Risk Of Catching Coronavirus Inside vs. Outside: Here's What To Know

Keep in mind this info on ventilation, air conditioning, heaters and indoor and outdoor transmission during the COVID-19 pandemic.

We’re arguably spending more time indoors than ever during the coronavirus pandemic. That means being in close quarters with others, using heating and cooling systems (depending on the season) and going outside when we can for a reprieve.

So does all this put us at a higher risk for contracting COVID-19? What’s the deal with ventilation, and why is it so important? And what’s the risk difference between being outside and being inside?

Here’s what you should know:

The virus does spread more easily indoors.

The virus is more easily transmitted when you’re in closed areas, where there’s less ventilation or room for airflow. This is especially true for small spaces, like elevators.

“In such a tightly enclosed space without vigorous air movement for a short period of time, I’m afraid you might be exposed,” William Schaffner, a professor of infectious diseases at Vanderbilt University, told Business Insider.

Most official guidelines suggest that the primary mode of transmission is through person-to-person contact, or when someone comes into contact with large respiratory droplets containing the virus. But we also know now that COVID-19 is airborne, meaning the virus can travel when an infected person expels smaller droplets and they remain suspended in the air (sometimes for several hours). This type of transmission is even more likely in indoor settings because of the lack of ventilation and air movement.

Generally, your risk of contracting the virus is remarkably lower when you’re outside. This is because there’s more space to separate from others and there’s more natural airflow.

Some evidence suggests sunlight may also be useful, but more research needs to be done. A study in The Journal of Infectious Diseases showed that 90% of coronavirus particles deactivated within 10 minutes when exposed to UV rays from sunlight, according to a report from HuffPost UK. (This isn’t to be confused with temperature or warmth from the sun, which doesn’t have an effect, according to the World Health Organization. And you shouldn’t just assume that the sun will disinfect everything and make you safe.)

Health experts recommend people get outdoors when they can as a way to improve their physical and mental health, especially during a time that’s incredibly taxing on the mind and body. Just avoid group activities, like team sports, crowded playgrounds, etc. Walking, running and biking are all good ways to reap the benefits while still staying pretty safe (just be sure to still wear a face mask if you’re going to be near others).

HVAC systems and fans can theoretically spread COVID-19...

How do air conditioners, heaters and fans play a role?

Many people freaked out when a study of an air-conditioned restaurant in Guangzhou, China, found the coronavirus spread to separate families eating in the restaurant. Each group was sitting near the air conditioner, and the direction of the airflow seemed to correspond with who got sick. Approximately 10 patrons became ill.

This can happen in more confined spaces, including your home. Indoor circulation of the virus typically occurs if you’re in range of the respiratory droplets a person expels into the air when they sneeze, cough or even just talk. These respiratory droplets are often “heavy,” meaning they can float in the air for up to a few feet before they fall to the ground.

As Mount Sinai Health System notes: “If someone in the house who is infected with the virus is coughing and sneezing and not being careful, then tiny virus particles in respiratory droplets could be circulated in the air. Anything that moves air currents around the room can spread these droplets, whether it is an air conditioning system, a window-mounted AC unit, a forced heating system, or even a fan.”

COVID-19 is airborne indoors, but there are some precautions you can take to reduce your risk.
nadia_bormotova via Getty Images
COVID-19 is airborne indoors, but there are some precautions you can take to reduce your risk.

But, as mentioned above, the virus may also be aerosolized, meaning tiny, lighter-weight particles of the virus can mix into the air with dust and dirt and spread through the air ― and potentially through an AC system ― rather than falling to the ground pretty quickly because of their size.

A July report in the National Review also found that while evidence on AC and the coronavirus is still mixed, there is some data that suggests an indoor cooling system could play a role in virus transmission.

In a piece for The Harvard Gazette, Edward Nardell, a professor of environmental health and of immunology and infectious diseases at the Harvard T.H. Chan School of Public Health, pointed to Southern states in the U.S. as an example. A host of factors have played a role in the rise in cases in states like Florida and Texas, like mass reopenings and a lack of mask-wearing. However, Nardell also said that the hot weather luring more people to stay inside in air-conditioned spaces could be an additional contributor.

“The states that, in June, are already using a lot of air conditioning because of high temperatures are also the places where there’s been greater increases in spread of COVID-19, suggesting more time indoors as temperatures rise,” Nardell said. “As people go indoors in hot weather and the rebreathed air fraction goes up, the risk of infection is quite dramatic.”

The same could potentially be said for fans.

“When you have a relatively large room and a system blowing air, there’s a chance a fan can blow droplets from one person toward somebody else,” James Lo, an assistant professor of architectural engineering at Drexel University, previously told HuffPost. “So, that is the key concern of how an HVAC system might impact airborne transmission.”

...but that risk is generally low, and can be easily mitigated by following recommended guidelines.

Experts emphasize that all of this shouldn’t cause panic. The risk of getting sick because of an HVAC system ― either in your home or in a public building ― isn’t something that alarms experts.

“The likelihood is that the dilution and filtration that occurs as air recirculates is likely sufficient to prevent most of the dispersion of virus in that way that would pose a threat to people,” Rick Martinello, medical director for infection prevention at Yale New Haven Health, previously told HuffPost.

Also, most importantly, your risk of catching the virus indoors can be lowered if you take some precautions.

Experts say that opening windows can help flush out virus particles and keep the air more free-flowing. Opening blinds and shades may also help your mission to reduce transmission. And if you have control over your AC, make sure your filtration is in top shape.

“One thing you can do if you are a homeowner and have a forced-air heating and cooling system is to ensure that the air filter in your unit is replaced according to the filter instructions ... Some filters are designed to remove particles such as respiratory droplets,” according to guidance from Mount Sinai. (Your indoor air quality is also important to your health, regardless of the coronavirus.)

Additionally, your proximity to an infected person matters. This is why social distancing is so important, whether you’re out on your daily walk, at the grocery store or taking care of a sick person at home.

The closer you are to someone who is contagious, the more likely you are to come in contact with respiratory droplets that can make you ill. Experts recommend keeping 6 to 8 feet of space between yourself and people who are not in your household. (Even without feeling sick, many people can still be COVID-19 carriers.) And, of course, wear a mask when you’re around other people.

Finally, keep practicing good hygiene. We know now that the virus isn’t mainly spread by touching contaminated surfaces ― so there’s no urgent need to wipe down every piece of mail ― but you should still wash your hands frequently. It’s also advisable to wipe down high-touch surfaces, like doorknobs, countertops and handles.

Taking care of yourself, even if it feels like an overreaction, is the best way to reduce your risk.

This story has been updated to include new information about indoor and outdoor spread.

Experts are still learning about COVID-19. The information in this story is what was known or available as of publication, but guidance can change as scientists discover more about the virus. Please check the Centers for Disease Control and Prevention for the most updated recommendations.

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