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Is the coronavirus airborne? Here’s what we know.

One reason why measles — a notoriously contagious disease — is so difficult to contain is because its infectious viral particles can linger in the air for up to two hours. Can the coronavirus do the same?

It’s a question health officials appear to be grappling with: On Thursday, the San Francisco Department of Public Health said people must wear masks if they are within 30 feet of someone not in their household, a far greater distance than the widely recommended 6 feet of social distancing. Meanwhile, the Centers for Disease Control and Prevention website reads, “It is unknown how long the air inside a room occupied by someone with confirmed COVID-19 remains potentially infectious.”

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While scientists say it is possible that the coronavirus can drift through the air, many note there’s no evidence these tiny bits of virus are enough to make people sick.

To understand how the virus travels by air, it’s important to know whether it’s hitched a ride on a jumbo jet — or a paper airplane.

“It’s basically a size difference,” said Dr. Ronald Collman, referring to the size of the droplets that contain viral particles. Collman is a professor of medicine in the Division of Pulmonary, Allergy and Critical Care at the University of Pennsylvania’s Perelman School of Medicine.

When a person with COVID-19 coughs or sneezes, they spew relatively large droplets — at least 5 micrometers in diameter — into the air. These droplets are filled with viral particles.

Studies have demonstrated how those globs of saliva and sputum are so big and heavy — compared to other respiratory emissions, that is — that they generally come into contact with another person’s face, or fall to the ground or surfaces, within about a six-foot radius.

The CDC believes those respiratory droplets are responsible for the majority of COVID-19 transmissions.

But some studiesshow viral particles can get stuck in tiny aerosols less than 5 micrometers in diameter. They’re too light to fall to the ground, thus becoming airborne.

This is much like the water vapor you can see in your breath when it’s cold, wafting through the air before slowing dispersing.

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The potential for the coronavirus to linger in the air is concerning to scientists who study aerosols. Kimberly Prather, a distinguished professor in atmospheric chemistry at the University of California, San Diego, addressed the possibility in a paper published this week in the journal Science.

“Aerosol transmission of viruses must be acknowledged as a key factor leading to the spread of infectious respiratory diseases,” Prather and her co-authors wrote.

But evidence is lacking that this particular virus is indeed infectious in aerosolized form, Prather told NBC News.

“How long does it live when it’s in the air? We still have a lot of work to do to answer that,” Prather said.

Other experts agreed.

“Just because some viral element is detected does not mean it is infectious,” said Dr. Aditya Shah, an infectious disease fellow at Mayo Clinic in Rochester, Minnesota.

That’s because when it comes to making a person sick, viruses tend to get their strength in numbers.

Any pathogen — including SARS-CoV-2, the type of coronavirus that causes COVID-19 — generally requires larger amounts of particles to become infectious. The smaller the particle, scientists say, the less likely it is to carry enough virus to survive a breezy journey into another person’s body.

“I think it is plausible that there could be some level of aerosol transmission,” Collman said. “But if it occurs, it’s probably not very important or common.”

Still, as evidence grows that people can spread the virus without having any symptoms, doctors overwhelmingly support the use of face masks to reduce the risk for infecting others with emissions of all sizes.

“People who don’t wear face masks think that they’re saying, ‘I’m tough. I’m strong. I’m not afraid of getting COVID,'” Collman said.

“What they’re really saying is, ‘I don’t give a damn about other people.'”

Face masks at home?

While the CDC recommends wearing face coverings in public places to cut the risk of spreading potentially infectious droplets, a study published Thursday in the journal BMJ Global Health suggests benefits to masking at home.

Researchers in China surveyed 124 families in which at least one person had been diagnosed with COVID-19. Households that were able to isolate the ill family member, as well as use disinfectants, had a lower chance of viral spread.

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What’s more, the study suggested wearing face masks in the home was 79 percent effective in curbing spread of COVID-19 among family members.

“This is the first study to show effectiveness of precautionary mask use, social distancing and regular disinfection in the household,” the study authors wrote.

The research also illustrated the elevated risks of transmitting the virus in close quarters, such as sitting around a dinner table or watching TV together. Still, it appears to be too soon for physicians to recommend wearing masks inside the home routinely.

Dr. Katie Passaretti, medical director for infection prevention at Atrium Health in Charlotte, North Carolina, said it may be reasonable to wear a mask inside the home if one family member is often exposed to members of the public, or perhaps is caring for a vulnerable person.

“What’s the biggest bang for your buck?” Passaretti asked. “It probably makes the most sense to encourage [masks] in households with individuals who are at higher risk for more severe disease,” such as older adults or people who have weakened immune systems.

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