere in an apartment building. There’s also some evidence that infected individuals can generate these kinds of viral particles simply through breathing or talking, but there’s no evidence that such particles are viable or, if they are, that they would be released in large enough amounts to be infectious. 

While any system that moves air around buildings has the potential to spread contaminants, HVAC systems in large buildings like yours provide both ventilation and filtration, which reduce risk. Out of the thousands of papers and articles published since the virus was first identified at the end of 2019, there have been no reports of space-to-space transmission through an HVAC system. And since we know that COVID-19 viral particles disperse, or break apart, relatively quickly in the air, even if an infectious amount of airborne viral particles was released in one apartment, it’s unlikely that these particles would be picked up at an air-return vent, pass through the air-handling unit and filters, mix with fresh air drawn into the system from outdoors, and somehow arrive, intact and infectious, in another apartment.

In short, you probably should not worry about using your air conditioner as needed. Of course, during cooler nights or days when the outside temperature is comfortable, you may want to open your windows for a little extra ventilation. 

Remember, your biggest risk comes from close contact with other people, something that can be challenging to avoid when you live in a high-rise building. Wash your hands often, practice social distancing (avoid crowded lobbies and elevators), and cover your face when you leave your apartment.


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