In the wake of the COVID-19
pandemic, the U.S. Centers for Disease Control and Prevention recommends that
people wear masks in public. Because N95 and surgical masks are scarce and
should be reserved for health care workers, many people are making their own
coverings. Now, researchers report in ACS Nano that a combination of cotton
with natural silk or chiffon can effectively filter out aerosol particles -- if
the fit is good.
SARS-CoV-2, the new coronavirus
that causes COVID-19, is thought to spread mainly through respiratory droplets
when an infected person coughs, sneezes, speaks or breathes. These droplets
form in a wide range of sizes, but the tiniest ones, called aerosols, can
easily slip through the openings between certain cloth fibers, leading some
people to question whether cloth masks can actually help prevent disease.
Therefore, Supratik Guha at the University of Chicago and colleagues wanted to
study the ability of common fabrics, alone or in combination, to filter out
aerosols similar in size to respiratory droplets.
The researchers used an aerosol
mixing chamber to produce particles ranging from 10 nm to 6 μm in diameter. A
fan blew the aerosol across various cloth samples at an airflow rate
corresponding to a person's respiration at rest, and the team measured the
number and size of particles in air before and after passing through the
fabric. One layer of a tightly woven cotton sheet combined with two layers of
polyester-spandex chiffon -- a sheer fabric often used in evening gowns --
filtered out the most aerosol particles (80-99%, depending on particle size),
with performance close to that of an N95 mask material. Substituting the
chiffon with natural silk or flannel, or simply using a cotton quilt with
cotton-polyester batting, produced similar results.
The researchers point out
that tightly woven fabrics, such as cotton, can act as a mechanical barrier to
particles, whereas fabrics that hold a static charge, like certain types of
chiffon and natural silk, serve as an electrostatic barrier. However, a 1% gap
reduced the filtering efficiency of all masks by half or more, emphasizing the
importance of a properly fitted mask.
The authors acknowledge use of
the U.S. Department of Energy's Center for Nanoscale Materials user facility at
Argonne National Laboratory and funding from the U.S. Department of Defense's
Vannevar Bush Fellowship.