What assurance would you have that a product "mostly works" other than a certification by professionals? If we rely on common sense, we might as well all drink bleach.
Policies written with a more holistic view would say "lets spot check these masks, and for masks we find that is sub-par we will fine the importer some smallish amount that approximates the economic effect of the deficiency".
For example, if the standard says "must reduce particulates by 95%", yet the mask reduces particulates by 94%, then it might be reasonable to fine the importer (6-5)/5 = 20% of the value of the goods.
The key is to try to encourage all products to be as good as possible, without introducing months of delay and tens of thousands of dollars of paperwork for every product.
For example, if the standard says "must reduce particulates by 95%", yet the mask reduces particulates by 94%, then it might be reasonable to fine the importer (6-5)/5 = 20% of the value of the goods.
The key is to try to encourage all products to be as good as possible, without introducing months of delay and tens of thousands of dollars of paperwork for every product.