'Using an industry-standard test, he measured the particulate matter outside the mask, then checked how much made it through the device and into the space around his nose and mouth. Percentage-wise, he told me, “it removes 99.99 … I didn’t measure how many nines; it was working so well.” On broader scales, too, the protective math plays out: Well-fitting masks can curb smoke-related hospitalizations; studies back up their importance as a firefighting mainstay.'

https://www.theatlantic.com/health/archive/2023/06/mask-wildfire-smoke-n95-effective/674325/