“Considering that smaller particles are airborne longer than large droplets (larger droplets sink faster), the use of such a mask might be counterproductive,” the study’s researchers wrote.

Face masks are most effective when worn by all

In recent months, public health experts have stressed the importance of wearing face masks as a way to deter the spread of the coronavirus — especially by asymptomatic carriers, or people who don’t know they have the virus because they never develop symptoms. Face coverings act as a barrier by helping to keep respiratory particles from escaping an infected individual and landing on another person.

“Just speaking lets these particles get out, and these particles can carry viruses,” including the virus that causes COVID-19, explains Eric Westman, M.D., an associate professor of medicine at Duke University School of Medicine and a coauthor on the study. And knowing which masks are best at blocking these particles can help guide consumer decisions that impact the spread of the virus, he adds. The researchers designed a setup consisting of a box, a laser, a lens and a cellphone camera to test the masks.