2 per pack P-Series filter intended for removal of any particle including oil-based liquid aerosols. 3M recommends that P-series filters should be used and reused for no more than 40 hours of use or 30 days, whichever occurs first, in atmospheres that contain only oil aerosols unless the filter needs to be changed for hygiene reasons, is damaged, or becomes difficult to breathe through before the time limit is reached. At least 99.97% filter efficient when tested with -0.3 um DOP (Dioctyl Phthalate) aerosol. 3M makes replaceable filters and filtering facepiece respirators in this category. May be used for any solid or liquid particulate airborne hazard.