TORONTO -- More than half a year after doctors in Wuhan reported patients with mysterious cases of pneumonia, there is still a great deal that remains unknown about SARS-CoV-2, even as scientists, researchers and doctors around the world scramble to learn as much as they can about the virus that causes COVID-19.

Whether someone can contract the virus through the air is one question that keeps cropping up, especially among those living in multi-unit dwellings concerned about the possibility of getting sick through shared air ducts and ventilation systems.

“What we’re seeing right now is this virus is probably not spread through the air as a main route of transmission,” said Dr. Yves Longtin, who chairs the infection prevention and control unit at Jewish General Hospital in Montreal.

If airborne transmission does exist, it is likely a secondary or tertiary method, he said in an interview with, and very minor compared to the main mode of transmission: contact with droplets from an infected individual.

Still, there is enough uncertainty that some scientists are not ruling out the possibility entirely and suggest taking simple precautionary measures.

“I don't think it's too serious an issue, but I think it brings up the importance of trying to be as proactive as possible,” infectious disease specialist at Toronto Western Hospital, Dr. Abdu Sharkawy, told CTV News’ Chief Anchor and Senior Editor Lisa LaFlamme in April.

“I would say that if the ventilation system is one in a high-rise apartment, for example, that it is unlikely that someone on the top floor will come in contact with the same virus that went through the ventilation system on the bottom floor. However, if it's a smaller shared dwelling, there is the possibility it could run through the ventilation system.”

Despite how quickly information and knowledge on the virus can evolve in this pandemic, Sharkawy, an assistant professor of medicine with the University Health Network, maintained that view in May.

“It’s always a good idea to open windows to optimize air flow and reduce the risk of a virus being confined to an indoor space,” he added in an email to

There is no concrete evidence so far that someone can be infected through contaminated indoor air travelling over a great distance or through indoor vents, but there are at least indications that the virus can be found in the air somehow.

“To my knowledge, there is no evidence supporting the transmission of COVID-19 through ventilation systems … although researchers are still exploring the possibility,” said Bin Zhao, an expert on ventilation and a professor in the Department of Building Science, School of Architecture at Tsinghua University in Beijing, over email.

“There is still no direct evidence for the airborne transmission of the virus, however, some reported cases of infection occurred in poor ventilated spaces, such as restaurants.”

In one well-known restaurant example from China, members from three different families sitting at neighboring tables likely contracted COVID-19 from one individual who began showing symptoms later that day. A study on the outbreak concluded that the most likely cause was droplet transmission, with the strong air blowing from A/C sending the droplets further. The A/C itself tested negative for the virus, and staff and diners who were sitting away from that air current were not infected.


Part of the difficulty in getting a definitive answer on the question lies in distinguishing how scientists define “airborne” --and not all of them even agree on that -- and what the public considers “airborne.”

Most scientists and public health officials define “airborne” under the technical “aerosol” definition -- particles small enough to remain floating in the air over longer periods and distances. But most people have a simpler concern: can the virus be carried through the air and still infect me?

We know for certain the disease spreads through small droplets expelled from the nose or mouth during talking, sneezing or coughing. These droplets are too heavy to linger in the air for long before they are pulled down by gravity and land on a surface. You can be infected if you touch something contaminated with the virus and then touch your mouth, nose, or eyes.

The World Health Organization updated its position on the virus's modes of transmission in July to include the possibility of aerosols, saying that outside of medical facilities, certain types of outbreak reports have suggested that possibility exists in combination with droplet transmission. It cited examples including choir practice, restaurants and fitness classes, where "short-range aerosol transmission, particularly in specific indoor locations, such as crowded and inadequately ventilated spaces over a prolonged period of time with infected persons cannot be ruled out."

Previously, the WHO said in late March only that airborne transmission “may be possible in specific circumstances and settings in which procedures that generate aerosols are performed,” such as when health-care workers intubate infected patients, for example. But it had also explicitly stated that “COVID-19 is NOT airborne” in a tweet around the same time.

All of this can create confusion for the public looking for answers.

“What I mean by airborne would be that the virus would easily transmit over extended distances through air currents,” explained Longtin, who is also an associate professor of medicine at McGill University. This means a patient four rooms down a hospital corridor could be infected with an airborne virus simply by breathing the same air, he said.

“We have to remember that airborne transmission remains what I would call a microbiologic aberration. Out of the hundreds and hundreds of microbes that can infect humans, there’s only three that are truly airborne - measles, varicella, and … tuberculosis.”

Even the virus that causes the common cold is not truly “airborne” under that definition, he added.

Airborne pathogens are incredibly contagious, Longtin said, and very short exposure -- such as passing through a room of 30 people with a single infected case of the measles -- can lead quickly to disease, but this is not what health-care workers have seen with COVID-19.

Specialists around the world are actively investigating whether SARS-CoV-2 can be found in the air. And if so, how long can it stay in the air? How infectious is it? How far can it travel?

Researchers in bio-aerosols have been sampling vast quantities of air to answer these questions and more. So far, they have detected pieces of the virus’s genome in the air.

A team with the University of Alberta's Mechanical Engineering Department received a $444,000 Canadian Institutes of Health Research government grant to study how the virus might travel as an airborne particle through a building's heating, ventilation and air-conditioning (HVAC) systems, and how to improve these systems to mitigate the potential spread of the virus.

Some studies have raised the possibility that the virus may linger in the air longer and travel over longer distances than previously thought. In one case, the virus could also be found in air pollution particles.

But it is unclear from these studies whether the virus is viable, Longtin noted, and if so, whether there is enough to infect an individual breathing that air.

The air current generated from simply walking through a room can be enough to raise dust from the ground, so it is possible that what is being sucked from the air is actually dead virus. Scientists have tried viral cultures to see if the virus is alive without success, Longtin noted.

Many of these studies also come with an important caveat: they are “preprints,” which means they have yet to be peer-reviewed and published in a journal -- a process that can take months -- and some have been criticized for their flawed methodologies.

The WHO noted that the results of one study examining the potential airborne qualities of the virus were generated under controlled laboratory conditions using high powered machines that do not reflect normal human cough conditions.

Because of the urgency of the pandemic, the wider research community, racing to understand as much as itcan about the virus, has welcomed the flood of preprints on COVID-19. But the public dialogue surrounding these papers also means that poorly done studies can be quickly picked up and disseminated by the media and public.

The reality is that definitive answers may take years -- as they did for SARS -- prompting some experts to recommend taking more proactive measures in the meantime.

Earlier in April, researchers at the Queensland University of Technology and the Chinese Academy of Sciences published a paper in Environment International urging health authorities around the world to recognize airborne transmissions as a likely mode of infection and to implement measures to mitigate its spread indoors. It pointed to research from the 2003 SARS outbreak, outbreaks on cruise ships, and the general science around viral infection spread as evidence the novel coronavirus is likely spreading through the air.

“When the epidemic is over and retrospective data demonstrates the importance of airborne transmission it will be too late,” the authors wrote. Lidia Morawska, one of the authors and an expert on air quality, human exposure, and aerosol science, told Nature: “In the mind of scientists working on this, there’s absolutely no doubt that the virus spreads in the air...This is a no-brainer.”

Other anecdotal instances -- such as the spread of COVID-19 among members of a choir group in Washington state that attended a rehearsal in early March -- already has some health officials concluding that the virus, while not necessarily airborne under the strict scientific definition, can be transmitted through air. In the choir’s case, no singers showed any symptoms at the time and the group made an effort to avoid touching during practice, but by the end of the month, some threequarters of the group who attended practice were confirmed to have COVID-19 or had symptoms, several were hospitalized, and two were dead.

And part of the protracted debate around masks was also about whether the virus was in the air or not.


During the 2003 SARS outbreak, a cluster of cases was identified in an apartment complex in Hong Kong that was traced back to faeces. More than 300 residents were infected. Through an interconnected pipe system involving the bathroom floor drain and the sewage pipe, air containing a high concentration of SARS aerosols were drawn into bathrooms by the exhaust fan, which in turn also drew the contaminated air outside.

“Computer modelling on fluid flow has suggested this plume of gas from the exhaust fan rose up and affected neighboring buildings,” Wallace Leung, a filtration expert and professor at Hong Kong Polytechnic University, explained to via email.

Leung said the wind carried the plume of virus to the upper floors of other surrounding buildings, but spared the lower floors as well as buildings that were outside the wind pattern.

It took investigators several years to reach their findings.

There was fear that history could be repeating itself again when another old housing complex in Hong Kong confirmed two COVID-19 infections in February, but an investigation found that this situation was unique, Leung said. One of the infected residents had made alterations to their bathroom that left a toilet venting pipe disconnected and venting gas into the room, he explained. That pipe connected to a common vent, and investigators suspected that the resident may have been exposed to an infected individual 10 floors above as a result.

“COVID-19 ... can be aerosolized as demonstrated in the example above,” despite disagreement from experts on the airborne mechanism of the virus, Leung said.


There are several ways that air might travel through a multi-unit dwelling, according to Jeffrey Siegel, an expert on indoor air quality and a professor with the Department of Civil and Mechanical Engineering at the University of Toronto.

In a typical condominium or apartment, there are several ways air might travel into a unit:

  • Corridor ventilation - the fresh air that flows into the hall and flows into a residence through the undercuts of the front door
  • When windows are opened, air from one unit can flow into another
  • Toilet, plumbing, exhausts and vents
  • If you can smell your neighbour’s cooking, that means air is flowing from their apartment into yours

While you should be aware of the airflow in your home, said Siegel, becoming infected through the ventilation is unlikely, in part because of the way buildings and homes are designed. The connection from one unit or room to another is rarely a straight one.

“Even if there is transfer of airflow, it doesn’t mean the virus will transfer too. It depends on a lot of details around the particle size … as well as the kind of path it takes,” Siegel told


Siegel and Zhao both stress there is no need to panic over home or building ventilation. Screening asymptomatic infectors and making sure they are self-isolating are more important preventative measures, said Zhao, adding that wearing a mask to protect others as well as avoiding confined spaces with poor or no ventilation were also key. Siegel added that above all else, proper hand washing, wearing masks, and physical distancing, both indoors and outdoors, are the most important measures to staying healthy.

But there are still things residents can do to help mitigate the possibility of becoming infected, Siegel said.

If there is someone sick or suspected to be sick in your household:

  • Try to keep them in their own space and isolate that space.
  • That part of the home should be as isolated from the rest of the home as possible. That means keeping the door to that room closed, and weather permitting, making sure there is a lot of ventilation in that room by opening the windows.
  • If possible, avoid sharing a bathroom.
  • If possible, put a fan by an open window and have it blowing out; ideally that keeps the room under slightly negative pressure with respect to the rest of the home
  • A portable HEPA filter air cleaner (not an ionizer or other non-filtration air cleaner) could also be considered to reduce the particles and droplets the person is producing in the isolated space.

How effective these steps are, however, really depends on the details of the building or house, the weather, and other factors, Siegel said, “but it doesn’t hurt.”

“The bottom line is, it’s almost impossible to really isolate one space in a home from another space.”

What about turning on the air-conditioning? If the A/C serves multiple units (for example, a single family home that’s been converted into multiple units), then Siegel suggests being more cautious. If it only serves one unit, then he would want to know if more air is being sucked into the unit from the outside or the corridor. If yes, then consider running the A/C only when necessary. But he cautioned residents not to create bigger health risks by not running the A/C just to mitigate the very small risk of contracting the coronavirus.

Zhao also recommended that building HVAC systems be run in "all fresh air" mode, with the ventilation rate increased to as high as the system can support.

What about someone who is healthy, but vulnerable or concerned about becoming infected by a sick family member? The advice is almost the same as that given for isolating someone who is sick in a home, Siegel said. The only difference is the fan by the window should be blowing into the space instead, in order to create a little bit of positive pressure in the room. Negative pressure allows air to flow into the room, but not escape from the room. Positive pressure does the opposite, allowing air to flow out of the room.

These steps can also apply to those living in a multi-family building, he added, except he would be much more cautious about opening the window.

“To my knowledge, there has been no reported transmission from out one window or vent into another, but everything I know about aerosol science says, you know, there’s a possibility there, so why risk it?” Siegel said.

“But I don’t want anyone to think that fans, filters, and ventilation are more important than hand washing, surface cleaning, all those other things that public health authorities are telling us to do.”

Edited by producer Sonja Puzic