Ongoing debate about the role of airborne transmission of SARS-CoV-2 is complicated by a host of variables that include quantity and infectivity of virus, room ventilation, distance, and personal protective equipment.

It is widely accepted that healthcare workers may be infected if they are not adequately protected during aerosol-generating procedures on patients with COVID-19. In the absence of aerosol-generating procedures, the public health consensus has been that large droplet particles emitted by a patient do not travel beyond six feet. However, studies have shown that smaller particles can travel farther distances, although there is some question whether they are infectious.

The World Health Organization (WHO) recently stated that it is “actively discussing and evaluating whether SARS-CoV-2 may also spread through aerosols in the absence of aerosol-generating procedures, particularly in indoor settings with poor ventilation. … These theories suggest that a number of respiratory droplets generate microscopic aerosols (< 5 µm) by evaporating, and normal breathing and talking results in exhaled aerosols.”1

The hypothesis is people could be infected by the airborne route if the virus is in sufficient quantity to cause infection within the recipient. “However, the proportion of exhaled droplet nuclei or of respiratory droplets that evaporate to generate aerosols, and the infectious dose of viable SARS-CoV-2 required to cause infection in another person are not known, but it has been studied for other respiratory viruses,” the WHO stated.

The WHO cited studies that showed aerosol production through coughing and talking, with the volume of the latter related to increased rates of particle emission.2,3 The transmission of SARS-CoV-2 by this type of aerosol route has not yet been demonstrated. More research is needed given the possible implications of such route of transmission, the WHO concluded.

Crowds and Clusters

“Just because there are no random control trials in the medical literature regarding airborne transmission of COVID-19, does not mean it does not happen. The absence of evidence is not the evidence of absence,” says Gabor Lantos, P.Eng, MBA, MD, president of Occupational Health Management Services in Toronto and a consultant at several hospitals and universities in the area.

COVID-19 infections have occurred primarily in clusters, in crowded conditions and poorly ventilated environments such as transit, nursing homes, and — as noted in recent research — a bus of people traveling to a Buddhist temple, he notes.

“The attack rate on an enclosed bus containing an individual with COVID-19 was 34.3% while no one on a second, similar bus was infected,” the authors reported. “Through further epidemiologic investigation, we determined that the possibility of transmission by direct contact on the exposed bus was low. The absence of a significantly increased risk in the part of the bus closer to the index case suggested that the risk was fairly evenly distributed throughout, and thus, airborne spread of the virus may, at least partially, explain the extraordinarily high attack rate observed.”4

The finding has important public health significance. Future efforts at prevention and control should consider the potential for airborne spread of COVID-19, particularly in enclosed spaces with re-circulating air conditioning systems, the authors concluded.

“COVID-19 is a coronavirus,” Lantos says. “The precautionary principle would infer that the biophysical and aerodynamic properties of COVID-19 be presumed to be the same as for other coronaviruses. SARS-1 [sudden acute respiratory syndrome] was known to have spread via the airborne route — ditto MERS [Middle East respiratory syndrome].”

After working on Toronto’s response to SARS in 2003, Lantos says healthcare workers are at risk, but the virus can be controlled with full precautions including N95 respirators.

“I have maintained ever since SARS ... a healthcare worker in the same room as a patient with a respiratory disease (e.g., COVID) should be wearing a fit-tested [respirator] that is at least an N95,” Lantos says. “Sick patients repeatedly cough and sneeze, and the quantity of airborne particles that remain suspended are sufficient to be infectious when the healthcare worker enters the room.”

The U.S. pandemic response has been undermined by a shortage of N95 respirators, which are recommended — if available — for the care of patients with known or suspected COVID-19. The WHO recommends airborne precautions with a respirator only for aerosol-generating procedures.

No Airborne Transmission

The authors of a recently published paper reported that early in the pandemic — when some of the first COVID-19 patients were treated without full precautions — healthcare workers were not infected even when they performed aerosol-generating procedures.5

In separate incidents, two patients with COVID-19 were admitted in February and March 2020 to community hospitals and the University of California, Davis (UC Davis) Medical Center. They were treated without contact or droplet precautions, and underwent several aerosol-generating procedures.

“A total of 421 healthcare workers were exposed, and the results of the case contact investigations identified eight secondary infections in healthcare workers,” the authors reported. “In all eight cases, the staff had close contact with the index patients without sufficient personal protective equipment.”

Somewhat surprisingly, none of the infections were traced back to exposures during aerosol-generating procedures. “These observations suggest that, at least in a healthcare setting, a majority of SARS-CoV-2 transmission is likely to take place during close contact with infected patients through respiratory droplets, rather than by long-distance airborne transmission,” the authors concluded.


Hospital Employee Health sought further insight on the study from lead author Bennett Penn, MD, PhD, an infectious disease physician at UC Davis Medical Center.

HEH: These exposures occurred at the very beginning of the outbreak in the United States?

Penn: Yes, it was at a time when COVID was not suspected in these patients. [One] patient was actually the first patient diagnosed with community-acquired COVID in the United States. It took about a week for the CDC to be willing to test her because she had no travel history. She was off precautions for several days and was not on precautions at all at the initial medical center. Then, she was on limited precautions at the second medical center when she transferred to our care. COVID was not suspected for about the first seven days of her illness and before the testing was done. The second patient was several weeks later and came in with a very complex medical history and other issues that clouded the clinical picture. COVID was not suspected on his initial hospitalization either for about 14 days into his [care] when the testing was done. A number of healthcare workers were exposed during those windows prior to the COVID testing.

HEH: Which aerosol-generating procedures were performed? How did you determine no transmission occurred by this route?

Penn: Both patients had high-flow nasal cannula up to 40 liters per minute, which generates aerosols. Both patients underwent intubation, and both went underwent bronchoscopy. There was extensive contact tracing done of all healthcare workers who had contact with the patients, as well as surveillance in healthcare workers on the wards these patients had been on. All symptomatic healthcare workers — whether they had direct contact or not with the patient — underwent SARS-CoV-2 testing. Of the roughly 200 COVID tests that were run on the staff on those wards, the only positive tests were of healthcare workers who had direct bedside contact with these patients. None of the other nurses, staff, or patients on the ward developed COVID.

HEH: What are the implications of your study for the ongoing debate about the role of airborne transmission?

Penn: To state the obvious, it’s controversial. I would say the data are inconclusive. Our study indicates in these couple of patients with eight secondary infections there wasn’t airborne transmission. At this point, there have been a number of articles published on hospital outbreaks that come to that same conclusion, where there has not been airborne transmission. On the other hand, there have been a couple of cases that have been widely publicized; for example, one at a choir practice in the Seattle area where there something like 40 or 50 secondary infections from a single index patient.6 That’s one of the outbreaks that get cited for the possibility of airborne infection. I would say that is possible, but it is hard to be certain. People were in a small, enclosed room, mingling with one another — probably well within the six-foot radius — and sharing snacks. It’s hard to know whether the transmission events were airborne. The large number of secondary infections raises the possibility that the instance might have been airborne infection.

A handful of cases that have been studied, largely in healthcare settings, have shown absence of airborne transmission, and a couple of instances of potential airborne transmission. My feeling is that [airborne transmission] is not going to be the norm, but it may be possible in a select number of instances. For example, in patients with extremely high viral loads, or in situations like that choir practice where there was active singing for a long period in a confined space, generating a huge number of particles. My overall interpretation of the data is that airborne transmission is probably not likely to happen in the majority of cases. But in a small fraction of cases, there probably can be some airborne transmission.


  1. World Health Organization. Transmission of SARS-CoV-2: Implications for infection prevention precautions. Scientific Brief. July 9, 2020.
  2. Somsen GA, van Rijn C, Kooij S, et al. Small droplet aerosols in poorly ventilated spaces and SARS-CoV-2 transmission. Lancet Respir Med 2020;8:658-659.
  3. Asadi S, Wexler AS, Cappa CD, et al. Aerosol emission and superemission during human speech increase with voice loudness. Sci Rep 2019;9:2348.
  4. Shen Y, Li C, Dong H, et al. Airborne transmission of COVID-19: Epidemiologic evidence from an outbreak investigation (May 11, 2020).
  5. Bays DJ, Nguyen MH, Cohen SH, et al. Investigation of nosocomial SARS-CoV-2 transmission from two patients to health care workers identifies close contact but not airborne transmission events. Infect Control Hosp Epidemiol 2020 Jul 3;1-22.
  6. Hamner L, Dubbel P, Capron I, et al. High SARS-CoV-2 attack rate following exposure at a choir practice — Skagit County, Washington, March 2020. MMWR Morb Mortal Wkly Rep 2020;69:606-610.