Cloth masks widely used by Chinese to protect themselves against persistent air pollution, especially in large cities like Beijing, vary widely in effectiveness and could be giving users a false sense of security, especially in highly polluted areas.
A new study by environmental health scientists at the University of Massachusetts Amherst is probably the first study that rigorously tested disposable surgical masks and washable cloth masks widely used in Asia and Southeast Asia for personal protection against airborne particulate matter, said researchers Richard Peltier, Kabindra Shakya and colleagues.
Their study shows "wearing cloth masks reduced the exposure to some extent," but "the most commonly used cloth mask products perform poorly when compared to alternative options available on the market."
This has clear public health risk, according to the study. Peltier said users in the developing world should not assume these masks convey protection, "especially if an individual makes personal choices not to avoid high concentration environments because they assume they are protected from these contaminants."
"We found ourselves wondering how effective these masks are. I was shocked that we couldn't find any research studies investigating them," said Peltier.
Standard industrial hygiene mask such as the N95 respirators aren't readily available in most developing countries, and at $3 or $4 each would be too expensive for most consumers. In contrast, reusable cloth masks cost 10-15 cents and can be washed and worn for months.
In a series of experiments with an experimental mannequin, the team tested four masks. One was the pleated surgical type; two cloth and one cone-shaped cloth with exhalation flaps.
They tested for several variables and effectiveness in filtering out five different synthetic aerosol particle sizes plus three particle sizes of diluted whole diesel exhaust, which simulated real-world conditions.
Among the cloth masks, the one with exhaust valves performed fairly well, removing 80-90 percent of synthetic particles and about 57 percent of diesel exhaust.
Plain cloth masks were "only marginally beneficial" in protecting people from particles smaller than 2.5 micrometers, often considered more harmful than larger particles because they can penetrate the lungs more deeply.
The least expensive cloth masks removed just 39 percent to 65 percent of standard particles of 30-, 100-, and 500-nanometers, and 1- and 2.5-micrometers. All masks performed worse for diesel combustion particles compared to monodispersed particles.
Filtration efficiency of cloth masks for particles emitted from diesel combustion ranged 15 to 57 percent for total particle concentrations (for 30-, 100-, and 500-nm) and 13 to 40 percent for total particulate mass.
Cloth masks did offer measurable reduction in particle counts, but results were highly variable. The cloth mask with exhalation valve performed better than those without. The two worst performing masks performed better for larger particle size, but poorly on the more harmful smaller particle sizes.
"What became clear to us is that millions of people probably wear these masks and feel safer, but we worry that this is potentially making things worse, if they stand next to a diesel truck and think they are protected by the mask, for example," said Peltier.