Maximum Use Concentration (MUC)
means the maximum atmospheric concentration of a hazardous substance from which a responder can be expected to be protected when wearing a respirator. This is determined by using the APF of the respirator or class of respirators and the exposure limit of the hazardous substance. The MUC can be determined mathematically by multiplying the APF specified for a respirator by the required OEL, STEL, or ceiling limit. When no OSHA exposure limit is available for a hazardous substance, an employer shall determine an MUC on the basis of relevant available information and informed professional judgment.
Source: API RP 98, Personal Protective Equipment Selection for Oil Spill Responders, First Edition, August 2013. Global Standards