A good option: a mask made of two layers of a thick-weave fabric with a built-in pocket where you can place a filter, says May Chu, an epidemiologist at the Colorado School of Public Health who co-authored a paper published on June 2 in Nano Letters on the filtration efficiency of household mask materials. ... "If you go to Walmart, you look for Oly-fun, which is the brand name of that fabric. It's also called spunbond," says Chu, who's a scientific adviser to the World Health Organization and helped craft its recent detailed guidance on cloth masks.