This study, conducted by scientists at the National Institute of Standards and Technology (NIST) and the Smithsonian's Museum Conservation Institute, was published in ACS Applied Nano Materials.
An earlier study by the same research team showed that dual-layer masks made of tightly woven cotton fabrics with a raised nap, such as flannels, are particularly effective at filtering breath. That study was conducted under relatively dry conditions in the lab, and its main finding still stands.
"Cotton fabrics are still a great choice," said NIST research scientist Christopher Zangmeister. "But this new study shows that cotton fabrics actually perform better in masks than we thought."
The researchers also tested whether humidity makes the fabrics harder to breathe through and found no change in breathability.
The Centers for Disease Control and Prevention (CDC) recommends that people wear masks to slow the spread of COVID-19. When worn correctly, those masks filter out some of the virus-filled droplets that an infected person exhales and also offer some protection to the wearer by filtering incoming air.
The filtration efficiency of cotton fabrics increases in humid conditions because cotton is hydrophilic, meaning it likes water. By absorbing small amounts of the water in a person's breath, cotton fibers create a moist environment inside the fabric. As microscopic particles pass through, they absorb some of this moisture and grow larger, which makes them more likely to get trapped.
Most synthetic fabrics, on the other hand, are hydrophobic, meaning they dislike water. These fabrics do not absorb moisture, and their filtration efficiency does not change in humid conditions.
For this study, the team tested fabric swatches, not actual masks. First, they prepared dual-layer fabric swatches by placing them inside a small box where the air was maintained at 99% humidity - roughly the same as a person's exhaled breath. For comparison, a second set of swatches were prepared at 55% humidity. After the fabrics reached an equilibrium with the humidified air, the researchers placed them in front of a pipe that emitted air at about the same velocity as exhaled breath. That air carried salt particles in a range of sizes typical of the droplets that a person exhales when breathing, speaking and coughing. This salt particle method is recommended by the CDC's National Institute for Occupational Safety and Health (NIOSH) for measuring the filtration performance of mask-making materials.
MEDICA-tradefair.com; Source: National Institute of Standards and Technology (NIST)
