That paper says "when you put a mask on, and we immediately ask you to exhale onto a plate, we don't detect virus on that plate".

Parent poster isn't asking whether masks trap particles or not (clearly, they do), they're asking a much more interesting and useful question which is if we make everyone wear a mask does that prevent the spread of disease or not?

So far, we really don't have much evidence. In this thread, where lots of people are posting links, there's only one decent link posted by gameswithgo here:

https://news.ycombinator.com/item?id=24589569

I replied in place.