Blumberg, speaking on a college livestream about the coronavirus, said that a “standard rectangular surgical mask ... will decrease the risk of infection to the person wearing the mask by about 65%” and that homemade masks also “should work quite well.”
N95 masks are the most effective but should be reserved for medical personnel, he added.
The masks mainly provide a physical barrier to respiratory droplets that are about one-third the size of a human hair, he said. Those drops are one of the major ways the virus is transmitted.
“People who say 'I don't believe masks work' are ignoring scientific evidence,” Blumberg said. “It's not a belief system. It's like saying, 'I don't believe in gravity.'
“You're being an irresponsible member of the community if you're not wearing a mask. It's like double-dipping in the guacamole. You're not being nice to others.”
But even surgical masks are not airtight enough to create an effective barrier against much smaller aerosol particles, which are about 1/100th the size of a human hair, he said. The best defense against aerosol particles is social distancing and interacting with people outdoors.
“Studies in laboratory conditions now show the virus stays alive in aerosol form with a half-life on the scale of hours. It persists in the air,” said William Ristenpart, PhD, a professor of chemical engineering at UC Davis. “That's why you want to be outdoors for any social situations if possible.”
Enclosed places like bars are especially troublesome, he said, because “The louder you speak, the more expiatory aerosols you put out.”
Blumberg said scientists' opinions about the effectiveness of masks has evolved since the pandemic began months ago.
Although more states and cities are issuing mask mandates as cases continue to surge in the U.S., the issue remains controversial. Wearing them was not universally recommended during the early days of the pandemic, partly to ensure health care workers had enough protective gear while shortages existed.