High-efficiency masks up to six times better at filtering aerosols than cloth, surgical masks: Canadian study

Toronto -

A recent study from engineering researchers in Ontario has found that high-efficiency masks are up to six times better at filtering aerosols compared to more commonly used cloth and surgical masks.

Researchers at the University of Waterloo looked into how effective different types of masks are at filtering out aerosol particles, which are solid or liquid particles approximately 0.001 millimetres in diameter and suspended in the air. They published their findings in the journal Physics of Fluids on July 21.

The team put masks over a CPR mannequin that could simulate a person's breathing and exhale aerosol droplets, which were made using olive oil, and measured the amount of aerosols that would be built up in a large, unventilated room. The measurements were taken from two metres away, the Public Health Agency of Canada (PHAC)'s recommended distance for physical distancing.

The researchers found that N95 masks were able to filter out 60 per cent of exhaled aerosols, and KN95 masks could filter out 46 per cent.

On the other hand, cloth masks and surgical masks only filtered out 10 per cent and 12 per cent of exhaled aerosols, respectively.

“There is no question it is beneficial to wear any face covering, both for protection in close proximity and at a distance in a room,” said lead author Serhiy Yarusevych in a press release​. “However, there is a very serious difference in the effectiveness of different masks when it comes to controlling aerosols.”

Researchers say cloth and surgical masks are prone to air leakage at the top of the mask, where the mask meets the bridge of the nose.

In the early days of the pandemic, health officials in Canada and around the world said that that the SARS-CoV-2 virus was thought to primarily spread through droplets created when people cough, sneeze or talk. But in November, PHAC updated its advice to acknowledge aerosol transmission of the virus.

A growing number of scientists and doctors also believe that aerosols are the most dominant mode of transmission of COVID-19. A paper published in The Lancet in April pointed to long-range COVID-19 spread in quarantine hotels as well as air samples in hospitals that contained viable SARS-CoV-2.

Yarusevych's team also examined ventilation and found that even modest ventilation can significantly reduce the amount of aerosols.

When the researchers conducted the experiment with an unmasked mannequin and a room that had a ventilation rate of 1.7 room volumes per hour, the ventilation managed to eliminate 69 per cent of aerosols. At 3.2 room volumes per hour, 84 per cent of aerosols were gone.

Yarusevych says his data shows that wherever possible, high-efficiency masks should be paired with proper ventilation in indoor settings such as schools and workplaces.

“A lot of this may seem like common sense,” he said. “There is a reason, for instance, that medical practitioners wear N95 masks – they work much better. The novelty here is that we have provided solid numbers and rigorous analysis to support that assumption.”​