A growing number of studies, including one published this week in the journal Nature, have found evidence that the coronavirus can remain suspended in the air in aerosol particles. That raises anew the question of whether and to what extent the virus can be transmitted as an aerosol — although the evidence is far from conclusive and no such infections have been documented.

The consensus so far is that the virus, although very contagious, spreads through respiratory droplets generated when people breathe, speak or cough and doesn’t infect people through particles that can linger in the air for hours, in the way that measles and some other viral diseases can.

But the research is fueling a scientific debate over one of the most basic questions about the novel coronavirus — how it spreads — and doing so at a time of high anxiety and rattled nerves. Outbreaks linked to crowded indoor environments such as prisons, meatpacking plants, a cramped call center and a restaurant may serve as warnings about the perils of reopening.

The scientific literature is full of alarming questions: Could ventilation systems spread the virus? Could removing clothing shake virus particles back into the air?

Research has shown that the virus typically is transmitted from person to person through relatively large respiratory droplets that travel only a few feet before falling to the floor or ground. People can also become infected by touching contaminated objects — known among scientists as fomites — and then, for example, touching their face.

“Airborne spread has not been reported for COVID-19 and it is not believed to be a major driver of transmission based on available evidence,” concluded a comprehensive report by researchers from China and the World Health Organization published in February.

But that report went on to say that some medical procedures in health-care facilities, such as intubating a patient, can generate virus aerosols. And research at the Rocky Mountain Laboratories, part of the National Institute of Allergy and Infectious Diseases, showed that virus particles aerosolized with laboratory equipment remained viable — still capable of growing in a cell culture — for up to three hours while suspended in the air.

The Nature paper, authored by scientists at Wuhan University in China, reported that aerosolized traces of viral genetic material, called RNA, were found in two hospitals — particularly in poorly ventilated spaces. The highest concentration of the aerosol particles were found in a single-person mobile toilet that lacked ventilation. Viral RNA was also found in an area where hospital workers took off their protective gear.

The report, however, did not establish whether the airborne coronavirus samples were viable — that is, capable of generating a new infection.

“Although we have not established the infectivity of the virus detected in these hospital areas, we propose that SARS-CoV-2 may have the potential to be transmitted via aerosols,” the authors wrote. “Our results indicate that room ventilation, open space, sanitization of protective apparel, and proper use and disinfection of toilet areas can effectively limit the concentration of SARS-CoV-2 RNA in aerosols.”

The public should view the new China airborne virus study with caution, because the PCR test cannot distinguish viable virus from genetic fragments that aren’t infectious, said Andrew Noymer, an epidemiologist at the University of California at Irvine.

“The test for viable virus is a much-more-painstaking cell culture test, which the present study did not perform,” Noymer said.

An alarming report from a restaurant in Guangzhou, China, showed that one infected person who had not yet developed symptoms infected nine other diners. Researchers suggested that an air-conditioning unit recirculating the air could have spread droplets, carrying the virus between tables.

“This just demonstrates the terrible confusion that is created by the common misconception that there’s somehow a bright line between aerosols and respiratory droplets,” said Donald Milton, professor of environmental health at the University of Maryland School of Public Health. “First of all, they are all respiratory droplets — some are larger and some are smaller, all the way down to microdroplets less than a single micron in diameter. It is true that larger droplets will behave as aerosols as the velocity of air increases, countering the pull of gravity so that they don’t fall out.”

A study from an 11th-floor South Korean call center found that 94 people were infected in a single outbreak, most of them clustered in one half of the office. The authors wrote that the outbreak highlights that the virus is “exceptionally contagious in crowded office settings such as a call center.”

That does not necessarily mean aerosolized particles caused the infections.

“I’m skeptical. To my knowledge we haven’t really seen evidence of transmission occurring that way,” said Angela Rasmussen, a virologist at the Columbia University Mailman School of Public Health. “The transmission studies from the call center and restaurant suggest that regular airborne droplet transmission in enclosed or poorly ventilated spaces is itself a transmission risk without needing to worry about small particle aerosols, too.”

To test whether aerosolized particles are spreading the virus, researchers will have to grow live virus from those samples and not simply recover viral RNA, which could just be residual genetic material not capable of seeding new infections.

A report, not yet peer-reviewed, from the University of Nebraska Medical Center found virus RNA on the surfaces of cellphones, toilets, bedside tables and exercise equipment used by 13 patients with confirmed cases of covid-19. Samples from the hallway outside patient rooms were also positive for viral RNA, suggesting that aerosols could be spreading the virus, but again the research did not conclude that the hallway samples were infectious.

Outside researchers remain skeptical of this fragmentary data.

Vincent Munster, a virologist at the Rocky Mountain Laboratories who led the research on aerosolized coronavirus, said Wednesday it is possible aerosolized particles are capable of spreading the virus — but this could be a very minor element in the pandemic.

“The question is, ‘What is currently driving the pandemic?’ If it’s mostly droplet transmission, and if we do have airborne or aerosol transmission, what is the relative role? These are all big questions that I still think need to be addressed,” Munster said.

Milton is planning studies with a contraption that measures the droplets expelled when infected people point their heads into a cone-like device called the Gesundheit Machine. Those studies could help definitively answer questions about whether fine aerosolized particles are routinely generated when people breathe or cough, and whether they are likely to spread disease.

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