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caption: Melissa grocery shops at Fred Meyer on Monday, November 16, 2020, on Northwest 45th Street in Seattle. New statewide restrictions were announced by Gov. Jay Inslee on Sunday to curb the rapid spread of Covid-19.
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Melissa grocery shops at Fred Meyer on Monday, November 16, 2020, on Northwest 45th Street in Seattle. New statewide restrictions were announced by Gov. Jay Inslee on Sunday to curb the rapid spread of Covid-19.
Credit: KUOW Photo/Megan Farmer

It’s all about air flow: How to keep coronavirus at bay indoors

Proper mask-wearing, hand-washing, and other precautions taken at the individual level can help reduce person-to-person transmission of the coronavirus. But another, less visible variable is also at play: Ventilation.

Here’s what you should know about the risk of Covid-19 exposure while in indoor spaces, according to the experts.

Limit time indoors around non-household members to mitigate coronavirus transmission, health officials have said throughout this pandemic.

But many people, by necessity, still patronize grocery stores and work in-person. Many also still engage in not-so-essential indoor activities, such as receiving a professional haircut, going in for a manicure, or getting a new tattoo — all of which are still allowed under Washington’s latest statewide social restrictions.

There are several measures that, when used in tandem, can help cut down on person-to-person coronavirus transmission: Wearing a mask over your nose and mouth, frequently washing your hands, and only congregating with people whom you share a household, for instance.

But there's a less tangible factor that also plays a role in spreading the virus: Air flow.

Smaller respiratory droplets known as aerosols can linger in the air for several hours, increasing the odds of airborne coronavirus transmission. No two indoor spaces are the same when it comes to ventilation.

Some, like airplanes and hospitals, tend to have advanced ventilation systems that can quickly filter out potentially dangerous air. Others are bound to have less sophisticated ventilation systems, making coronavirus spread more likely.

Here’s what you should know about the risk of Covid-19 exposure while in indoor spaces, according to the experts.

Out with the old, in with the new

When it comes to assessing the specific risks associated with different indoor facilities, it’s not so simple, said Dr. Shirlee Tan, a toxicologist with Public Health — Seattle & King County.

“Every building is different, so it's really hard to predict the risks of a given situation,” she said.

Tan offers this advice: “If you walk into a space, and it seems stuffy, get out. That's a good sign that there's not very good ventilation.”

Earlier in the pandemic, public health wisdom was that maintaining six feet of distance was ideal for reducing coronavirus spread via infected respiratory droplets, produced during speech, breathing, coughing, or sneezing, for example. But more recent research points to coronavirus containing aerosols traveling way beyond that.

“Just being six feet away from somebody doesn't necessarily mean that you're protected from these small particles,” said Dr. Marissa Baker, a professor at the University of Washington’s Department of Environmental and Occupational Health Sciences who specializes in industrial hygiene.

“And depending on the quality of the mask you're wearing, how well you're wearing it, and how well it fits, these particles can also get out and around masks as well.”

A study recently published in the Journal of Korean Medical Science recreated a scenario in which three Covid-19 cases tied to an indoor restaurant surfaced. Through interviews, cell phone location data, and surveillance footage, researchers determined that the infector’s airborne respiratory matter infected two others who were sitting approximately 16 and 21 feet away, respectively.

The same study also suggests that it doesn’t take prolonged contact in order to catch the virus. One of those two people became infected after just five minutes of exposure.

Maintaining a good flow of fresh, outdoor air is key to moving those infectious particles out of a building. Having six air changes — or instances in which stagnant air is replaced with new air — per hour is ideal, Baker said.

“That would mean that every 10 minutes the air in a space is turned over,” she added. “Anything that's in there is only going to be in there for 10 minutes.”

Absent a high-level ventilation or air quality system, the air changes of any space can be increased by simply opening doors and windows, Baker said.

“If you're going to be inside doing things, it's better to be inside bundled up with the windows open than it is to keep those windows closed,” she said, adding that indoor businesses should keep their doors open whenever possible.

If it’s not possible to keep a door open at all times, Baker advises opening the doors each hour to purge out stale air. The same goes for when people outside your household enter your vehicle or home.

A number of factors, including a building’s age, design, and heating ventilation and air conditioning (HVAC) system, will affect its capacity to maintain sufficient air flow and filtration in the time of Covid.

“If your HVAC system has that capability, then bringing in the fresh air from outside is going to be better than recirculating whatever air is in the space — unless you're also using filtration,” Baker said. “When we're talking about filtration, it's pretty much similar to thinking about like a mask for your HVAC system: If the air is recirculating, it draws the air past that.”

The Centers for Disease Control and Prevention recommends using MERV filters, ranging from 13 to 16, which are able to capture infectious aerosols at relatively high efficiency rates — anywhere between 50% and 95% for 0.3 to 1 micron.

The CDC also endorses the use of HEPA filters, which the agency says are at least 99% efficient at capturing respiratory droplets containing the coronavirus. HEPA filters are primarily used in air purifiers, but can also be used for some HVAC systems.

Tan also pointed to the use of upper-room ultraviolet germicidal irradiation systems, composed of lamps that emit ultraviolet light akin to that from the sun for the purpose of sterilizing indoor air. The CDC proposes using them to supplement other ventilation mechanisms to reduce the spread of Covid-19.

Baker also cited a growing body of research suggesting that Covid-19 is less likely to be transmitted in warm, humid settings than it is in cool, drier ones. Thus, she said, adding a humidifier to your indoor air cleaning arsenal isn’t a bad idea.

Tan warned of various products on the market that make unverified assertions about their ability to eliminate coronavirus from the air.

“We do encourage people to avoid things that produce ozone, that do ionizing technologies, plasma technologies, or UV other than upper-room germicidal UV units,” she said, because “you may have a false sense of security and certain things may not be doing what they claim to do.”

Helping or hurting?

Baker also called attention to the potential for some common Covid mitigation strategies to actually get in the way of fresh air flow, such as the use of plexiglass screens by many businesses. She also pointed out that using ceiling fans can be helpful for diluting coronavirus-carrying particles, so long as they’re used in tandem with bringing in fresh air.

The World Health Organization advises only using table or pedestal fans to circulate indoor air among people sharing a household, citing the potential for them to actually increase the odds of airborne coronavirus transmission by moving infectious aerosols around and into others’ bodies.

One way to gauge indoor air quality as it relates to Covid is to track the carbon dioxide levels in a space using an air quality monitor. A buildup of carbon dioxide, which we exhale, is an indicator that fresh air isn’t getting in — that’s assuming the amount of people present hasn’t increased.

“You're not getting rid of that exhaled air, which not only is CO2 but also contains the coronavirus,” Baker said.

It’s not an exact science, and there’s no consensus about exactly how much carbon dioxide is too much. But a presence of anywhere from 1,000 to 2,000 parts per million is considered unhealthy.

But ventilation strategies notwithstanding, both Baker and Tan emphasized that mitigating the spread of Covid-19 is best done through avoiding indoor crowds and gatherings to the greatest extent possible.

“If there are ways to do things without being in a space with a lot of other people, then choose that first — if you can be outdoors, if you can be doing takeout rather than dining in,” Tan said. “Choose spaces that seem to be ventilated and where people are doing all the other measures that make you comfortable — distancing, wearing masks.”