
The 3M company, which makes valve masks for such occupations, illustrates on its website how they work: inhaled air is filtered through the fabric part of the mask, and hot, humid exhaled air goes ou… [+1169 chars]

The 3M company, which makes valve masks for such occupations, illustrates on its website how they work: inhaled air is filtered through the fabric part of the mask, and hot, humid exhaled air goes ou… [+1169 chars]