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

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

Valves will keep you more comfortable, but endanger those around you.
In 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 …

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