The science is clear: Face masks can prevent coronavirus transmission and save lives.
A preliminary analysis of 194 countries found that places where masks weren't recommended saw a 55% weekly increase in coronavirus deaths per capita after their first case was reported, compared to 7% in countries with cultures or guidelines supporting mask-wearing. A model from the University of Washington also predicts that the US could prevent at least 45,000 coronavirus deaths by November if 95% of the population were to wear face masks in public.
But not all masks confer equal levels of protection.
The ideal face mask blocks large respiratory droplets from coughs or sneezes — the primary method by which people pass the coronavirus to others — along with smaller airborne particles, called aerosols, produced when people talk or exhale.
The World Health Organization recommends medical masks for healthcare workers, elderly people, people with underlying health conditions, and those who have either tested positive for the coronavirus or show symptoms. Healthy people who don't fall into these categories should wear a fabric mask, according to the WHO. The Centers for Disease Control and Prevention also recommends cloth masks for the general public.
But even cloth masks vary, since certain types are more porous than others.
"It depends on the quality," Dr. Ramzi Asfour, an infectious-disease physician in Marin County, California, told Business Insider. "If you're making a cloth mask from 600-thread-count Egyptian cotton sheets, that's different than making it from a cheap T-shirt that's not very finely woven."
Scientists have been evaluating the most effective mask materials for trapping the coronavirus over the last few months. Here are their results so far — the list goes in order from most to least protective.