Reusable sheet masks last after a year of washing, drying

Reusable cloth masks that people have been using for a year or more can look a little worse for wear. But new research from the University of Colorado at Boulder shows that washing and drying them does not reduce their ability to filter virus particles.

“This is good news for sustainability,” said lead author Marina Vance, assistant professor in the Paul M. Rady Department of Mechanical Engineering. “That cotton mask that you washed, dried and reused? It’s probably still fine, don’t throw it away.”

The study, published in the journal Aerosol and air quality research, also confirms previous research that layering a cotton mask over a surgical mask – snug against the face – offers more protection than fabric alone.

Science for sustainability

Since the start of the pandemic, around 7,200 tonnes of medical waste has been generated every day, much of which is disposable masks.

“We were really bothered at the start of the pandemic, when we were going on a hike or going downtown, and seeing all these disposable masks littering the environment,” said Vance, who is also a faculty member. of the environmental engineering program.

So Vance was eager to join forces when scientists from the nearby National Renewable Energy Laboratory (NREL) approached her to study the impact of washing and drying on reusable sheet masks.

Their process was quite simple: create double-ply cotton squares, subject them to repeated washes and dryings (up to 52 times, the equivalent of a weekly wash for a year), and test them every 7 cycles of. cleaning approx.

While the masks weren’t tested with real people, instead, they were mounted on one end of a steel funnel through which the researchers could monitor a constant flow of air and airborne particles, the researchers tested the masks under realistic to real-life conditions, with humidity levels and high temperatures to mimic the impact of our breathing on the mask.

While the cotton fibers began to fall apart over time after repeated washing and drying, the researchers found that this did not significantly affect the filtration efficiency of the fabric.

The only noticeable change was that the resistance to inhalation has increased slightly, meaning the mask may feel a bit harder to breathe after some wear and tear.

Mask fit is essential

A key caveat is that they performed the tests using a “perfect fit” in the lab.

“We assume there is no space between the mask material and the person’s face,” Vance said.

The shape of each person’s face varies considerably. So, depending on the shape of a mask and how the person fits it, it may or may not fit well. Previous research has shown that an ill-fitting mask can pass up to 50% of the airborne particles we inhale and exhale, as well as the virus.

So which mask to wear?

This study is not the first to find that fabric masks offer less protection than surgical masks or a layered combination of surgical and fabric masks.

Measuring the filtration quality of the air breathed by the mask – protecting the person wearing the mask, not reducing transmission from the source – this study found that cotton fabric masks filtered up to 23% of the smallest particle size (0.3 microns) the virus can travel on. The bandanas filtered even less, at only 9%.

In comparison, surgical masks filtered between 42% and 88% of the tiny particles, and cotton masks on top of surgical masks achieved a filtration efficiency of almost 40%. Unsurprisingly, the KN95 and N95 masks performed the best, filtering out 83-99% of these particles.

But while this study found that cloth masks on their own offer less protection against the virus than a layered approach or disposable masks, such as surgical masks, KN95 and N95, there remains important information. for those who rely on fabric for its comfort, affordability, and reuse, says Vance.

“I think the best mask might be the one you’re actually going to wear,” Vance said. “And it will fit your face perfectly without being too uncomfortable.”