The Hindu Explains | Is airborne transmission of COVID-19 a risk?

In a letter, what have scientists told the World Health Organization ? How does the disease pass from one person to another?

Updated - July 12, 2020 02:12 pm IST

Published - July 12, 2020 12:02 am IST

The story so far: On Monday, July 6, 239 scientists from 32 countries put their signatures on an open letter that said COVID-19 is also transmitted via aerosols. Titled It is Time to Address Airborne Transmission of COVID-19 , and addressed to the World Health Organization (WHO), they said there was enough evidence to show that viruses are released during exhalation, talking, and coughing as micro droplets small enough to remain aloft and pose a risk of exposure at distances beyond 1-2 metres from someone who is infected (over the 3 feet–6 feet recommended for physical distancing between people to avoid transmission).

What do the scientists believe?

The scientists, led by Lidia Morawska of the International Laboratory for Air Quality and Health, WHO Collaborating Centre, Queensland University of Technology, Brisbane, Australia, appealed to “the medical community and the relevant national and international bodies to recognise the potential for airborne spread of COVID-19. There is significant potential for inhalation exposure to viruses in microscopic respiratory droplets (microdroplets) at short to medium distances (up to several metres, or room scale), and we are advocating for the use of preventive measures to mitigate this route of airborne transmission.”

Also read | Indoor airborne spread of coronavirus possible, says WHO

The letter came at a time when most public health organisations, including WHO, do not “recognise airborne transmission except for aerosol-generating procedures performed in healthcare settings.”

Responding to the letter, in Oxford Academic — Clinical Infectious Diseases , Dr. Benedetta Allegranzi, WHO’s technical lead for infection prevention and control, was cited in media reports on Tuesday as saying, “ there was evidence emerging of airborne transmission of the coronavirus , but that it was not definitive.”

She went on to say that the “… possibility of airborne transmission in public settings — especially in very specific conditions, crowded, closed, poorly ventilated settings that have been described, cannot be ruled out.” She added, “However, the evidence needs to be gathered and interpreted, and we continue to support this.”

What are aerosols? How different are they from respiratory droplets?

In common understanding, aerosols are minute particles that are expelled under pressure, as in the case of fine mist from a jar of perfume, or a can of roach repellent. However, aerosol is a term used to broadly refer to particles suspended in the air; they could include fine dust, mist, or smoke. In the context of transmission of viruses, as in this case, aerosols are read as micro droplets, much smaller (5 microns or less) than respiratory droplets, and take a longer time to drop to the floor. They will be expelled by people breathing, laughing or singing, as against respiratory droplets that are expelled with forceful acts such as sneezing or coughing. As per the open letter, “at typical indoor air velocities [5], a 5 micron droplet will travel tens of metres, much greater than the scale of a typical room, while settling from a height of 1.5 m to the floor.”

Also read | India ‘watching’ WHO alert on airborne spread of coronavirus

As they remain suspended in the air for longer, an individual who is COVID-19 positive is likely to infect people standing even at a distance of 1-2 m in a small, poorly ventilated room. “This poses the risk that people sharing such environments can potentially inhale these viruses, resulting in infection and disease,” the signatories endorsed.

That respiratory droplets transmit COVID-19 infection has dominated the discourse from nearly the beginning of the epidemic, and has guided the path that interventions have taken thus far, including wearing masks, keeping distance, and hand washing routines.

Is there evidence to prove that aerosols transmit SARS-CoV-2?

In its response, WHO did say that there was need to watch the area for possible exposure to aerosols causing the infection, but insisted that the evidence was not yet entirely compelling, except in health-care settings where aerosol emission is common.

The bar of proof has been set high for aerosol transmission; even the scientific reluctance to accept this theory has been couched in belief that it would trigger widespread panic in the community.

A Reuters report cited Jose-Luis Jimenez, a chemist at the University of Colorado Boulder who signed the paper, trying to explain the historical reluctance to accept the notion of aerosol transmission.“If people hear airborne, healthcare workers will refuse to go to the hospital,” he said. Or people will buy up all the highly protective N95 respirator masks, “and there will be none left for developing countries”.

Does the open letter to the World Health Organization present enough proof?

It says: “Airborne transmission appears to be the only plausible explanation for several superspreading events investigated which occurred under such conditions… and others where recommended precautions related to direct droplet transmissions were followed.”

Further, the letter says, “It is understood that there is not as yet universal acceptance of airborne transmission of SARS-CoV-2; but in our collective assessment there is more than enough supporting evidence so that the precautionary principle should apply. In order to control the pandemic, pending the availability of a vaccine, all routes of transmission must be interrupted.”

The signatories agreed that “the evidence is admittedly incomplete for all the steps in COVID-19 micro droplet transmission,” but pointed out that it is “similarly incomplete for the large droplet and fomite modes of transmission.” Further they advanced the point of view that “airborne transmission mechanism operates in parallel with the large droplet and fomite routes, that are now the basis of guidance”.

They relied heavily on several retrospective studies conducted after the SARS-CoV-1 epidemic, demonstrating that airborne transmission was the most likely mechanism explaining the spatial pattern of infections. “Retrospective analysis has shown the same for SARS-CoV-2,” the letter said. “In particular, a study in their review of records from a Chinese restaurant, observed no evidence of direct or indirect contact between the three parties. In their review of video records from the restaurant, they observed no evidence of direct or indirect contact between the three parties.”

Earlier, a letter in The New England Journal of Medicine, titled “Aerosol and Surface Stability of SARS-CoV-2 as Compared with SARS-CoV-1” (, suggested that SARS-CoV-2 remained viable in aerosols for up to three hours, but the generation was done via a high-powered machine that is unlikely to be replicated in real-life situations.

The signatories of the letter said many studies conducted on the spread of other viruses, including respiratory syncytial virus (RSV), Middle East Respiratory Syndrome coronavirus, and influenza, show that viable airborne viruses can be exhaled and/or detected in the indoor environment of infected patients. “This poses the risk that people sharing such environments can potentially inhale these viruses, resulting in infection and disease. There is every reason to expect that SARS-CoV-2 behaves similarly, and that transmission via airborne micro droplets is an important pathway.”

Says Prof. T. Jacob John, retired Professor of Clinical Virology, Christian Medical College, Vellore, “The airborne aerosol transmission theory is rather hyped up, in my opinion.” And yet, he says, in closed spaces without ventilation where people tend to crowd around, one must take precautions. Wearing the mask at all times, even indoors, if others are present in the circumstances as described in the open letter, would be recommended, he adds.

What is the future?

As WHO waits for more robust evidence on the principle of aerosol transmission, the authors are pushing only to address every possible pathway to slow down the transmission of COVID-19. Providing sufficient and effective ventilation as far as possible in public buildings, schools and hospitals, avoiding overcrowding in public buildings and transportation systems are recommended, besides, supplementing general ventilation with airborne infection controls such as local exhaust, high efficiency air filtration, and germicidal ultraviolet lights.

The authors, recommending a zero-tolerance approach to COVID-19 transmission, add: “The measures we propose offer more benefits than potential downsides, even if they can only be partially implemented.”

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