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Air conditioning and Covid-19: The scientific factors you should know

"Changing to 100 percent outside air is much more important than anything else."

CSA Images, zhangshuang

As summer heats up, many people are blasting the air conditioning units that fill their homes. Meanwhile, recently reopened restaurants, buildings, and offices are weighing how to cool public spaces without heightening the risk of Covid-19 transmission.

As people search for the safest way to cool off, experts say air conditioning systems and ventilation play a pivotal role in how people contract and spread Covid-19.

Air conditioning systems partially filter coronavirus-containing viral particles, alter airflow patterns, and often, recirculate indoor air — all shifts that can influence Covid-19 transmission.

A.C. also increases ventilation within a stagnant room or building, which can disperse coronavirus-containing viral droplets, and lower the chance of infection.

Based on current evidence, concern about Covid-19 and air conditioning hinges on the airflow patterns, not the air itself, Qingyan Chen tells Inverse. Chen is a mechanical engineer at Purdue University who studies how airflow influences viral transmission.

"The virus is not brought in by the air conditioning system, but the system will enhance recirculation in the room," Chen explains. Recirculating indoor air can cause problems transmission-wise, by exposing people to viral particles.

If you have the choice between firing up the A.C. or opening a window, choose the latter, Chen says.

"Just open a window"

"Just open a window because natural ventilation is much more effective than air conditioning systems," he advises.

Breathe in, breathe out — When people sneeze, speak, or cough, they're also spewing hundreds — even thousands — of droplets into their immediate vicinity. Covid-19 mainly spreads through these respiratory droplets, the Centers for Disease Control explains, primarily through close contact with infected individuals and sometimes, contaminated surfaces. Masks help cut off the source and spread of Covid-19 by providing a physical block between people and these viral particles.

Emerging evidence suggests that while many coronavirus-containing viral droplets drop to the ground or dissolve quickly, some can spread by air. Studies show small droplets can become airborne and linger for longer than 8 minutes in stagnant air. In some cases, small droplets can stick around for hours.

Air conditioning and ventilation methods can influence Covid-19's aerodynamics — altering how and where viral droplets spread through an area.

How does air conditioning work?

Air conditioning systems function by filtering, cooling, and circulating air (often a mixture of outdoor and indoor air) through a complicated chemical and mechanical process.

"Almost all air conditioning systems use filters," Chen says. "However, lots of filters are not high-efficiency filters."

Typical air conditioning systems filter 20-30 percent of viral droplets (mainly large particles) that could infect people with Covid-19, Chen adds.

It's unlikely that buildings with central air conditioning systems, like commercial office buildings, would spread Covid-19 droplets from one space to another and infect office workers. But it is possible.

Even low doses of Covid-19 exposure over a long duration, like a typical eight-hour workday, could make someone sick, Chen says. If possible, people should work remotely as long as Covid-19 is with us or until there is an effective treatment, Chen says.

"When we talk about going back to office buildings in the future, you will stay there at least eight hours per day. And then there will be risks," Chen says.

Jane Khomi

To keep people healthy and limit spread, scientists, including Chen, advise maximizing ventilation and minimizing the recirculation of indoor air.

In public spaces, cooling systems should be set to maximum airflow, Chen says, which can dilute viral particles and maximize filtration rates. If financially feasible, Chen suggests replacing low-efficiency filters with HEPA filters, like the ones used on airplanes and in hospitals.

"Changing to 100 percent outside air is much more important than anything else," Chen says. "That will ensure the air sent into different spaces is very clean."

Luckily, making this change is often as easy as flipping a switch, he adds. Most air conditioning units are designed with this feature in mind, although, it may take more energy to rely on outdoor air than recirculated air.

"It will cost a little bit more energy, but you can easily calculate the trade-off between energy saving and health risk," Chen says. "During the pandemic season, we should take health more importantly than saving a couple of dollars in energy."

On especially hot days, air conditioning units may have trouble cooling off spaces with outside air alone, than with recirculated air. But Chen adds that it's worth it to put up with temperature swings — at least for now, as we wait for an effective Covid-19 vaccine and widely accessible treatment.

It's even easier to open a window. Natural ventilation keeps air cleaner than air conditioning systems.

What does the research say about air conditioning and Covid-19?

A research letter published by the Centers for Disease Control suggests air conditioning systems can shape the direction of airflow, and potentially heighten Covid-19 transmission. In the letter, researchers describe how strong airflow from air conditioners propelled viral droplets table-to-table in a restaurant in Guangzhou, China, fueling an outbreak of 10 people. To prevent the spread of the virus in restaurants, the letter's authors recommend increasing the distance between tables and improving ventilation.

Another study, which analyzed the level of aerosol coronavirus present in two hospitals in Wuhan China, found that levels of airborne SARS-CoV-2 RNA (Covid-19 viral particles) in most public areas was undetectable, except in two areas that were prone to crowding. Levels were higher in the toilet areas used by patients. Based on the findings, the team concludes room ventilation, open space, and disinfection of toilet areas can help contain Covid-19's aerosol spread.

To date, large scale aerosol studies haven't been conducted, so the full scope and scale of airborne Covid-19 isn't clear yet.

What's next — At this point in the pandemic, some researchers say existing evidence is "sufficiently strong to warrant engineering controls targeting airborne transmission as part of an overall strategy to limit infection risk indoors."

Engineer controls, on top of social distancing, isolation measures, and hand hygiene, would be important measures globally to manage Covid-19, the researchers add.

This means building managers and business owners should ensure sufficient and effective ventilation. They should also consider employing particle filtration and air disinfection, as well as avoid air recirculation and overcrowding, the team writes. These controls are crucial for places like hospitals, offices, schools, libraries, cruise ships, public transport, and restaurants.

"We don't know how long Covid-19 will last," Chen says. "So if we don't have a vaccine and then, Covid-19 will be with us for a few years, I recommend that we should change air conditioning systems in such a way that we do not try to mix air in indoor spaces."

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