Cotton masks stand test of time, study says

'That cotton mask that you have been washing, drying and reusing? Don’t throw it away.'

Dave Yasvinski 4 minute read September 13, 2021
cotton masks covid

Cotton masks continue to protect against COVID, even after many cycles of washing and drying.

They may no longer look pretty at this point in the pandemic, but a new study has found reusable cotton masks can take a licking and keep on ticking.

The research, published in the journal Aerosol and Air Quality Research, says that while cotton coverups aren’t as effective at filtering out viral particles as their surgical counterparts, a year’s worth of washing and drying does not diminish their effectiveness. With an estimated 7,200 tons of medical waste generated every day since the start of the pandemic — the majority in the form of disposal masks — that’s a shiny silver lining for a planet that’s grown accustomed to storm clouds.

“It’s good news for sustainability,” said Marina Vance, lead author of the study and an assistant professor in the Paul M. Rady Department of Mechanical Engineering at the University of Colorado Boulder. “That cotton mask that you have been washing, drying and reusing? It’s probably still fine — don’t throw it away.”

Vance, who is also a faculty member in the school’s environmental engineering program, jumped at the opportunity to team up with scientists from the National Renewable Energy Laboratory (NREL) who were testing the longevity of the masks that have become a pervasive part of everyday life. “We were really bothered during the beginning of the pandemic, when going out on a hike or going downtown, and seeing all these disposable masks littering the environment,” she said.

The team delved into the durability of the masks by creating a series of double-layered cotton squares and washing and drying them up to 52 times — the equivalent of once a week for a year — and analyzing their effectiveness every seven cycles. They did not test the masks on real people but subjected them to real-world situations by mounting the squares on the end of a steel funnel that allowed them to control the flow of particles in the air and alter humidity and temperature levels to simulate the effect of breathing.

While the cotton fibers began to degrade after repeated cycles, researchers found this wear and tear did not significantly diminish the fabric’s filtration ability. The only detectable difference was an increase in inhalation resistance, meaning the masks gradually became more difficult to breathe through over the course of study.

The fit of the mask was of key importance to its ability to guard against airborne particles and remain effective over time, according to the study. Previous work has found a poorly fitted mask can let as many as 50 per cent of airborne particles — including the virus — in and out. “We’re assuming there are no gaps between the mask material and the person’s face,” Vance said.

In terms of protecting the person wearing the mask, the study found reusable cotton filtered out up to 23 per cent of the smallest particles (0.3 microns) on which the COVID-19 virus can travel from being inhaled. Bandanas were even less effective, preventing just nine per cent of these particles from passing through.

Surgical masks, on the other hand, filtered out between 42 and 88 per cent of particles, with KN95 and N95 masks in particular stopping 83 to 99 per cent of particles. Using cotton on top of a surgical mask resulted in 40 per cent filtration effectiveness.

Although the benefits of making a surgical selection over a cotton coverup appear clear, Vance said there are other factors — including comfort, affordability and reusability — that factor into the decision. “I think the best mask might be the one that you’re actually going to wear,” she said. “And that is going to fit snugly against your face without being too uncomfortable.”

Dave Yasvinski is a writer with Healthing.ca