It's all a bit complicated, but an article in the Laborers Lifelines magazine attempts to explain it:
The respirator supplied in various situations is based on the hazard faced (e.g. dust respirators for exposures to dusts and gas and vapor respirators for exposures to gases) and the expected level of exposure. The goal, according to OSHA, is to make sure exposure inside the mask does not get above the OSHA permissible exposure limits (PELs). Thus, if a standard allows exposures to 1 mg/m3 of dust and exposures are expected to be less than 10 mg/m3, then a mask with an APF of 10 should provide enough protection.The Lifelines article also explains how this new announcement is a sell-out to industry:
The most controversial part of the new APFs is the lack of any distinction between traditional rubber (elastomeric) masks with cartridge filters and new paper masks (filtering facepieces) that breathe through the entire mask surface. Both are given an APF of 10. This decision was based primarily on data from the respirator manufacturers. Labor unions, including LIUNA, and many other groups testified that filtering facepiece masks should only get an APF of 5 because they do not seal as well against the face to keep out contaminants. A 5 would mean they only protect half as well. Yet, OSHA sided with the manufacturers who have been promoting the filtering facepiece masks which have become a larger share of their business.
Thanks Jordan!
