Isn't containing coughs and sneezes (along with hand washing, etc) one of the most fundemental parts of our understanding of how to prevent the spread of infectious diseases. The NHS has had the "catch it, bin it, kill it" motto for years as a way to prevent flu / common cold. There may not be studies specifically into covid-19 on this topic. But we do know that this is one of the primary spread vectors for basically every other similar respiritory disease. It's worth studying covid specifically just to be sure, but it would be bizarre if covid was different in this respect. And IMO it's completely wrong to say we have no evidence tht masks work.