A screen door doesn’t keep the rain out. FIfty times the screens still don’t keep the rain out but may slow it down for awhile. Thing is, when you’re in a submarine, a screen door should be near the bottom of the list as a tool to repair your hull. You know?
Stopping 50% of the particles is not good. If you’re exposed to 50,000 potentially infectious particles per breath, reducing that to 25,000 isn’t exactly a happy day. Inhaling particles is like inhaling flying bullets. Only getting shot half the time isn’t exactly a winning situation for the guy getting shot.
So again, I’m going to call your attention to Table 1. It’s in Chapter 2.2, “Filter materials,” which you really can’t miss unless you’re trying to be difficult.
The qF score is a measure of the penetration efficiency of particles (expressed as 1-η) divided by the delta pressure drop P. So high pressure drops (read: difficulty breathing through the filter) result in low scores. Being bad at stopping particles also means a bad score.
The best on the list is the BestAir Household air filter, with a qF of 7.2.
A Jinjiang-brand KN95 mask has a qF of 5.9.
A vacuum-cleaner bag from Hoover rates a 3.09.
Some activated carbon nonsense rates a 1.22.
The best coffee filter tested rated a 0.34, and the only thing that scored worse than that was another coffee filter, at 0.29.
Even a Levi’s bandana did better than the coffee filters, with 0.37.
So there’s a bit of science for you, and we can conclude that of the various things tried, the coffee filters are the worst. I can’t really make that any clearer. Even if you find the mythical perfect coffee filter and stack them 30-deep, they’re still only as good as your ability to seal them on your face anyway.
At the least, tie the filters on your face with a bandana. The bandana can do better filtration and you could use the coffee filters to actually filter some coffee! Pour some grounds in there, everything would smell nice and you could tell people you were the Coffee Bandit. Win/win there, as far as I’m concerned.