That's a good lesson in data science (the author is one). He has all the numbers right but did he forget some things ? I didn't see anything about re-usability

That's a good lesson in data science (the author is one). He has all the numbers right but did he forget some things ? I didn't see anything about re-usability of the masks (no one has an endless stash of mask with a fresh one for every use)? What about using them right when putting them on/off and while wearing them (masks are efficient only when worn correctly, or not?)?