The Perfect Enemy | How long does coronavirus stay in the air after someone with COVID leaves the room?
August 11, 2022

How long does coronavirus stay in the air after someone with COVID leaves the room?

How long does coronavirus stay in the air after someone with COVID leaves the room?  San Francisco ChronicleView Full Coverage on Google News

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Dear Advice Team: How long does COVID stay in the air after someone with COVID (who is not wearing a mask) leaves the room? Assuming the windows are closed and there’s no filtration system, how long do you have to be concerned that you can get COVID from being in the room without a mask?

Welcome to Pandemic Problems, an advice column that aims to help Bay Area residents solve their pandemic and post-pandemic conundrums — personal, practical or professional.
As COVID evolves into an endemic issue, we know readers are trying to navigate the “new normal.” Send your questions and issues to pandemicproblems@sfchronicle.com.

Today, The Chronicle’s Kellie Hwang fields this question about the risk of COVID indoors.

Dear Reader:

Coronavirus case levels in the Bay Area are still high as the highly infectious BA.5 variant makes its rounds, and local officials say they have no plans to reinstate mask mandates, which were lifted in California in February. So your questions about COVID transmission indoors from particles in the air are good ones.

We briefly touched on this topic in a recent Pandemic Problems column about the risk of catching COVID from touching contaminated surfaces. We concluded from experts that the risk of surface transmission is quite low (1 in 100,000) compared with the primary means of transmission — namely, inhaling aerosolized particles. According to the experts, you are 1,000 times more likely to contract the coronavirus by breathing these tiny, lightweight viral particles suspended in the air after an infected person talks, coughs or sneezes than you are to get it via touch.

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However, what you specifically seem to be asking for is a set amount of time — minutes or hours — that coronavirus aerosols emitted by an unmasked person linger in the air in an indoor space, posing the risk of infecting others in the room.

And unfortunately, the experts I talked to said there is no set number. That’s because the dynamics of aerosol transmission are complicated, and each indoor setting is different, involving a large number of variables.

Generally, it’s true that once an infected person leaves the room, “the concentration of virus-laden aerosol particles in the room will decline,” said Richard Corsi, dean of UC Davis’ College of Engineering, who has extensive knowledge about indoor air quality.

Beyond that, it quickly gets very complicated.

The length of time the virus remains in the air depends on a number of factors, including the size of the droplet or aerosol carrying the virus, and the amount of clutter and air motion in the room, according to an aerosol protection FAQ by nearly a dozen experts from universities across the world, including Corsi.

The coronavirus has been found in tiny aerosols — even smaller than 1 micron — that can stay in the air for more than 12 hours. A 2021 study from Building and Environment, an international engineering journal, found that aerosols from speaking (not necessarily containing SARS-CoV-2) can stay in the air for up to nine hours.

But, these tiny particles usually “leave a building in the air faster than they settle on indoor surfaces and the virus can decay during this time,” even if the aerosols continue to float in the air, according to the FAQ.

How fast air leaves a room depends on how quickly outdoor air can enter and mix with the indoor air. According to the FAQ, a 95% replacement can take anywhere from 30 minutes to 10 hours in a residence, 12 minutes to two hours in a public building, and as little as five minutes in a highly ventilated area of a hospital, like an isolation ward.

The decline is marked by three factors, or what is known as “sinks” in the indoor air quality field: ventilation (the provision of fresh air in a room), filtration (the capture of air particles by filters) and deposition (the process in which aerosol particles collect or deposit themselves on indoor surfaces).

“The relative significance of each sink depends on the magnitude of ventilation, or air change per hour, the effectiveness of filtration,” size of the particles, “and degree of mixing of room air,” Corsi wrote in an email.

“Filtration and deposition to surfaces can be converted to what is called ‘equivalent air changes per hour,’ which basically mean how much additional ventilation would be needed to reduce virus-laden aerosol particles in indoor air to the same extent as filtration or deposition to surfaces does,” Corsi explained.

In the situation you described with no filtration, Corsi estimates the ventilation rate is two air exchanges per hour. Using that calculation, the time for a 95% reduction of aerosol particles in the air is about 90 minutes. With an increased ventilation rate of six air changes per hour, the time drops to 30 minutes.

“So how long a virus can stay in the air indoors is highly dependent upon the indoor environment,” the experts wrote in the FAQ.

It also depends on what the infected person was doing. Corsi said breathing is usually the least significant emission source, unless the person is breathing heavily — for example, from activity such as exercising. Talking raises emission levels, especially when elevating one’s voice such as in a noisy restaurant or bar.

Coughing also can emit large amounts of virus-laden respiratory aerosol particles, Corsi said — “but the impact depends on the frequency of coughing.”

Some studies have attempted to quantify the length of time aerosolized coronavirus particles may linger in the air.

Abraar Karan, an infectious disease doctor at Stanford, pointed to a 2020 New England Journal of Medicine study that found “viable virus for at least three hours in aerosols under experimental conditions.”

An air purifier inside a classroom at McClymonds High School in Oakland.

Santiago Mejia/The Chronicle 2021

UCSF infectious disease expert Peter Chin-Hong shared a January 2022 pre-print study out of the United Kingdom, not yet peer-reviewed, which looks at COVID infectivity in an environment that is more representative of a person being near another person who is breathing, talking or coughing, using a specially made device. The results show that the virus can decay more quickly than previously thought.

“Studies mimicking more real world settings suggest that 20 to 30 minutes is the time that the virus can linger in the air in a poorly ventilated space,” Chin-Hong wrote in an email. “Ninety percent or more of the virus’ potency is lost after about 20 mins.”

But Corsi said he was a bit dubious of the study’s results and called the figures an “underestimate.”

“Early work on earlier viruses suggested much slower decay rates than the 2022 paper, and we know that virus decay rate is dependent on relative humidity and a couple of less significant factors,” he said. “So, it is difficult for me to say that I agree with these results, particularly since the variant we are now dealing with is different from that studied by the authors.”

So, with the scientific debate continuing, and with all the variables involved, what should you do if you’re worried about catching COVID-19 indoors?

The best way to protect yourself, experts say, is to social distance and wear high quality masks such as an N95, KN95 or KF94 mask while in indoor public spaces, in addition to being up to date on your COVID vaccinations.

“We seem to have lost that point along the way as we transitioned from a more ‘community’ based effort to reduce spread, to a ‘protect yourself’ situation we are in now,” Corsi said.

But if everyone wore high quality masks indoors, COVID transmission risk would drop “dramatically,” he said. “The infector is also wearing a mask and so emissions are reduced substantially and thus levels of virus-laden aerosol particles in room air decrease substantially.”

“Pandemic Problems” is written by Chronicle Advice Team members Annie Vainshtein and Kellie Hwang, combining thorough reporting and guidance from Bay Area experts to help get answers and find a way forward.