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 out through the valve. The system may be what you want when tearing out a kitchen for remodeling, but the valve defeats the purpose when you’re trying to slow the spread of a virus.
Public health experts have been recommending mask-wearing to prevent respiratory droplets from spreading into the air when you exhale, speak, cough or sneeze, and the valves allow those droplets through.
Medical masks, you’ll notice, do not have valves.
In its guidelines for mask-wearing, San Francisco stipulates that masks with valves do not meet its standards.
“Any mask that incorporates a one-way valve (typically a raised plastic cylinder about the size of a quarter on the front or side of the mask) that is designed to facilitate easy exhaling allows droplets to be released from the mask, putting others nearby at risk,” the order says.
The Centers for Disease Control and Prevention recommends simple cloth masks for the public to prevent the spread of covid-19. A few layers of cotton prevent most of the potentially infectious respiratory droplets from escaping into the air around you, and they are also much cooler than the form-fitting N95 masks.