Masks aren't used to protect the wearer. They are used to protect others. I asked my daughter about it just last Friday. She is the lab manager for two hospitals in western MN. The virus can become airborne and is microscopic and can pass through the mask. The mask captures larger moisture droplets and prevents them from spraying out away from the wearer. This is why healthcare workers are one of the highest risk categories.Its affecting some US hospitals now. Masks are now in such short supply that the CDC has told health care workers doing the tests to use bandannas or scarves over their mouths for their protection.