Cotton face masks are proving more effective against the spread of the coronavirus than previously thought, a new study found.
Even under humid conditions — which is typically the case, as the mask covers the mouth and nose — 100% cotton performed significantly better than synthetic fibers, including nylon, polyester and rayon.
In fact, the humidity provided by one’s breath was shown to actually improve filtration in cotton masks. Across the nine types of cotton flannel masks researchers tested, humidity increased efficacy on average from 12% to 45%.
Meanwhile, scientists saw no improvement among the six synthetic mask swatches included in the experiment, most of which are relatively hydrophobic — meaning they do not lock in moisture the way cotton and other natural fibers do.
Examining the various mask types under a microscope revealed their very different weaves, which show how and where viral particles may enter or escape through the cloth barrier.
Cotton has long been the top choice among physicians and researchers in lieu of disposable, polypropylene-based N95 respirators, which filter at about the same rate — 95% — in dry and humid conditions. Masks found in a clinical setting are generally made of electrostatic polypropylene fiber, which is pressed, and not woven, into a fibrous mass that leaves little room through which the coronavirus can pass.
However, in lieu of N95 respirators, cotton is considered key to prevention.
"Cotton fabrics are still a great choice," affirmed Christopher Zangmeister, a research scientist at the National Institute of Standards and Technology, an arm of the US Department of Commerce. "But this new study shows that cotton fabrics actually perform better in masks than we thought."
Currently, the Centers for Disease Control and Prevention recommends that masks be worn at all times in public. Their own latest study on face masks concluded that 2-ply fabrics or double masking is ideal for prevention, with the masks fitted as close to the face as possible for best results.
NIST researchers have shared their findings with the American Society for Testing and Materials (ASTM) International, an organization charged with setting standards for manufacturing materials, and published a report in the American Chemical Society’s journal Applied Nano Materials.
The results were surprising, but researchers believe that the moisture that collects in cotton face masks from the wearers’ breath — which is only about 150 milligrams, or two droplets’` worth, of water — helps to expand the hydrophilic fibers, making the microscopic pathways through which viral particles may pass even more difficult to penetrate.
Researchers also discovered that the typical level of humidity in one’s breath — about 99% — is not enough to affect change in a cotton mask’s breathability, confirming that exercise while wearing a cotton mask is perfectly safe for most people. However, masks that become soaked are not recommended, as the bloated fibers decrease breathability.
Zangmeister added that the NIST team was careful to design an experiment that closely mimicked the real-life conditions of people wearing face masks.
"To understand how these materials perform in the real world," he said, "we need to study them under realistic conditions."