Washington Post: “In guidance updated late last week, the Centers for Disease Control and Prevention warned against wearing masks with exhalation valves or vents, a type of face covering made for hot and dusty construction work that has become a popular pandemic accessory because of its seemingly high-tech design. “The purpose of masks is to keep respiratory droplets from reaching others to aid with source control,” the agency’s guidance reads. “However, masks with one-way valves or vents allow air to be exhaled through a hole in the material, which can result in expelled respiratory droplets that can reach others. This type of mask does not prevent the person wearing the mask from transmitting COVID-19 to others.vent.” 3M, which makes valve masks for construction work, 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 recommend 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…”