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N95 filters are rated based on how many 0.3-micron particles they can block. They are rated for that particle size because that particle size is the hardest particle size to block and therefore serves as a proper test of the filter's efficiency.

If you look at a graph of the blocking efficiency of an N95 filter vs particle size, that graph is a curvy "V" shape, with the bottom of that "V" being at 0.3 microns.