In a final rule on Assigned Protection Factors (APFs), OSHA revised its existing Respiratory Protection standard to add definitions and requirements for APFs and Maximum Use Concentrations (MUCs). (See 63 FR 1152; 29 CFR 1910.134; 71 FR 50122, August 24, 2006.) This guidance document focuses on the mandatory selection provisions of the APFs, MUCs, and their role in the overall Respiratory Protection standard. The provisions can only be used when respirators are properly selected and used in compliance with the full Respiratory Protection standard. The final Respiratory Protection standard (29 CFR 1910.134 and 29 CFR 1926.103) applies to general industry, construction, longshoring, shipyard, and marine terminal workplaces. The Agency developed the final APFs after thoroughly reviewing the available literature, including workplace protection factor studies, comments submitted to the record, and hearing testimony. The final APFs provide employers with critical information to use when selecting respirators for employees exposed to atmospheric contaminants found in industry. Proper respirator selection is an important component of an effective respiratory protection program. Accordingly, the final APFs are necessary to protect employees who must use respirators to protect them from airborne contaminants.