Air purifying respirators with filters provide respiratory protection against aerosols by removing dusts, mists, fumes fibers and other particulates. Filters do not remove gases or vapors. End of service life for filters is normally determined by the increase in breathing resistance sensed by the user. When it becomes difficult to breathe comfortably, the filters should be replaced. 3M offers an extensive line of replaceable filters that provide different levels of filtration efficiency along with nuisance levels of a combination acid gas and organic vapors. All particulate filters are NIOSH approved, meet 42 CFR 84 requirements, when used with 3M air-purifying respirators. 502 is a retainer used for the 2000 series filters. 501 are a retainer used for both 5N11 and 5P71 filters.