How to stay safe from COVID-19 when you're inside

More time indoors with people you don’t live with means more potential for viral spread.

As winter approaches and the pandemic surges, staying safe from COVID-19 while sharing space indoors is more important than ever.

Because SARS-CoV-2 spreads through the air, being outdoors mitigates some of the risk — especially when distanced from others — as virus particles either fall to the ground or dissipate in the open air.

Indoors, however, those particles can recirculate and linger, potentially for 24 to 48 hours, says Reza Ronaghi, MD, a pulmonologist with UCLA Health.

“If you are spending time indoors with somebody, that’s where the danger can happen and you can get infected,” he says, especially since COVID-19 has a “dormant phase” during which an infected person feels fine, exhibits no symptoms and doesn’t realize they’re sick – but is still spreading the virus.

The Centers for Disease Control and Prevention advises Americans to avoid in-person holiday gatherings this year, particularly those held indoors. Los Angeles County public health officials issued new guidelines recently urging residents to avoid situations where people from different households might mix.

If you must share indoor space with people who don’t live with you, staying at least six feet apart and wearing a mask is essential, Dr. Ronaghi says. While masks aren’t 100% effective at preventing infection, they do offer two-fold protection, he says: They reduce the number of virus particles a sick person exhales into the environment, and they prevent the inhalation of virus particles by trapping them within the mask’s fibers.

“That’s why indoor dining is such a big issue, because when you’re eating, obviously it’s not possible to have a mask on,” Dr. Ronaghi says. “That means that as you talk or eat, the virus can be expelled into the environment, where it can linger around and potentially infect others.”

Reducing exposure

Richard J. Jackson, MD, a professor emeritus at the Fielding School of Public Health and a former director of the CDC’s National Center for Environmental Health, notes that masks and physical distancing help prevent infection by reducing viral exposure. “Your risk of becoming ill depends in a lot of ways on how much virus you are exposed to,” he says. “If you’re pretty robust, you’ll probably need a larger exposure to get sick. If you’re quite frail, very small exposures might make you sick.”

Air purifiers can be helpful by capturing some virus particles, experts say, but only if equipped with high-efficiency particulate air, or HEPA, filters. “Not any filter is actually able to take the virus and filter it out,” Dr. Ronaghi says. “But what have been tested and shown to be effective are the ones that contain the HEPA filter.”

Because these machines are designed for different room sizes or volumes of air, it’s important to choose one, or several, suited to the area, he says. A small machine intended for an area the size of a desk, for instance, may not effectively purify the air in a large living room.

“An air purifier doesn’t mean you can get together and have a big party and everything is safe,” Dr. Ronaghi says. “You should still follow the protocols. However, it does filter the air – it makes it so the air inside isn’t stagnant and it’s able to filter out the virus if it has the HEPA filter and is the right size for the room.”

Dr. Jackson says investing in a high-quality air purifier may be a good idea because it can also filter out dust, pollen and smoke from wildfires, which are likely to continue as climate change worsens. He adds that virus particles often cling to dust and soot in the air, and a HEPA filter can reduce these indoors.

“One truism of public health, particularly environmental health,” he says, “is that a good intervention solves multiple problems.”