How is the coronavirus really transmitted in the air?

21 September 2021

Part 1: Large vs small droplets

Confusing statements about the modes of transmission of the SARS-CoV-2 virus have been disseminated in news outlets, and in press releases since the beginning of the pandemic. Early in the crisis, the world health organization (WHO) announced that large droplets (>5 microns) and contact were the two main modes of transmission, dismissing the possibility of airborne transmission outside of very special circumstances (for ex. when fine droplets and aerosols are the result of medical procedures). In a press release on 29 March 2020 the WHO wrote 1:

According to current evidence, COVID-19 virus is primarily transmitted between people through respiratory droplets and contact routes.

However, it has long been known that viruses (e.g. influenza) can also be transmitted from one person to another through the air in the form of fine airborne particles and small droplets, also called aerosols. The scientific literature on aerosol contamination is rather rich, and the physics that explains how droplets (including the ones containing viruses, e.g. a coronavirus) can propagate through the air has been studied for several decades. A google scholar search returns over 14,400 articles on the subject (including more than 4,000 papers published from 1990 to 2010).

It was established well before 2020 that aerosols are one of the main modes of transmission of the influenza A virus.2 So why not take a cautious step and assume that the SARS-CoV-2 virus could also be transmitted in the form of aerosol (of course while waiting for studies either corroborating or dismissing this assumption)? In fact, many scientists have been seriously discussing the possibility of COVID transmission and infection in the form of aerosols (not just droplets) since the beginning of the pandemic. For example, a news article written in the journal Nature in April 2020 states that:

“The World Health Organization says the evidence is not compelling, but scientists warn that gathering sufficient data could take years and cost lives… In the mind of scientists working on this, there’s absolutely no doubt that the virus spreads in the air”. 3

In another article published in June 2020, the authors wrote that:

“The virus is transmitted in the air NOT only in large drops (e.g. cough), but ALSO in small drops (aerosol).” 4

Only recently (30 April 2021), the WHO updated their statement to include the possibility of virus transmission through aerosols:

Current evidence suggests that the virus spreads mainly between people who are in close contact with each other, typically within 1 metre (short-range). A person can be infected when aerosols or droplets containing the virus are inhaled or come directly into contact with the eyes, nose, or mouth.1

However, these statements are very confusing to most people, and it is difficult to understand the insistence of the WHO and other official organizations to only acknowledge that the virus mainly spreads and infects people at short distances (1m). Clearly scientific evidence shows otherwise.

In a coming series of posts, I will discuss in more detail what is known about coronavirus transmission, the spreading of small and large droplets in the air, and what it means in terms of protection (masks, etc …). But first, an important question to ask is how airborne transmission occurs, and how far away small droplets are transmitted through the air. 

Be sure to stay tuned for more important and informational content!

References

1) WHO press releases, check the WHO website for updated information:

https://www.who.int/emergencies/diseases/novel-coronavirus-2019/media-resources

2) Cowling BJ, Ip DKM, Fang VJ, et al. Aerosol transmission is an important mode of influenza A virus spread. Nat Commun. 2013; 4: 1935. 

3) News article, Nature 580, 175, 2 April 2020.

4) Morawska L. & Cao J., Environment International, Volume 139, 105730,

June 2020.

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