Researchers asked volunteers with COVID-19 to breathe into a contraption known as the Gesundheit II Machine, developed by Milton and colleagues to measure viruses in exhaled breath. Participants breathed into the machine for 30 minutes at a time and also repeated the alphabet, sang “Happy Birthday” and even repeatedly shouted “Go Terps!”—first with masks on, and then without. “Data from our study suggests that a mildly symptomatic person with COVID-19 who is not wearing a mask exhales a little over two infectious doses per hour says first author Jianyu Lai, a postdoctoral researcher at the PHAB Lab. “But when wearing an N95 mask, the risk goes down exponentially.”