[hamburglar]

Well, the actual mask's performance is 95% effective, but you can't really make a guarantee about the overall worst case effectiveness when deployed by humans. N95 masks are not 95% effective on someone who has a big fluffy beard preventing a proper seal, for example. Or with people who use their gut feeling to decide where and when to put them on (like deciding if there's nobody in the elevator, you don't need it, ignoring the fact that someone may have gotten out of the elevator 20 seconds ago and left particulates in the air).