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Droplet, aerosol, airborne: The confusion over how COVID-19 spreads

Click to play video: 'Droplet, aerosol, airborne: The confusion over how COVID-19 spreads' Droplet, aerosol, airborne: The confusion over how COVID-19 spreads
Conflicting messages from public health authorities and experts have fueled a great deal of confusion and debate over COVID-19, particularly regarding its transmission. Brittany Greenslade reports – May 6, 2021

Conflicting messages from public health authorities and experts have fuelled a great deal of confusion and debate over COVID-19, particularly regarding its transmission.

The terms “droplet,” “aerosol” and “airborne” are used when trying to explain how viruses are spread.

For more than a year, health officials and experts have said COVID-19, a respiratory virus, is spread via droplets.

Droplets are large mucus or saliva particles heavier than air that fall toward the ground and droplet transmission typically occurs when a droplet containing a virus comes in contact with another person’s eyes, nose or mouth.

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“By far, COVID-19 is spread through droplet transmission,” Manitoba’s chief provincial public health officer Dr. Brent Roussin said Monday.

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“So when someone speaks, coughs, sneezes, those droplets go into the air. They usually drop fairly readily and that’s why physical distancing is effective,” he said.

Droplets vary in size, from large droplets that fall to the ground rapidly (within seconds or minutes) near the infected person, to smaller droplets, sometimes called aerosols, which linger in the air under some circumstances.

Aerosol is a catch-all term for any solid or liquid particle so tiny and lightweight it can become suspended in air and float. Smoke and dust are common examples.

“When someone exhales cigarette smoke, it floats in the air for a while,” epidemiologist and University of Toronto professor David Fisman said.

“Closeness is still important. You can have more smoke near to the person’s mouth and farther away. So the density of that smoke diminishes as you get further from the person.”

There is now evidence to support that COVID-19 can be transmitted both ways.

The spectrum between droplets and airborne transmission

Typically, viral spread is looked at in two ways, droplet or airborne transmission, specifically in hospital settings. But the current debate lies in whether COVID-19 transmission outside sterilized settings isn’t as cut and dried.

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When you are in close contact with an infected person, COVID-19 can be transmitted by droplets.

“They usually drop fairly readily and that’s why physical distancing is effective,” Roussin said.

But with airborne diseases, physical distancing plays a much smaller role.

Read more: Experts find more evidence COVID-19 is airborne, that we need to rethink indoor spaces

“Airborne transmission is defined as the spread of an infectious agent caused by the dissemination of droplet nuclei (aerosols) that remain infectious when suspended in air over long distances and time,” according to the WHO.

Three diseases — measles, tuberculosis and chickenpox — have been designated to have airborne transmission.

However, there’s a growing debate, even among experts, on whether COVID-19 should be added to that list.

“The debate really at its core is about whether it’s closer to what we deem airborne and therefore the small aerosols are more predominant, or is it more along the other end of the spectrum, where larger droplets, for example — which is likely the case with the flu — are the predominant way that people will acquire the virus,” said Dr. Alon Vaisman, an infection control and infectious diseases expert at University Health Network.

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The World Health Organization said current evidence suggests the virus is primarily transmitted between people via these larger respiratory droplets and the surfaces they contaminate.

But on Friday, the WHO made an update to include aerosols in the spread of COVID-19.

“What the WHO identified in the most recent update is the possibility that in poorly ventilated settings or (where aerosols are found) during procedures, that transmission is more likely to be on the aerosol end of things,” Vaisman said.

“And so, again, the debate goes back to what about routine care,” he said. “What about just standard interactions with patients with COVID? What is the predominant method of transmission?”

The debate among experts

A highly polarized debate is happening among experts and health officials about which type of transmission is dominant in driving COVID-19 infections.

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“There’s been reports of transmission wider than that two-metre distance,” Roussin said. “We wouldn’t classify it necessarily as airborne.”

Roussin again pointed back to the three previously classified airborne viruses.

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“Just being in the same room for really any amount of time, somebody would be considered a contact,” he said. “That’s not the case with COVID. So there is a there is a gradient. By far, it’s spread by droplet.”

The Centers for Disease Control and Prevention agrees.

“COVID-19 is primarily transmitted from person-to-person through respiratory droplets,” the group said.

However, other experts say there is growing evidence to support that airborne transmission plays a much greater role and is the dominant driver of infection.

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For Fisman, there is no doubt that COVID-19 is not just spread by aerosols but is, in fact, airborne.

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A Lancet report from epidemiologists and experts across both Canada, U.S. and the U.K., including Fisman, said there is an overwhelming amount of evidence that the disease is spread through aerosols.

“That explains why we have these huge explosive outbreaks in places like church choirs and karaoke bars and spin studios, where people are vocalizing and creating a lot of aerosols,” he said.

Fisman said it’s much less likely we are seeing a spectrum of different modes of transmission, but instead points to a difference in the viral load a person has.

“If I have a higher viral load and if I’m bellowing or singing or shouting or laughing or if I cough, I’m going to extrude a much larger volume of these small particles,” he said. “(And) we have no way of knowing who’s got a high viral load and who doesn’t.”

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Ventilation is key

Whether someone is indoors or outdoors or in a well-ventilated space can affect whether or not the virus is transmitted.

“What we call buoyant particles can hang for up to about three hours. So that means if I’m an infectious case, I can cough in the room, maybe contaminate that poorly ventilated space, take off and infect someone who arrives there an hour later,” Fisman said.

What all experts do agree on is that proper ventilation is key to helping stop the spread of the virus.

“In areas less, very poorly ventilated … that’s where you’re more likely to see the stagnation of the air … more likely to see more transmission through these small aerosols,” Vaisman said.

“So if the air exchanges are poor, then these smaller droplets can persist in the air for longer.”

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That is why doctors say the focus needs to be on ventilation, keeping clean, fresh air circulating and engaging in more outdoor activities.

“We can open windows, we can use our air cleaners, we can focus on fixing up ventilation in poorly ventilated schools, and we can also think about things like respirator masks,” Fisman said.

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