If there’s one simple technology that has come to the forefront during the coronavirus pandemic, it’s the face mask. Special masks called respirators are designed to prevent doctors and nurses from catching the virus when they treat infected people. But not all respirators do what they’re labelled to. Fakes and shoddy products abound. And you can’t always tell how many particles a mask can filter just by looking at it. Naomi Kresge reported on how you catch a fake mask – and the lengths one German company is going to, to fight the problem.