Why simple cloth masks without valves are better at stopping the spread of COVID-19

Those face masks you see with coin-sized valves on the front may look intriguing but they are not as good at preventing the spread of the novel coronavirus as the seemingly lower-tech, non-valved masks.

Some masks designed for hot, dusty construction work - where the intent is to filter out dust before it hits the wearer's lungs - have "exhaust" valves that allow the exhaled air to flow out more easily, to keep the mask-wearer cooler.

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.

https://www.sfgate.com/news/article/Why-simple-cloth-masks-without-valves-are-better-15371959.php