Here’s why aerosol transmission of COVID matters to us all

by Raywat Deonandan

This article was published in The Ottawa Citizen on Oct 19, 2021.


Eighteen months into the pandemic, we continue to learn about COVID-19 transmission. While there is still some disagreement, it’s becoming clear that so-called aerosol transmission is an important and overlooked infectious route.

There are at least five ways that viruses can infect people: via (1) vectors, like mosquitoes that bite an infected person then transfer infection to a new person via another bite; (2) the oral-fecal route, which is exactly what it sounds like; (3) direct contact, such as sexual intimacy; (4) so-called “fomite” transmission, in which an infected person touches a doorknob or other object, which is subsequently touched by another person who then acquires that infection; and (5) via large droplets, which are small bits of exhaled fluid, typically larger than 5 microns, which gravity drags harmlessly to the ground within 1-2 metres.

COVID-19 is not known to be vectorborne. And while traces of the SARS-CoV2 virus have been found in the feces of infected people, oral-fecal transmission has not been observed. The same is true for sexual contact: the virus has been detected in the semen of infected men but has never been observed to infect others through sexual fluids.

Early on, there was panic around the prospect of fomite transmission, with many people disinfecting their groceries and mail. While there is some evidence for fomite transmission, cases are vanishingly rare and are definitely not the means by which COVID has stifled the world.

Instead, public health officials contend that large droplets are the primary infection route. Hence the reliance on social distancing, plexiglass barriers, and simple masks, which serve as “windbreaks” that shorten the distance that droplets can travel.

However, since at least summer of 2020, it was becoming clear to many scientists that COVID-19 has a significant aerosol transmission component. Aerosols are small droplets that can be 1-5 microns or even smaller, and that can linger in the air for hours. People naturally produce respiratory aerosols by several means, including breathing, talking, and coughing. Some medical procedures, like CPR and tracheotomy, can artificially generate them, as well.

Whereas large droplets are responsible for short range transmission and are typically what we see in diseases like influenza, aerosols are responsible for the long range transmission that we see in Measles and Chickenpox, and now COVID.

One way to visualize aerosol transmission is to think about cigarette smoke, which is made up of aerosol particles 0.1-1 microns in diameter. Imagine a COVID-infected person exhaling a cloud of such “smoke”. See how it floats out from the creases of a poorly-fitted mask and over the tops of plexiglass barriers. See how it’s thickest nearer to its source, but still present many metres further away. Notice how it lingers for hours in an enclosed space, even after its source has left the premises.

Aerosol transmission explains some of the deeper mysteries of this pandemic. Transmission between people who have never been in the same room, as in occupants of quarantine hotels or cruise ships, can now be explained by their shared ventilation systems.

With the reality of aerosol transmission comes the need to invest in different mitigation tools. Droplet dogma holds that poorly fitted surgical masks and social distancing are sufficient to slow most population spread. But aerosols are best controlled by ventilation and high quality fitted masks, such as N95 respirators.

One could argue that controlling droplet transmission is largely the burden of the people, who are responsible for wearing masks and keeping their distance. But controlling aerosols is more the responsibility of government and employers: providing N95 masks to forward-facing staff, and requiring good ventilation in schools and workplaces.

To safely navigate out of this pandemic, we must acknowledge the significance of aerosol transmission and take the appropriate mitigation steps. This means a focus on well fitted, high quality masks and proper indoor ventilation.

Raywat Deonandan is an Epidemiologist and Associate Professor with the Faculty of Health Sciences, University of Ottawa.