Frequently asked questions

An assigned protection factor is the workplace level of respiratory protection a class of respirators is expected to provide when used within an effective respiratory-protection program. OSHA lists APFs in 29 CFR 1910.134, Table 1. For example, a half-mask air-purifying respirator has an APF of 10 and a full-facepiece one has an APF of 50.

Multiply the respirator’s APF by the occupational exposure limit (OEL): 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 can never exceed the substance’s IDLH value, so it is capped at the IDLH when the product is higher.

The hazard ratio is the measured concentration divided by the exposure limit (concentration ÷ OEL). A respirator is adequate only if its APF is equal to or greater than the hazard ratio. If the concentration is 150 ppm and the PEL is 50 ppm, the hazard ratio is 3, so you need a respirator with an APF of at least 3.

Under OSHA Table 1: a half-mask air-purifying respirator is APF 10; a full-facepiece air-purifying respirator is APF 50; a half-mask PAPR is 50; a full-facepiece PAPR is 1,000; a loose-fitting PAPR or hood is 25 (up to 1,000 only with manufacturer performance evidence). A full-facepiece pressure-demand SCBA is 10,000.

IDLH means Immediately Dangerous to Life or Health. When the concentration is at or above the IDLH value, 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 full-facepiece pressure-demand supplied-air respirator with an auxiliary self-contained air supply, may be used.

A higher APF provides more protection, but respirators with higher APFs are heavier, costlier, and more burdensome. Good practice selects the least over-protective respirator that still meets or exceeds the hazard ratio, which is why this tool lists the lowest adequate option first.

Divide the concentration by the exposure limit to get the hazard ratio, then pick any respirator whose APF meets or exceeds it — provided the concentration is below the IDLH. Enter your values in the calculator and it returns the full compliant shortlist with each respirator’s MUC.

No tool replaces a qualified exposure assessment or the written respiratory-protection program required by 29 CFR 1910.134. This calculator is a reference and educational aid that applies OSHA’s published APF table and MUC method. Always verify inputs and results against current OSHA and NIOSH sources and involve a qualified professional.

ACGIH Threshold Limit Values are copyrighted and cannot be redistributed, so they are not bundled. You can still use a TLV by entering it yourself in the exposure-limit field; the calculator works with any OEL you provide.

Use whichever unit your exposure limit is expressed in, and enter the measured concentration in the same unit. The hazard ratio is unitless, so as long as concentration and OEL share a unit the result is valid. Converting between ppm and mg/m³ requires the substance’s molecular weight and is not performed automatically.