masks are a type of semi-disposable dust mask used to help keep
dust and debris out of the mouth and nose.
Valved masks do not provide any protection from airborne chemicals
and solvents - so don't use them for painting, instead buy a respirator.
Valved masks help keep contaminants like sawdust, metal filings,
dust from rust, etc out of the mouth and nose.
The valve or valves - which are usually located either in the center
of the mask, or with one in either side of the mask - are designed
to help vent air faster during exhalation. This has the effect of
reducing the humidity inside the mask.