Yulia ShaihudinovaGetty Images Did you know homemade face masks made of cotton are about 20 percent more effective at filtering out small particles like SARS-CoV-2, the virus that causes COVID-19, than silk or flannel? And have you heard that after two weeks of wearing any of these three fabrics over your nose and mouth, you could potentially prevent up to 19 or 20 people from contracting the virus? Two friends in Poland are on a mission to make sure you're aware of these facts—and to dispel misinformation surrounding protective face coverings. And the best way to do that is with a calculator.