What is the relationship between OSHA permissible exposure limits/maximum use concentrations and assigned protection factors?
OSHA’s Hazard Communication Standard (1910.1200, Appendix D) requires that Safety Data Sheets list not only the relevant OSHA permissible exposure limits, but also the American Conference of Governmental Industrial Hygienists threshold limit values and any other exposure limits used or recommended by the chemical manufacturer, importer or employer preparing a hazardous materials SDS.
Historically, most of the PELs contained in the Z-Tables of 29 CFR 1910.1000 were adopted from existing federal standards for general industry. These, in turn, were adopted from the 1968 TLVs of ACGIH.
NIOSH offers a pocket guide that lists its recommended exposure levels as well as the OSHA PELs.
Additionally, some respirator manufacturers provide selection guides that list occupational exposure limits, including but not limited to the OSHA PELs, ACGIH TLVs, NIOSH RELs, etc. Think of these manufacturers as “the NIOSH Approval-Holders.”
To ensure effective respiratory protection, zero in on all current edition OELs knowing a PEL may or may not exist for a specific compound. This is essential to calculating the maximum use concentration and the appropriate level of protection for the wearer.
Definitions: OSHA 29 CFR 1910.134
“Assigned Protection Factor” is the workplace level of respiratory protection that a respirator or class of respirators is expected to provide to employees when the employer implements a continuing, effective respiratory protection program as specified by this section.
“Maximum Use Concentration” is the maximum atmospheric concentration of a hazardous substance from which an employee can be expected to be protected when wearing a respirator, and is determined by the APF of the respirator or class of respirators and the exposure limit of the hazardous substance.
The MUC usually can be determined mathematically by multiplying the APF specified for a respirator by the PEL, short-term exposure limit, ceiling limit, peak limit or any other exposure limit used for the hazardous substance. The MUC for respirators is calculated by multiplying the APF for the respirator by the PEL. The MUC is the upper limit at which the class of respirator is expected to provide protection. Whenever the exposures approach the MUC, the employer should select the next higher class of respirators (higher APF) for workers. Beware that some publications may refer to calculating the “hazard ratio” vs. MUC.
“Employers must not apply MUCs to conditions that are immediately dangerous to life or health (IDLH); instead, they must use respirators listed for IDLH conditions,” states OSHA Ref: 29 CFR 1910.134 – MUC.
Additionally, in today’s changing world, the same respirator face piece may be assigned to an individual for multiple uses, including but not limited to toxic industrial materials and/or for response to an IDLH environment and/or use in chemical, biological, radiological, nuclear and explosive applications.
Once a respirator has been selected with an adequate APF, the program manager of the respiratory protection program must ensure effective training is executed, as well as fit testing, in accordance with OSHA 29 CFR 1910.134. Follow-up training and fit testing must be scheduled as well to maintain user confidence and protection. Finally, do not hesitate to solicit support from OSHA, NIOSH and the manufacturers of respirators and fit-testing apparatus. These sources are willing to contribute their knowledge to your respirator program to make it a successful one.
-Bruce Schorer, Consultant, OHD LLLP