niosh air filtration rating niosh air filtration rating the niosh air filtration rating is the u.s. national institute for occupational safety and health (niosh)'s classification of filtering respirators. the ratings describe the ability of the device to protect the wearer from solid and liquid particulates in the air. the certification and approval process for respiratory protective devices is governed by part 84 of title 42 of the code of federal regulations (42 cfr 84). respiratory protective devices so classified include air-purifying respirators (apr) such as filtering facepiece respirators and chemical protective cartridges that have incorporated particulate filter elements. a filtering facepiece n95 respirator half-face air-purifying respirator with combination p100 particulate filter (magenta) and organic vapor (black) cartridge a firefighter wearing an n95 filtering facepiece respirator. note that, in this picture, the respirator is being worn incorrectly because the top strap should be above the ears and behind the head. the classifications only cover the filtration of particles or aerosols, not the air-purifying respirator's ability to remove chemical gasses and vapors from air, which is regulated under 42 cfr 84 subpart l. for more information see cartridge (respirator). the classifications assume that the respirator is properly fitted. niosh classifications