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Scientists Probe How Coronavirus Might Travel Through The Air

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When researcher Josh Santarpia stands at the foot of a bed, taking measurements with a device that can detect tiny, invisible particles of mucus or saliva that come out of someone's mouth and move through the air, he can tell whether the bedridden person is speaking or not just by looking at the read-out on his instrument.

"So clearly the particles that that person is putting out are being breathed in by someone that is five feet away from them, at the foot of their bed," says Santarpia, who studies biological aerosols at the University of Nebraska Medical Center. "Do they contain virus? I don't know for sure."

He and his colleagues are doing their best to find out. Already, using another contraption that looks like a fancy dustbuster, they've sucked up air samples from 11 isolation rooms that housed 13 people who tested positive for COVID-19 infection, all of whom had a variety of mild symptoms.

In those air samples, researchers found the genetic fingerprint of the virus. "It was more than half of the samples that we took. It was fairly ubiquitous," says Santarpia, "but the concentrations were really pretty low."

Finding the genetic material doesn't necessarily mean that there's viable virus that could potentially make someone sick, he cautions. Some preliminary evidence indicates that this might be the case, but the team wants to do more work "and try and be as certain as we possibly can whether or not certain samples had infectious virus in them or not."

They want to know that with a high degree of confidence because the question of whether or not the coronavirus can be "airborne" is extremely contentious right now — and it's a question that has real implications for what people should do to avoid getting infected.

Investigating 'Bioaerosols'

Now, citing Santarpia's results along with other studies, a committee of independent experts convened by the National Academies of Sciences, Engineering, and Medicine has weighed in, in response to a question from the White House Office of Science and Technology Policy about whether the virus "could be spread by conversation in addition to sneeze/cough-induced droplets."

"Currently available research supports the possibility that SARS-CoV-2 could be spread via bioaerosols generated directly by patients' exhalation," says a letter from the committee chair. By bioaerosols, they are referring to fine particles emitted when someone breathes that can be suspended in the air rather than larger droplets produced through coughs and sneezes.

Even if additional research shows that any virus in such tiny particles is viable, researchers still won't how much of it would need to be inhaled to make someone sick. But the committee experts also caution that uncertainty about all this is almost a given—because there's currently no respiratory virus for which we know the exact proportion of infections that come from breathing the virus in versus coming into contact with droplets in the air or on surfaces.

"I personally think that transmission by inhalation of virus in the air is happening," says Linsey Marr, an aerosol scientist at Virginia Tech. But she says so far, health experts have largely discounted the possibility of transmitting this coronavirus in this way.

"I think the public is concerned about this, but the World Health Organization says 'no, the disease is not airborne,'" says Marr.

"From an infection prevention perspective, these things are not 100% black and white. The reason why we say 'droplet' versus 'airborne' versus 'contact' is to give overall guidance on how to manage patients who are expected to be infectious with a specific pathogen," said Dr. Hanan Balkhy, assistant director-general for antimicrobial resistance at WHO, in an interview with NPR earlier this week.

As an expert who worked to contain an outbreak of the deadly MERS coronavirus in Saudi Arabia, she believes that this new virus should behave similarly to other severe coronaviruses — and that means, unless health-care workers are doing invasive procedures like putting in breathing tubes, the virus is expected to primarily spread through droplets.

Droplets are larger respiratory particles that are 5 to 10 micrometers in size. Those are considered "big," even though a 5 micrometer particle would still be invisible to the naked eye. Traditionally, those droplets are thought to not travel more than about three feet or so after exhalation. That would mean the virus can only spread to people who get close to an infected person or who touch surfaces or objects that might have become contaminated by these droplets. This is why public health messages urge people to wash their hands and stand at least 6 feet away from other people.

An "airborne" virus, in contrast, has long been considered to be a virus that spreads in exhaled particles that are tiny enough to linger in the air and move with air currents, letting them be breathed in by passersby who then get sick. Measles is a good example of this kind of virus — an exhaled measles pathogen can hang suspended in a room for a couple hours after an infected person leaves.

The reality of aerosol generation, however, is far more complex than this "droplet" versus "airborne" dichotomy would suggest, says Marr. People produce a wide range of different-sized particles of mucus or saliva. These particles get smaller as they evaporate in the air and can travel different distances depending on the surrounding air conditions.

"The way the definitions have been set up, this "droplet" vs "airborne" distinction, was first established in the 1950s or even earlier," says Marr. "There was a more limited understanding of aerosol science then."

Even a 5 micrometer droplet can linger in the air. "If the air were perfectly still, it would take a half hour to fall from a height of 6 feet down to the ground. And, of course, the air isn't perfectly still," says Marr. "So it can easily be blown around during that time and stay in the air for longer or shorter."

What's more, coughs and sneezes create turbulent clouds of gas that can propel respiratory particles forward.

"For symptomatic, violent exhalations including sneezes and coughs, then the droplets can definitely reach much further than the 1 to 2 meter [3 to 6 feet] cutoff," says Lydia Bourouiba, an infectious disease transmission researcher at MIT, referring to the distance typically cited as safe for avoiding droplet-carried diseases.

How Far Can Droplets Travel?

In fact, studies show that "given various combinations of an individual patient's physiology and environmental conditions, such as humidity and temperature, the gas cloud and its payload of pathogen-bearing droplets of all sizes can travel 23 to 27 feet," she wrote in a recent article published online by the Journal of the American Medical Association.

At a briefing by the White House's coronavirus task force on Tuesday, a reporter asked Dr. Anthony Fauci, head of the National Institute of Allergy and Infectious Diseases, about the potential for the coronavirus to travel 27 feet.

"This can really be terribly misleading," said Fauci, who then reared his head back and mimicked making an enormous sneeze. "If you go way back and go, 'Achoo!' And go like that, you might get 27 feet. So when you see somebody do that, get out of the way."

Fauci went on to say he was "disturbed" by headlines about the virus traveling such distances "because that's misleading. That means that, all of a sudden, the 6-foot thing doesn't work."

The virus traveling distances that might be achieved after a vigorous sneeze is "not what we're talking about" when it comes to social distancing, Fauci said, defending the 6-foot guideline.

WHO, in its general communications with the public on such platforms as Twitter, has stated that the virus "is NOT airborne" and that it is mainly transmitted "through droplets generated when an infectious person coughs, sneezes or speaks."

"Transmission of COVID-19 is through droplets, it is not airborne," said Dr. Maria Van Kerkhove, the WHO's technical lead for COVID-19, at a recent press briefing. "Therefore someone who has these small liquid particles that come out of their mouth; they travel a certain distance and then they fall so that's why we recommend the physical distance, to be separated so that you remove the opportunity for that virus to actually pass from one person to another."

And in its technical guidance on how the virus is transmitted, WHO says that protections against airborne transmission are only needed for health-care workers when they do medical procedures which might be anticipated to produce smaller respiratory droplets that could then be inhaled.

That position has been criticized by some experts on virus transmission, however, who say so little is known about this virus that it is better to be cautious and recommend that health-care workers use N95 masks and other kinds of more protective gear for all patient care whenever possible, especially given the emerging bits of evidence that airborne transmission is a possibility.

But WHO has defended its stance.

"This is not the WHO's opinion on its own. The WHO provides such guidelines through its networks of scientific experts from around the world," Balkhy told NPR.

"It can be inhaled if you are very close to the patient, if you are creating aerosolization," says Balkhy. But she said past clinical experience with other severe coronaviruses, SARS and MERS, supports the idea that these viruses mainly spread via droplet transmission.

In order to conclude otherwise, she said, "we need to have the evidence that it actually is being spread through a specific route that we're not seeing right now."

New Report On Airborne Transmission

Some of the strongest evidence that an airborne route of transmission might be possible for this virus comes from a report published last month by the New England Journal of Medicine that described mechanically generating aerosols carrying the SARS-CoV-2 virus in the laboratory. It found that the virus in these little aerosols remained viable and infectious throughout the duration of the experiment, which lasted 3 hours.

WHO mentioned this study in its recent review of possible modes of transmission and noted that "this is a high-powered machine that does not reflect normal human cough conditions ... this was an experimentally induced aerosol-generating procedure."

It may have been artificial, says Marr, but "the conditions they used in that laboratory study are actually less favorable for survival compared to the real world. So it's more likely that the virus can survive under real world conditions."

Humans definitely produce droplets that can dry down to the sizes that were tested in that study, says Santarpia. "Do ill people with this disease produce respiratory droplets containing the virus which dry down to those sizes? That I don't know," he says. "That's the sort of piece of data that I am trying to get right now."

Another important piece of information for understanding the risk of airborne transmission of this disease would be knowing how much of the virus a person has to inhale in order to actually fall ill.

"We're starting to answer the questions about how stable is this as an aerosol. We're starting to answer the questions about what kind of aerosols are people producing that have infectious virus in them," says Santarpia. "The last piece of that is the infectious dose, or how many virus particles do you have to ingest or inhale to become sick."

He thinks it is possible that some form of airborne spread is playing a role in the transmission of this virus.

"In my opinion, it's probably more than just droplet," says Santarpia, adding that intuitively it does not seem to him "that this is just like large droplets that just sort of have ballistic trajectories and land inside your mouth that are causing disease."

Still, he says, "one of the reasons we are doing the work that we are doing is that this is science and we look for proof."

The 6 Foot Recommendation

Generally speaking, researchers say, the 6-foot distance recommendation seems reasonable.

But if enough people gather in a room that has little or no ventilation, it's possible that even large droplets might accumulate in the air.

"Given what we know so far about this virus, 6 feet is OK if it's not crowded. But if you are in a room or somewhere where there is a person every 6 feet, then you could have enough virus in the air that I would be concerned," says Marr.

Something like that might have happened when a choral group in Washington state got together to sing, and ended up with dozens of cases of COVID-19 despite the fact that members of the group were careful not to touch each other or stand close.

"The simplest explanation is that it was being transmitted through the air," she says, noting that people should ventilate buildings and that concentrations of particles are less likely to build up outdoors.

She personally likes to stay 10 feet away from people, and is thinking of wearing some kind of mask for trips to the grocery store.

But even if scientific studies eventually prove that there's an airborne component to transmission, says Santarpia, that doesn't mean that you run the risk of contracting the disease from great distances, like if some stray, lonely virus particle floated into your path from way down the street.

"I just don't think that's realistic," he says, adding that he's assured his aunt that it's fine for his 95-year-old grandmother to sit outside on the porch to enjoy a sunny day.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

Nell Greenfieldboyce is a NPR science correspondent.
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