Air Apparent: How COVID-19 Is Being Transmitted

The evidence keeps piling up that airborne spread is driving this pandemic, which has ramifications for how we manage the ongoing risk.


When it comes to preventing COVID-19 transmission, we need to first understand how it spreads. And according to a growing body of evidence, the primary route could be through long-range aerosols — also known as airborne spread.

Ten pieces of evidence for this are outlined in an article published in The Lancet. Co-authors David Fisman, professor of public health at the University of Toronto, and Kimberly Prather, professor of chemistry and oceanography at the University of California, San Diego elaborated on the evidence at an event hosted by the O’Brien Institute for Public Health.

At the start of the pandemic, most scientists believed that COVID-19 is primarily spread through droplets. Droplets are also spread through the air, but they are much larger than airborne aerosols and so gravity pulls them to the ground within about six feet of an infected person. That mode of transmission is why physical distancing works.

But gravity is present everywhere on Earth, so the way that droplets behave doesn’t explain why gathering indoors or other enclosed spaces is a bigger transmission risk than gathering outdoors. It doesn’t explain how choir practices or spin classes can become the sites of superspreading events, even when physical distancing rules are followed at all times.

The extent of pre-symptomatic spread is also left unexplained, because infected people are not yet coughing and producing high volumes of droplets. The spread must be caused by particles generated when we are feeling healthy and going about our everyday activities.

Prather describes droplets as metaphorical cannonballs that are large and heavy and land close to their origin, but aerosols are more similar to the size of particles of smoke and they can float throughout a room. They can be inhaled at long range, and they can accumulate if there isn’t enough ventilation.

There is still resistance to a general consensus that airborne transmission is the main driver of infection, in part because it’s very hard to prove that infectious particles exist in the air. At longer distances from an infected person, aerosols are present at lower concentration, and viruses are easy to accidentally destroy when researchers try to collect samples. If that happens, then it becomes impossible to culture the virus to prove it is still infectious in the air.

That being said, the evidence still continues to grow. Researchers at the University of Florida published their gentle method of air sampling in the International Journal of Infectious Diseases.

They took samples from the hospital rooms of COVID-19 patients, beyond six feet of distance from the patient and in absence of aerosol-generated medical procedures. They were able to identify SARS-CoV-2 virus — the virus that causes COVID-19 — using PCR, the same method used in diagnostics. They went on to prove that they could culture the live virus from those samples, meaning they were still infectious.

The good news is that knowing this means we can change our policies, behaviour, and other precautions to reduce our risk.

Being outdoors dilutes long-range aerosols in effectively infinite space, and we can bring our indoor spaces closer to these conditions by improving ventilation and air filtration.

In addition to improving indoor air circulation, improving the fit of the masks we wear or upgrading to more effective masks like N95s that can protect against aerosols would also help.

Strict policies limiting the capacity in indoor spaces also lowers the accumulation of aerosols, and therefore the exposure risk.

Understanding COVID-19 transmission means we can use the right strategies and tools to block it, which is why the distinction between airborne and droplet transmission matters. Taking steps to boost our protective measures and infrastructure now will give us an edge not only on COVID-19, but on all respiratory illnesses where airborne spread is possible.


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Karyn Ho is a science animator and engineer who thrives at the interface between science, engineering, medicine, and art. She earned her MScBMC (biomedical communications) and PhD (chemical engineering and biomedical engineering) at the University of Toronto. Karyn is passionate about using cutting edge discoveries to create dynamic stories as a way of supporting innovation, collaboration, education, and informed decision making. By translating knowledge into narratives, her vision is to captivate people, spark their curiosity, and motivate them to share what they learned.