Protective masks have become a fact of life for millions of Americans in the face of COVID-19 — and with the pandemic spreading at a rapid pace, they’re more important than ever. There are thousands of options available to purchase, including medical-grade N95s reserved for frontline workers, Chinese KN95s and their Korean counterparts KF94s, nonmedical surgical masks, and factory or homemade cloth masks. They often come with claims quantifying how well they filter the air — but they aren’t all the same, and many don’t work as well as advertised. So, how do you know your mask actually works? Aaron Collins, a mechanical engineer specializing in aerosol science, explains.