New research suggests Covid-19 can travel 4 metres through the air in a hospital
- The new coronavirus typically spreads when droplets from an infected patient's coughs or sneezes land on or get inhaled by another person.
- According to the World Health Organisation, the virus is not airborne and does not spread between people who are more than 2 metres apart.
- Some research, however, suggests viral droplets can travel farther than 2 metres in certain conditions and that live coronavirus particles can persist in the air in aerosol form.
- Such aerosols pose a disproportionate risk to people in hospital settings.
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As researchers race to study the coronavirus, an important question continues to be debated: Can it spread through the air?
Scientists agree that the virus, which has infected nearly 2 million people worldwide, is primarily transmitted through droplets - particles larger than 5 micrometres - 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 Organisation says that's not the case for the coronavirus.
"FACT: #COVID19 is NOT airborne," WHO tweeted on March 28.
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," and a new CDC study found the virus could travel up to 4 metres as an aerosol in hospital settings.
Droplets versus aerosols
The recent CDC study found that particles of the coronavirus, whose scientific name is SARS-CoV-2, were "widely distributed in the air and on object surfaces" in the intensive-care unit and general ward of the Huoshenshan Hospital in Wuhan, China. The researchers identified live coronavirus in 35% of samples taken from the air in the ICU, and in 12.5% of the samples taken from the air in the general ward between February 19 and March 2.
"These results confirm that SARS-CoV-2 aerosol exposure poses risks," the study authors wrote, adding: "The maximum transmission distance of SARS-CoV-2 aerosol might be 13 feet (4 metres)."
They also found that coronavirus aerosols were most concentrated in patients' rooms, particularly near and downwind from them.
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 micrometres. 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 in March.
He added: "The epidemiologists say if it's 'close contact,' then it's not airborne. That's baloney."
Other research has also found that the coronavirus could travel farther than 6 feet (2 metres), 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 suggested that someone coughing or sneezing could emit particles that travel much farther than 6 feet (2 metres).
"The gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet, (7 to 8 metres)" the authors wrote.
But Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, called that study "terribly misleading."
During the a White House briefing on March 31, Fauci said that only "a very, very vigorous, robust, 'achoo' sneeze" would send droplets traveling as far as the study suggested.
'You could come into a room thinking everything's all right and then you inhale it'
"The Daily Show," host Trevor Noah also asked Fauci on March 26 on 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 aerosolised 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 is not the primary way the coronavirus spreads and reiterated that social distancing of 2 metres is 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 coronavirus could live on common surfaces and found that 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 aerosols, so they likely weren't identical to those produced by coughs.
- Linsey Marr, an expert on aerosol transmission at Virginia Tech, told The New York Times in March that an aerosol released at a height of about 6 feet (2 metres) 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 sanitisation 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.
Combined, all this evidence shows why medical staff are at higher risk of infection.
- "If you think you're doing aerosolising 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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