Wearing a face mask can protect you and others from Covid-19, but the type of material and how many fabric layers used can significantly affect exposure risk, found a new study from the Georgia Institute of Technology. The study measured the filtration efficiency of submicron particles passing through a variety of different materials. For comparison, a human hair is about 50 microns in diameter while 1 millimetre is 1,000 microns in size. The study was conducted during spring 2020 when the pandemic triggered a global shutdown of most institutions. Communities faced massive shortages of personal protective equipment, prompting many people to make their own homemade masks. Georgia Tech quickly set up the study since it already had “a great system for testing filtration efficiency using existing instruments in the lab,” Ng recalled. The study’s findings were used to shape homemade face mask recommendations here last April, with the comprehensive study findings published on March 22 in the journal Aerosol Science and Technology.