In the following, I write “should,” because not all masks work perfectly.
A chemical mask should work, because whatever is passing thru the mask, with the intake, or outlet, air, bonds chemically or physically.
A mechanical mask should work, because of some combination of its layers consisting of fibers, as a complex physical mesh composition -— all designed to cause intake, or outlet, air, to change directions more than once. At, or near enough to, the change of direction for air, objects and particles “suspended” in the air, fail to make the change of direction, because of their mass/weight, and are caught by the fibers.
A screen or fence should work, because its single layer of composition, is in the way of some object’s direction of travel.
There are degrees of combinations of the following:
- cleaned
- disinfected
- restored
- sanitzed
- sterilized
. . . that may apply to some masks that are intended to be re-used. Plus, the number of times that a mask can pass thru such processing, before the level of efficiency of a mask, falls below “satisfactory” performance (or the straps become frail and useless).
If 100 percent protection is expected, but not met, then a mask may be considered worthless. If levels of protection - levels of performance - are expected, then a mask has levels of value.
In general, masks should provide a “satisfactory” level of protection when an effort is made to respect the weaknesses of the mask.