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What You Should Know About COVID-19, Air Conditioning And Indoor Air Quality10:45
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A newly installed air conditioner sits in a window of a classroom in Bement, Ill. (David Mercer/AP)
A newly installed air conditioner sits in a window of a classroom in Bement, Ill. (David Mercer/AP)

The World Health Organization recently backed away from its stance that indoor transmission of the coronavirus was rare and largely confined to medical procedures like administering a ventilator.

Now the WHO says smaller particles that are spread by talking or breathing are a problem indoors. Shelly Miller, professor of environmental engineering who studies indoor air quality at the University of Colorado at Boulder, is one of the experts who petitioned the agency to change its position.

She says she wishes the WHO recognized the “major risk” of indoor spread earlier when scientists asked the agency to consider the problem back in March.

Aerosols, particles suspended in a gas and ranging in size, “are generated when we're speaking loudly and talking and singing and doing other activities that release particles from our respiratory tract through our mouths” that can then be inhaled by others, she says.

When these particles are released, they can evaporate or be transported up and away from the body, become airborne and move about the environment for a time period of minutes to hours, she says. She says this is a “perfect recipe for transmission” of the coronavirus if the particles are trapped indoors with poor ventilation and people not wearing masks.

“We really do think that being indoors is where most of the transmission is occurring,” she says. “And it's pretty rare now to see anything related to outbreaks from outdoor conditions.”

In these record-breaking hot summer months, coronavirus transmission through air conditioning is a point of concern for her, she says.

In a May study conducted in China, researchers connected a COVID-19 outbreak to an air conditioning system in a restaurant. University of Maryland environmental health professor Donald Milton told Here & Now that the study suggests air conditioners can blow infected particles through the air.

Central air conditioning recirculates air in a building to maintain a cool temperature. These cooling systems normally don’t bring much outside air because that air is hot, she explains.

“My concern is that in air-conditioned buildings, you don't have enough outside air and you don't have enough adequate filtration in your recirculation system to suppress and decrease the concentrations of the airborne virus that may be circulating in your environment,” she says.

She says she hasn’t identified any studies to date showing the virus could be spread through the central air conditioning system to other parts of a building.

The same issue occurs with in-room air conditioning units — outside air isn’t being pushed into a room to “dilute the virus concentration,” she says.

The risk is lower if you’re in an air-conditioned room with people within your quarantine group, she says.

“But if you're eating in a restaurant that has no outside air and is only recirculating the inside air through these air conditioning systems, that potential for transmission is much higher,” she says.

Miller says schools across the nation are in “critical situations” as they make the difficult decision whether or not to reopen for the fall. Many schools lack the resources for air cleaners or proper maintenance of HVAC systems, which poses a health threat to teachers and students in classrooms.

Miller and a team of researchers are conducting a six-month study into aerosol generation from playing musical instruments “so that we can better understand how to continue music education” at all educational levels during the pandemic.

For example, they’ve tested mitigation measures such as putting a bell cover of the end of a clarinet to see if it reduced aerosol emissions from the person playing the instrument.

Aerosol transmission can also apply to singing, she says. Researchers in her study are observing singers with and without wearing a mask.

“It's very uncomfortable to sing with a mask,” she says. “On the other hand, the potential to release aerosol when you're singing with a mask is reduced.”

Miller says more guidance from officials on the risks of coronavirus spread indoors versus outdoors is needed.

“It's just been so hodgepodge,” she says, “and you're having to rely on trying to find scientists like me to give this message when we really need leaders.”


Lynn Menegon produced and edited this interview for broadcast with Peter O'DowdSerena McMahon adapted it for the web.

This segment aired on July 15, 2020.

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