How to select a respirator by APF and MUC
Choosing the right respirator starts with a number: the hazard ratio, the measured airborne concentration of a contaminant divided by its occupational exposure limit (OEL) — typically the OSHA permissible exposure limit (PEL) or a NIOSH recommended exposure limit (REL). If workers are exposed to 150 ppm of a substance with a 50 ppm PEL, the hazard ratio is 3. The respirator you select must reduce exposure to at or below the limit, so its assigned protection factor must meet or exceed that hazard ratio.
What is an Assigned Protection Factor (APF)?
An assigned protection factor is the workplace level of respiratory protection that a properly functioning respirator, or class of respirators, is expected to provide when used within an effective respiratory-protection program. OSHA publishes APFs in29 CFR 1910.134, Table 1. A half-mask air-purifying respirator has an APF of 10; a full-facepiece air-purifying respirator, 50; a full-facepiece pressure-demand SCBA, 10,000. An APF of 10 means the respirator is expected to reduce the wearer’s exposure by a factor of ten. This calculator reads Table 1 directly and returns every respirator class whose APF is high enough for your hazard ratio.
What is the Maximum Use Concentration (MUC)?
The maximum use concentration is the highest airborne concentration at which a given respirator may be used. It is calculated as MUC = APF × OEL. A full-facepiece APR (APF 50) used against a substance with a 50 ppm PEL has an MUC of 2,500 ppm. The MUC must never exceed the substance’s IDLH(Immediately Dangerous to Life or Health) value — when the calculated MUC is higher than the IDLH, the MUC is capped at the IDLH. This tool applies that cap automatically when you supply an IDLH value.
IDLH atmospheres
When a measured concentration reaches or exceeds the IDLH, ordinary air-purifying respirators are not permitted. Under 29 CFR 1910.134(d)(2), only a NIOSH-certified full-facepiece pressure-demand SCBA (minimum 30-minute service life) or a combination full-facepiece pressure-demand supplied-air respirator with an auxiliary self-contained air supply may be used. The calculator flags IDLH atmospheres and restricts the shortlist accordingly.
Using this calculator
Pick a contaminant from the list to auto-fill common OSHA/NIOSH values, or enter your own. Choose your unit (ppm or mg/m³), enter the measured concentration and the exposure limit in the same unit, and optionally add the IDLH. The tool returns the hazard ratio, the minimum APF required, and a ranked shortlist — least over-protective first — with each respirator’s APF and MUC. It is built to be fast, accurate, and transparent, citing the exact OSHA and NIOSH sources behind every number so you can verify them yourself.
Reminder: this is a reference and educational calculator, not a substitute for a qualified exposure assessment or a written respiratory-protection program. Confirm every input and result against current OSHA and NIOSH sources for your specific substance and workplace. Read themethod and sources.