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Behind the Conflicting Advice on Coronavirus Safety

New research is upending early public health advice. That’s going to continue as researchers understand the virus better.

Bradley Kanaris/Getty Images
Crowds are seen for the first time in Brisbane, Australia, since the Covid-19 lockdowns.

The now-infamous event occurred on March 15: A choir gathered in Skagit County, Washington, for a socially distanced practice indoors. One choir member with coronavirus symptoms showed up to sing. Of the 60 members present, 52 people became infected with the coronavirus, and two died.

In the past few months, those following and researching the coronavirus pandemic have become well acquainted with this case. But in mid-May, the Centers for Disease Control and Prevention released a report that confounded public health messages on the coronavirus up to that point: The virus at the rehearsal had been airborne. This study is now one among several in recent weeks clarifying how and under what conditions the virus is most easily transmitted. And that, in turn, drives changes in how experts think about best practices—from the CDC’s pivot on mask-wearing to new theories about what sorts of daily activities may be relatively low risk or high risk as states reopen.

Originally, organizations like the CDC and the World Health Organization outright dismissed the idea that the virus could linger in the air in an infectious cloud. The agencies’ advice focused on avoiding large droplets from people coughing, as well as contaminated surfaces. Much of their advice is still centered on those forms of transmission. But the CDC report in May confirmed that vocalizations can create aerosols, or tiny droplets that hang in the air, which likely contributed to the wide spread of the virus at the choir practice. Another CDC research letter in May pointed to airborne transmission as one of the ways the virus was likely transmitted among meeting attendees in Germany.

Several studies have shown that singing, talking, and even sometimes breathing can produce a cloud of respiratory droplets that float through the air. Research published in The New England Journal of Medicine in April found that saying the words “stay healthy” in a normal, conversational voice emitted 347 droplets, many of them fairly small—small enough to remain suspended in the air. Even louder speech releases thousands of droplets that can drift in the air for eight to 14 minutes in an environment with stagnant air, according to research published in Proceedings of the National Academy of Sciences earlier this month. The researchers conclude that “there is a substantial probability that normal speaking causes airborne virus transmission in confined environments.”

Researchers at the National Academy of Sciences believe, based on a review of studies, that the coronavirus can also be spread by breathing in close proximity. One study they cited was a preprint—which has not been published in a journal or peer-reviewed officially—from researchers at the University of Nebraska Medical Center, who found viral particles in the air of rooms housing coronavirus patients, even when the patients weren’t coughing.

“Finding stuff in the air is what we expected,” Joshua Santarpia, the lead researcher on that study, told me. “I didn’t expect, maybe, to find quite so much of it.” The researchers found traces of the virus in the air, on windowsills, in air vents, and around toilets in the bathroom—everywhere they took samples. (Research also shows that the virus, like many viruses, can be aerosolized when poop is flushed down the toilet, although it’s not clear how infectious it is.)

The ubiquity of the virus samples makes sense, though, given what we now know about its transmission, Santarpia said. People who have the coronavirus are often contagious before they get sick, and as many as 40 percent of those who have the virus don’t show any symptoms at all. That means this pandemic is being driven, in part, by people who aren’t coughing or sneezing but are breathing, talking, and singing—likely at close distances, often in poorly ventilated spaces.


Scientists’ increasing conviction that airborne viral particles contribute to the high transmissibility of the coronavirus has played a role in several recent public health recommendations, including the increasing emphasis on mask-wearing. Originally, health organizations recommended against most people using masks to avoid the virus. The U.S. surgeon general tweeted in late February that masks were “NOT effective” in keeping the general public from catching the coronavirus. In April, however, the CDC began strongly recommending masks, and in early June, researchers from the United Kingdom’s Cambridge and Greenwich Universities published a study suggesting widespread mask usage could in fact prevent new waves of infections. Notably, in the aforementioned study where people said “stay healthy,” simply putting a damp washcloth over their mouth brought the spray of droplets down to zero.

The slow evolution of public health advice has perplexed some researchers. Lisa Brosseau, an expert on respiratory protection and infectious diseases, wrote back in March that airborne virus particles were likely an important but overlooked aspect of the pandemic. “Inconsistent and conflicting” advice from public health experts could hurt the public’s ability to respond to the virus, she said. In June, Lidia Morawska and Junji Cao, researchers on air quality and physics, wrote in Environment International that “the world should face the reality” of airborne transmission of the coronavirus. Doing so, they said, would allow officials to implement measures to slow the spread of the virus.

Linsey Marr, a professor of civil and environmental engineering at Virginia Tech, characterized the early CDC reticence on acknowledging airborne spread as a case of conflicting priorities: “They were motivated mainly by conserving supplies of respirators and surgical masks for healthcare workers because of the shortages.” And airborne diseases tend to have higher panic potential. “I’m not in the public health field, but I can see how they’re walking a fine line between convincing people to take appropriate measures through distancing and wearing masks but also not causing panic, because panic could be even worse.”

Even at this point in the developing scientific consensus, public health experts have been reluctant to talk about airborne spread. “This is a virus that is primarily spread through respiratory droplets and secretions—that cough, that sneeze,” Saskia Popescu, a hospital epidemiologist and infection preventionist, told me. While yelling or singing, as in the case of the choir, can also put out more droplets, she said, aerosols are primarily created in medical settings, like when a patient is intubated. There are some diseases, like measles and tuberculosis, that are primarily spread through clouds of viral particles, Popescu said, but that doesn’t seem to be the main way this coronavirus is spread.

A research letter published in The New England Journal of Medicine found that SARS-CoV-2 can remain viable in the air for up to three hours, but that experiment was done by artificially aerosolizing the virus, Popescu pointed out—not by testing air samples in patients’ rooms, for instance. The big questions that remain for scientists are how viable naturally released virus particles are and how many viral particles in an aerosol cloud would be infectious. In other words: Can they make you sick, and if so, how big a dose does it take?

“Previous studies have shown that other viruses survive equally well, if not better, in suspended aerosols compared to large, stationary droplets,” Marr and Joseph Allen, an assistant professor of exposure assessment science at the Harvard T.H. Chan School of Public Health, wrote in a recent article that hasn’t been peer-reviewed or officially published. One test of these aerosols’ viability, experts have said, would be growing viruses from the particles found floating through the air. Santarpia hopes to answer this question in his next study.

It is, of course, extremely difficult to tell how someone was infected with a virus—through large or small droplets or through contact with a contaminated surface or item. There have been no studies confirming one route of coronavirus transmission over the other, Marr pointed out. But she believes the airborne route is just as likely as droplet spread. “I’ve seen at least as much evidence supporting airborne transmission at short range, close to someone, compared to what they call droplet spray transmission,” she told me. “If you happen to be infected, then you could be releasing clouds of virus into the air.”


As each aspect of this potential transmission chain is better understood, public health recommendations could change significantly—and get very specific about what sorts of activities might be high risk and what other activities might be relatively low risk.

Such is the case with the emerging theory that some forms of public transit may not be as dangerous as originally thought—particularly if riders are silent and wearing masks, and the cars and buses are well ventilated and not too crowded. Going to a crowded bar, experts agreed, is extremely high risk; going on a walk through the park is lower risk.

“You have to understand three factors: intensity, frequency, and duration,” Allen said. Imagine a smoker. If they exhale right next to you, you’ll get a pretty heavy hit of the smoke. This is especially true inside, when there’s not a lot of fresh air moving around. But if you’re standing a few feet away, you’re less likely to get a face full of smoke; stand 10 feet away, outside, and you may not even notice. “If we think about the airborne particles, we might get scared: Oh, they linger for a long time,” Allen said. “But we have to remember that they dilute in time and space.”

Indoor environments with poor air circulation, where the infectious clouds are less likely to be diluted, are particularly risky. Good ventilation systems have air filters specifically designed for catching tiny particles like these; bad ventilation just recirculates the air. In one case from Guangzhou, China, for instance, an asymptomatic person sat in a restaurant in front of an air conditioning vent. Three families at tables seated downwind from the vent got sick, while those seated outside of the stream of air didn’t. The air conditioning seemed to push viral particles through the air, even farther than six feet. Similarly, in an office building in South Korea, 94 of 216 employees working on the same floor contracted the virus, leading researchers to conclude that crowded office settings could lead to significant transmission. If it’s not possible to avoid crowded indoor environments, bringing in fresh air and filtering the rest could help; even portable HEPA filters can capture these airborne particles.

Staying outside isn’t a foolproof way to avoid the virus—if you’re in close contact with someone who has the virus, you’re likely to get it, no matter where you are. But having more open spaces makes it easier to keep a distance—and it may allow the virus particles to spread out in the air, so you’re less likely to be hit with a big viral dose.

“I think it should be empowering knowing how we’re exposed and what are the most high-risk locations, in the sense that then we can just apply prudent control measures to reduce risk,” Allen told me. “There’s no such thing as zero risk in anything we do. Certainly not during a pandemic. And so our goal is to reduce risk.” Allen recommends five ways to do so: stay home, especially if you’re sick; wear a mask when you go out; avoid large gatherings; freshen and filter indoor air; and keep a six or even 10-foot distance from others.

One of the advantages we have gained during the past few months is a much better understanding of this once-novel coronavirus. This new information doesn’t mean we can relax precautions: Don’t toss your mask aside on your way to a crowded restaurant. But the more we learn about the virus, the better and more effectively we can protect ourselves. As scientists continue making contributions at breakneck speed, public health advice and governmental reopening guidelines may change as well. The thing to remember, Santarpia said, is that this new knowledge doesn’t change how the virus works. “It’s not like once upon a time, it was droplet spread,” he told me. “It’s been this way the whole time. Our understanding of the virus is changing; the virus itself isn’t changing.”