What is a mask valve, and why are cities banning them?

n some states and major cities, it’s now mandatory to wear a mask in public to prevent the spread of COVID-19. That mask could be anything from a handkerchief to a designer cotton mask to a full-blown N95 respirator, so long as you have something between your mouth and nose and the world around you.

But in California’s Bay Area, there’s a new stipulation: You can wear any style of mask you want, so long as it doesn’t have a valve in it. What? Why? Because a mask with a valve may protect you from some pathogens in the air, but it doesn’t protect the people around you from your own breath.