Reusable cloth masks may be effective up to a year, as washing and drying does not reduce their ability to filter out viral particles, according to a study.
The finding, published in the journal Aerosol and Air Quality Research, also confirms previous studies that layering a cotton mask on top of a surgical mask -- properly fit on one's face -- provides more protection than cloth alone.
"It is good news for sustainability. That cotton mask that you have been washing, drying and reusing? It's probably still fine -- don't throw it away," said study lead author Marina Vance, an assistant professor at the University of Colorado Boulder in the US.
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Since the start of the pandemic, an estimated 7,200 tonnes of medical waste has been generated every day -- much of which is disposable masks, the researchers said.
"We were really bothered during the beginning of the pandemic, when going out, and seeing all these disposable masks littering the environment," said Vance.
The researchers created double-layered squares of cotton, put them through repeated washing and drying for a year, and tested them between about every seven cleaning cycles.
The masks were mounted on one end of a steel funnel through which researchers could control a consistent flow of air and airborne particles.
They tested the masks using realistic to real-life conditions, with high humidity levels and temperatures to mimics the impact on the mask from our breathing.
While the cotton fibres started falling apart over time after repeating washing and drying, the researchers found that did not significantly affect the cloth's filtration efficiency.
The only noticeable change was that inhalation resistance slightly increased, meaning that the mask may feel a bit more difficult to breathe through after some wear and tear.
A key caveat, the researchers noted, is that they conducted the testing using a "perfect fit" in the lab.
"We're assuming there are no gaps between the mask material and the person's face," said Vance.
The study found that the cotton cloth masks filtered out up to 23 per cent of the smallest particle size of 0.3 microns on which the virus can travel.
Bandanas filtered even less, at only 9 per cent.
In comparison, cotton masks on top of surgical masks reached close to 40 per cent filtration efficiency, the researchers said.
KN95 and N95 masks performed the best, filtering out 83-99 per cent of these particles, they said.
While this study found that cloth masks alone provide less protection from the virus than a layered approach or disposable masks, it remains important information for those who rely on cloth for its comfort, affordability and reusability, said Vance.
"I think the best mask might be the one that you're actually going to wear. And that is going to fit snugly against your face without being too uncomfortable," Vance added.