Several months into a pandemic that has claimed hundreds of thousands of lives and decimated economies around the world, scientists still lack a complete understanding of how the virus that caused it is transmitted. Lockdowns are already easing in some places, and people are preparing to return to a version of work and social life. But a crucial question stubbornly remains: Can the pathogen behind COVID-19 be “airborne”?

According to the U.S. Centers for Disease Control and Prevention and the World Health Organization, the novel coronavirus is primarily spread by droplets from someone who is coughing, sneezing or even talking within a few feet away. But anecdotal reports hint that it could be transmissible through particles suspended in the air. After attending a choir practice in Washington State in early March, dozens of people were diagnosed with or developed symptoms of COVID-19 even though they had not shaken hands or stood close to one another. At least two died. After dining at an air-conditioned restaurant in China in late January, three families at neighboring tables became sickened with the virus—possibly through droplets blown through the air.

To address the prospect of airborne spread of the novel coronavirus, it is first necessary to understand what scientists mean by “airborne.” The term refers to transmission of a pathogen via aerosols—tiny respiratory droplets that can remain suspended in the air (known as droplet nuclei)—as opposed to larger droplets that fall to the ground within a few feet. In reality, though, the distinction between droplets and aerosols is not a clear one. “The separation between what is referred to as ‘airborne spread’ and ‘droplet spread’ is really a spectrum,” especially when talking about relatively small distances, says Joshua Santarpia, an associate professor of pathology and microbiology at the University of Nebraska Medical Center.

Airborne spread has been hypothesized for other deadly coronaviruses, including the ones that cause severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS). A handful of studies suggest the new coronavirus, SARS-CoV-2, can exist as an aerosol in health care settings. But much remains unknown about whether the aerosolized virus is infectious and what amount of virus one needs to be exposed to in order to become sick, known as the minimal infectious dose. Even if aerosol transmission does occur, it is not clear how common it is, compared with other transmission routes, such as droplets or surfaces. Uncovering this information is vital, especially given the fact that people can spread SARS-CoV-2 when they have no symptoms.

Perhaps “Is the coronavirus airborne?” is the wrong question. COVID-19 may have the potential for airborne spread, says Stanley Perlman, a professor of microbiology at the University of Iowa. “But whether [this route is] important clinically is really the question one wants to know about,” he says.

Evidence for Aerosol Transmission

Some of the strongest evidence that airborne transmission of the new coronavirus may be possible comes from a study published late last month in Nature. In it, researchers measured the virus’s genetic material, or RNA, in aerosols sampled in February and March at two hospitals in Wuhan, China—the city where the outbreak is widely believed to have begun. The researchers found very low levels of airborne viral RNA in the hospitals’ isolation wards and in ventilated patient rooms. But there were measurably higher levels in some of the patients’ toilet areas. They also found high levels of viral RNA in places where medical workers remove protective gear, as well as in two crowding-prone locations near the hospitals. “Our study and several other studies proved the existence of SARS-CoV-2 aerosols and implied that SARS-CoV-2 aerosol transmission might be a nonnegligible route from infected carriers to someone nearby,” says study co-author Ke Lan, a professor and director of the State Key Laboratory of Virology at Wuhan University.

A preprint (not yet published) study led by Santarpia and his colleagues similarly found evidence of viral contamination in air samples and surfaces from rooms where COVID-19 patients were being kept in isolation. “I think there are a lot of us—myself included—who feel very strongly that the airborne route of transmission is very possible,” he says. “I would hesitate to call it proven by any means. But I think there’s mounting evidence to support it.”

Both the Nature study and Santarpia’s paper measured viral RNA, not actual virus, so it is not clear that the material found in aerosols was functionally infectious. “Finding RNA doesn’t tell you [that] you have aerosol spread,” says Perlman, who was not involved in either study.

Another paper, recently published in the New England Journal of Medicine, showed that infectious SARS-CoV-2 virus can remain in aerosols for at least three hours—and for several days on various surfaces—in a laboratory setting. But the amount of viable virus diminished significantly during that time. Scientists do not know the infectious dose of SARS-CoV-2. (For influenza, studies have shown that just three virus particles are enough to make someone sick.)

Overall, most of the evidence that SARS-CoV-2 can become airborne comes from clinical settings—which tend to have a lot of sick people and and may host invasive procedures, such as intubations, that can cause patients to cough, generating aerosols. It is not clear how representative of everyday environments these areas are. “There is not much convincing evidence that aerosol spread is a major part of transmission” of COVID-19, Perlman says.

That assessment does not mean it is not occurring, however. Benjamin Cowling, head of the division of epidemiology and biostatistics at the University of Hong Kong’s School of Public Health, says there is a popular misconception that if a virus can spread through the air at all, it must be able to spread over a long range. He gives the analogy of being in a restaurant where someone is smoking: “If the person on the other side of restaurant is smoking, you probably wouldn’t smell it, and you’d never even notice. That’s because the smoke would never reach you,” he says. “It doesn’t mean there’s not smoke produced.” In other words, just because SARS-CoV-2 may not be transmitted over a long range, that does not mean it is not airborne. Like cigarette smoke, aerosol particles spread around a person in a cloud, with the concentration being highest near the smoker and lower as one gets farther away.