He did explain it. He first recommended people not wear masks because we had limited supply and he didn't want everyone to buy all the masks (something like what happen with toilet paper) and not leave any for the medical workers. The evidence that they work is everywhere, it's common sense. They work for the same reason that you cover your nose when you sneeze. You didn't answer my question about the stop light. His professional opinion is backed by 50 years of experience. The real question is, why do you think you have the expertise to go against his professional opinion? What evidence do you have? What qualifications do you have?