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As I understand it even the N95s are only 80% effective.

Not quite. It depends what you're trying to filter out, but an N95 by definition is at least 95% effective for particles 300 microns or larger. (N99 is 99% minimum, and Ps mean oil-resistant.) As you get smaller, the masks lose effectiveness for sure. Waay back in my posting history (year or so, so I'm not digging it out), I posted an older study on mask efficiency from before they got politicized. Properly sealed N95s are generally ~98% effective at the ~300micron size.

But one problem is that 2% - if there's viri present, and you're getting a 2% dose of 'em, you're still getting some, perhaps enough to infect you. And the longer you're there, the more you get - one minute with a 50% effective mask is "safer" than an hour with a 2% mask.

And this completely ignores the BIIIIG factors - most people aren't even wearing sealed N95s, they're using a torn tshirt or a scarf or bandanna or those crappy useless blue surgical masks (vape or smoke a cigarette with one - how much of the smoke goes around the edges of the mask? Usually 60-80% of it!). They aren't following proper decon procedures - no biohazard disposal, people touch all over it (the filter is full of whatever got filtered out!), they take em off and put em back on (HUGE NO-NO), they don't refresh (new sterile mask) every hour or so, they don't replace the mask when they get slightly wet (compromises filtration efficiency), and so on.

Even if the filter was 100% effective, how people use these makes them essentially 0% effective, if not the opposite and actually causing harm.

“actually causing harm.”

Mask, without question, are causing harm and sickness. They become bacterial breeding grounds within minutes of warm sweaty breath on them. People then touch the masks and infect everything around them. Forget covid, they are spreading all kinds of germs.