Prime Minister Justin Trudeau's recent comment about how masks may protect people from you "breathing or speaking moistly" on them may conjure up a cringeworthy mental image. But some researchers think there's reason to believe it's a possible explanation for how COVID-19 spreads so quickly.

"There's a lot of evidence now that COVID-19 is actually transmitted from people who are asymptomatic — that don't have any symptoms and so therefore aren't sneezing or coughing," said William Ristenpart, who has studied the properties of airborne droplets.

"The question then is, 'Well if they're not sneezing or coughing, how are they transmitting it?''" he said in conversation with Quirks & Quarks' Bob McDonald.

Public health authorities have recommending that people stay about two metres away from each other as a physical distancing measure to reduce transmission of COVID-19. This is based on the idea that the SARS-CoV-2 virus spreads in large respiratory droplets that fall to the ground quickly, or by "contact routes" when you touch one of these droplets on a surface, and then touch your face.

But Ristenpart, a professor of chemical engineering at the University of California, Davis, and other scientists who study aerosol pathogens suggest there may be a third mode of transmission: tiny "airborne" microdroplets.

These ones that are emitted during speech and breathing are so small that very small fluctuations in the air currents help keep them up in the air for hours. - William Ristenpart, University of California, Davis

"A huge concern is the idea that actually whenever you speak, you're emitting lots of much smaller respiratory particles that you can't see with the naked eye, but that somebody nearby can inhale and become infected [from]," said Ristenpart.

When you cough or sneeze, you emit a cloud of droplets of different sizes — the larger ones you can see with the naked eye fall to the ground, but Ristenpart says the same isn't true for the smaller microdroplets.

"These ones that are emitted during speech and breathing are so small that very small fluctuations in the air currents help keep them up in the air for hours," he said.

Ristenpart teamed up with colleagues to write an editorial in the journal Aerosol Science and Technology, warning about potential airborne transmission of the COVID-19 coronavirus, and calling on those in the aerosol science community to study it.

Sneezing can release a turbulent gas cloud spewing droplets as far as seven to eight metres.

Suspicions based on prior microdroplet research

Research has shown that every time we open our mouths to exhale while talking or simply breathing, we release tiny respiratory microdroplets that are large enough to carry viral pathogens.

Ristenpart and his colleagues studied how many microdroplet particles people emitted as they spoke.

Their work found that the louder a person speaks, the more of these tiny particles they expel and that some individuals are "speech superemitters" who give off up to 10 times as many particles as others.

"On average, when you're just talking kind of a normal conversational voice, we find that it's about 10 particles per second are emitted. And so if you think about that over a 10-minute conversation, that means there's about 6,000 respiratory particles floating around in the air between you and your conversational partner," said Ristenpart.

The louder a person speaks, the more respiratory microdroplets they emit, Ristenpart and his colleagues found. (Hector Vivas/Getty Images)

A 2018 study in the journal PNAS suggests that approximately 39 per cent of the microdroplets in exhaled breath from people infected with influenza contain infectious virus particles that could infect another person should they be close enough to inhale it.

Ristenpart said a good way to think about how far the microdroplets can travel is to think about cooking in a domestic kitchen. The closer one is to the kitchen, the stronger the smell of the cooking food will be.

"Same thing for these respiratory particles — the further away you are, the lower their concentration and therefore the less likely you are to inhale one and potentially get infected," he said.

Preliminary evidence for SARS-CoV-2 airborne transmission

Scientists don't know if what they've learned about microdroplet spread means the COVID-19 coronavirus can be carried in respiratory microdroplets, but there is some early anecdotal evidence this may be the case.

Ristenpart points to the well-known spreading event at a choir practice in Washington state on March 6. About 60 choir members at a practice were being careful not to touch each other or shake hands, yet about 75 per cent of them became infected, possibly from airborne particles.

"When you sing, typically that's a very loud volume. So one potential interpretation is that even though they weren't touching, they emitted great quantities of these respiratory particles. There was at least one person there who was infected and over that two-hour exposure during singing, they infected a large number of their fellow choir members."

There are also indications of airborne particles carrying virus in health-care settings where COVID-19 patients were being treated.

Scientists in Singapore and Nebraska have found evidence of the SARS-CoV-2 coronavirus in air exhaust outlets and air samples . Ristenpart said "presumably the only way it got there was through the air emitted from the infected patient."

He also pointed to a recent study in the New England Journal of Medicine that found, under experimental conditions, the virus in its airborne form can potentially remain infectious for hours.

A woman is seen wearing a mask in the subway during morning commuting hours as Toronto copes with a shutdown due to the coronavirus. (Cole Burnston/Getty Images)

Biggest unknowns about COVID-19 coronavirus spread

This is all tantalizing evidence, but there are still many open questions when it comes to understanding conclusively whether the SARS-CoV-2 virus can be transmitted through airborne microdroplets. Ristenpart said this work "needs to be done soon."

These are among the significant unknowns for SARS-CoV-2:

If this virus is carried in microdroplets expelled via speech or breath.

How long the virus can remain viable in microdroplets in real-life conditions.

How much of the virus a person needs to inhale to get infected and sick.

Whether this kind of airborne transmission is a frequent event compared to other modes of transmission.

Until we know for sure what's happening with the COVID-19 coronavirus, "then I would definitely wear a mask" if you're going to a more crowded environment like a grocery store, Ristenpart advises.

He cautions that homemade masks might do a good job of preventing large respiratory droplets from escaping, but the microdroplets "are so small, they can just scoot right through the fibres."

"So I would also urge your listeners to be aware that even if you're both wearing masks, if you have a very close face-to-face conversation, don't count on the masks to block all these expiratory particles."

Produced and written by Sonya Buyting