3M model #2091 pre filter for use with 3M model #5000, 6000 and 7000 series respirators. Prefilters are used to help prolonge the life of the respirator by trapping dust, mists and particles. P100 filter is oil proof and 100% efficient. 2 filters per package. These prefilters must be used with 3M #502 filter retainer, which is sold separately.