If you are wearing a mask and it has a valve, you are doing it wrong. Valves allow you to exhale without any inhibition — the air flows without being filtered. Industrial masks have valves (not medical masks) as the concern is not about making people sick — coal does not get sick.
goatinamoat:
And a clarification — industrial N95 masks have valves — medical N95s do not. I now return you to your regularly scheduled nudity.