As researchers race to study the coronavirus, an important question remains hotly debated: Can it spread through the air?
Scientists agree that the virus, which has infected more than 950,000 people worldwide, is primarily transmitted through droplets — particles larger than 5 micrometers — when an infected person coughs, sneezes, or speaks.
Those droplets fly through the air before landing on another person. But scientists still aren’t sure to which degree a cloud of tiny viral particles — known as aerosols (these are smaller than droplets) — could linger in the air and infect the next person who walks through the same space. This is known as airborne transmission, and the measles virus is known to spread that way — it lives for up to two hours in an airspace where an infected person coughed or sneezed.
But the World Health Organization says that’s not the case for the coronavirus.
However, several recent studies have identified live coronavirus particles in the air. The Centers for Disease Control and Prevention says certain hospital procedures, like intubating a patient, “could generate infectious aerosols.”
Droplets versus aerosols
The main difference between droplets and aerosols is that the former are heavy and large, so they can’t stay aloft for long. The latter, called droplet nuclei by WHO, are smaller than 5 micrometers.
But some researchers are calling for the semantics to be swept aside to avoid confusion and better inform public-health responses to the pandemic.
“I think the WHO is being irresponsible in giving out that information. This misinformation is dangerous,” Donald Milton, an infectious-disease aerobiologist at the University of Maryland, told NPR.
He added: “The epidemiologists say if it’s ‘close contact,’ then it’s not airborne. That’s baloney.”
Some research shows the coronavirus can travel farther than 6 feet, the distance cited by the CDC as adequate social distancing.
“Vigorous coughing or sneezing, during which a patient gives their exhalation more energy, can send their microscopic particles beyond the 2-foot to 6-foot range,” William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University, told Business Insider.
A study published March 26 in the Journal of the American Medical Association said a cloud of virus emitted by someone coughing or sneezing could travel much farther than 6 feet: “The gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet.”
‘You could come into a room thinking everything’s all right and then you inhale it’
During a recent interview on “The Daily Show,” host Trevor Noah asked Dr. Anthony Fauci — the director of the National Institute of Allergy and Infectious Diseases — whether the coronavirus stays in the air.
“If you walk in to an elevator after somebody else, can coronavirus still be there?” Noah said.
Fauci replied that ill people sneezing and coughing could spread the virus by droplets and aerosols.
An aerosolized form of the virus “means the drop doesn’t go down right away; it hangs around for a bit,” Fauci said. “So you could come into a room thinking everything’s all right and then you inhale it.”
But he said aerosol transmission was likely not the primary way the coronavirus spreads and reiterated that social distancing of 6 feet was sufficient to protect yourself.
Schaffner agreed with Fauci about Noah’s elevator example. “In such a tightly enclosed space without vigorous air movement for a short period of time, I’m afraid you might be exposed,” he said.
But that’s a different situation than, say, a supermarket, which is fairly large and has air moving freely.
“The kinds of transient encounters walking up and down the aisle picking up peaches, not really hazardous,” Schaffner said.
In lab settings, the virus can linger in the air for 3 hours
A recent study from the National Institutes of Health looked at how long the new coronavirus could live on common surfaces and said it could live in the air as an aerosol for up to three hours. But those researchers used a high-powered laboratory machine to produce the coronavirus aerosols, so they likely weren’t identical to those produced by human coughs.
Linsey Marr, an expert on aerosol transmission at Virginia Tech, told The New York Times that an aerosol released at a height of about 6 feet should fall to the ground after 34 minutes. The amount of the virus that lingers in the air as an aerosol is also likely too small to infect someone, she said.
“It sounds scary,” she said. “But unless you’re close to someone, the amount you’ve been exposed to is very low.”
Aerosol transmission in hospitals
Because healthcare workers are exposed to higher concentrations of the virus, they face more risk of catching it from both droplets and aerosols.
When coronavirus patients need ventilators, doctors insert a tube into their airway, and that procedure inevitably generates infectious aerosols.
One study, which has yet to be peer-reviewed, examined two Chinese hospitals and found low levels of the coronavirus in the air in patients’ rooms. Higher airborne concentrations were recorded in medical-staff areas, particularly in places where doctors removed protective gear. The authors wrote that virus-laden aerosols were probably deposited on protective gear like masks and gowns while the doctors worked, then sent back into the air when the staff shook those items as they stripped down.
“Surface sanitization of the apparel before they are taken off may also help reduce the infection risk for medical staff,” the authors wrote.
Scientists also detected the virus in the air outside patients’ rooms at the University of Nebraska Medical Center.
“If you think you’re doing aerosolizing procedures in a healthcare setting, don’t use your surgical mask. Use an N95,” Schaffner said.
N95 masks filter out airborne particles smaller 0.3 microns. If the coronavirus weren’t airborne, those masks wouldn’t be as essential.
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