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Is air conditioning helping to spread coronavirus? In southern states, being indoors may be a factor | Coronavirus

An essential comfort for people living in southern states — air conditioning — has come under scrutiny for potentially aiding in the surging spread of coronavirus in areas with intensely hot summer weather.

With coronavirus cases rising sharply in Texas, Louisiana, Florida and other states in the southern part of the U.S., researchers and epidemiologists say that an increasing amount of time spent indoors, sometimes with recirculated air, may be one factor in the spread.

Dr. Edward Nardell, a Harvard University professor who studies airborne diseases, argued in the medical journal JAMA in June that air disinfection was overlooked in COVID-19 prevention. In an online presentation last week sponsored by the Massachusetts Consortium on Pathogen Readiness, he connected a rise in air conditioning use and time spent indoors in southern states with a rise in COVID-19 infections. 

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Hot temperatures, Nardell said, drive people inside, where coronavirus can linger for hours. In poorly ventilated buildings without outside airflow, the virus could get pushed around by currents created by air conditioning units. In many modern buildings, energy efficiency allows for little outside airflow, which could create a high concentration of virus-laden air that is continuously whipped around a room.

Some of Nardell’s conclusions have been echoed by other researchers, though many note that the increase in infections likely has more to do with people not wearing masks or failing to practice social distancing. Still, spending more time indoors with other people may be having an impact, they say.

Viruses typically die down in the summer, when children are not in school and families spend more time outdoors. 

But in the south, extreme heat is common. And for a virus that is spread through airborne droplets, enclosed spaces with recirculated air can be more infectious than well-ventilated buildings. 

Outside of New Orleans and some cities in California, the U.S. epicenters for the coronavirus in March and April were mostly in northern states, including New York, Michigan and Washington state. But as cases slowed in those regions due to restrictions and the season moved into spring and summer, more infections have appeared in the south and west.

Texas, Arizona and Florida have all posted their highest daily cases counts this week, with intensive care units in some areas of those states at capacity. 

In May, Louisiana and New York faced travel restrictions imposed by Florida. Now, the tables have turned and New York has imposed restrictions that require Florida and other states to quarantine after entering its borders. Cases are flat in New York while they have risen in Texas for 22 days and doubled overall. 

Conditioned air, whether it’s hot or cold, is usually the same air recirculated throughout one building. Many high-efficiency systems bring in a small amount of outside air to conserve energy in conditioning new air. Because coronavirus can linger in the air, one infected person in an enclosed space can contaminate more of the air — and then AC units can push that air around in a way that exposes more people. 

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A small study out of Wuhan, China published in Emerging Infectious Diseases found that one person infected nine other people at a restaurant — not just those sitting at the same table, but also those on either side. The study authors pointed to the air conditioning unit pushing the same infected air around the room as the mode of transmission. 

Not all air conditioning increases the likelihood of infection, and it is just one factor in potentially increasing cases, according to William Bahnfleth, an indoor air expert at Pennsylvania State University and chair of the epidemic task force at the American Society of Heating, Refrigerating and Air-Conditioning Engineers.

The Wuhan restaurant did not have a supply of outdoor air, which could contribute to the number of droplets in the environment. 

“It allows contaminants in the air to concentrate,” said Bahnfleth. “Imagine you’re cooking and there’s a fire, you’re getting foul smelling smoke off the stove. You open a window to dilute it.”

While the Wuhan restaurant study was speculative, according to Bahnfleth, it still highlights the risk of non-ventilated spaces that recirculate air, like many older buildings. 

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A connection between the rise of cases in southern states and air conditioning use is also still speculative, said Susan Hassig, a Tulane epidemiologist, who pointed to the overall hazards of being indoors among people, especially since mask-wearing isn’t always mandated by elected officials and has transformed into a political statement.

Air conditioning flow patterns in poorly ventilated buildings — without outside air being introduced — may contribute to infection rates, but higher rates can also be attributed simply to being inside after mingling with people outside of your quarantine ‘pod,’ said Hassig.

“It’s not from people in Phoenix sitting in their air conditioned homes, it’s from people not sitting in their air conditioned homes and going out and mixing with other people,” said Hassig. 

Transmission of COVID-19 in closed environments may be 18.7 times greater than an outdoor, open-air environment, according to a report from Japan awaiting peer review.

“That’s the whole idea of why indoor dining is more problematic than outdoor dining,” said Hassig. “It’s not just the air conditioning. It’s the enclosed air environment.” 

Bahnfleth said most building codes require a level of ventilation that is sufficient to introduce new air. For people who work in offices, he recommended checking with the building manager to make sure the building has outside ventilation. His organization also recommends upgrading filters to a denser material, from the standard MERV-8 filter to a MERV-13 filter, which can catch more particles. 

Nardell, the Harvard scientist, proposed an ultraviolet light installed near the ceiling of a room, where warm particles expelled from a human body would gather, as a way to sanitize buildings. UV light has been shown to reduce airborne tuberculosis transmission by 80%, but would have to be used carefully because it is a carcinogen. 

Overall, air conditioning and proper ventilation is just one piece of the prevention experts said will make a difference in infection rates. 

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“Air conditioning is a risk under conditions of inadequate ventilation,” said Bahnfleth. “But when you have normal ventilation, the risk is pretty low as long as you’re doing the other things you’re supposed to do. The risk from not distancing and not wearing masks is still the greatest.” 


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