How this calculator works

The three quantities

Hazard ratio. The measured airborne concentration divided by the occupational exposure limit (OEL): hazard ratio = concentration ÷ OEL. The OEL is usually the OSHA permissible exposure limit (PEL) or the NIOSH recommended exposure limit (REL), entered in the same unit as the concentration.

Federal vs. state exposure limits. The PEL values pre-filled on our substance pages are federal OSHA limits. About two dozen states run their own OSHA-approved plans, and some — California (Cal/OSHA) most notably — enforce lower or additional exposure limits. If a state limit applies to you, enter that value as the OEL; the calculator always uses whatever limit you provide. (Thanks to the r/WorkplaceSafety community for flagging this.)

Assigned Protection Factor (APF). The level of protection a class of respirators is expected to provide. A respirator is adequate only when its APF meets or exceeds the hazard ratio. APFs are fixed by OSHA in 29 CFR 1910.134, Table 1 (reproduced below).

Maximum Use Concentration (MUC). The highest concentration at which a respirator may be used: MUC = APF × OEL. The MUC must never exceed the substance’s IDLH value; when it would, the calculator caps the MUC at the IDLH.

Where the “measured concentration” comes from

This calculator starts from a measured airborne concentration — and that is the honest hard part, because you can’t select a respirator by guessing the exposure. The number comes from an exposure assessment: air sampling performed by a qualified person (typically an industrial hygienist) using a validated method — a sampling pump with the appropriate sorbent tube or filter analysed by a lab (e.g. OSHA or NIOSH NMAM methods), or a calibrated direct-reading instrument (PID, electrochemical or IR gas monitor) for real-time gases. If you don’t have sampling data, the right first step is an exposure assessment, not a respirator — until then, treat any result here as illustrative only.

Two more things the number alone won’t tell you, both essential to a compliant respirator program: the airborne contaminant must be matched to the correct cartridge or filter type (an APF is meaningless if the cartridge can’t capture that chemical — see each substance page and the cartridge & filter guide), and gas/vapor cartridges need a change-out schedule (you cannot rely on taste or smell). Respirator use also legally requires medical clearance and fit testing.

Worked example

Suppose carbon monoxide is measured at 600 ppm and the PEL is 50 ppm. The hazard ratio is 600 ÷ 50 = 12, so any respirator with an APF of at least 12 is adequate — a full-facepiece APR (APF 50), for example. Its MUC would be 50 × 50 = 2,500 ppm, but because carbon monoxide’s IDLH is 1,200 ppm, the MUC is capped at 1,200 ppm. Above 1,200 ppm the atmosphere is IDLH and only a full-facepiece pressure-demand SCBA — or a full-facepiece pressure-demand supplied-air respirator with an auxiliary self-contained air supply — may be used.

IDLH atmospheres

When the concentration is at or above the IDLH, air-purifying respirators are not permitted. Under 29 CFR 1910.134(d)(2), only a NIOSH-certified full-facepiece pressure-demand SCBA with a minimum 30-minute service life, or a combination full-facepiece pressure-demand SAR with auxiliary SCBA, may be used. The calculator detects this condition and restricts the shortlist accordingly.

OSHA Assigned Protection Factors — 29 CFR 1910.134, Table 1

This is the complete APF table the calculator uses:

Sources & last verified

• OSHA 29 CFR 1910.134(d)(3)(i)(A), Table 1 — Assigned Protection Factors — verified 2026-07-16.
• OSHA 29 CFR 1910.134(d)(2) — Respirators for IDLH atmospheres — verified 2026-07-16.
• NIOSH — Immediately Dangerous to Life or Health (IDLH) Values — verified 2026-07-16.
• OSHA Permissible Exposure Limits — 29 CFR 1910.1000 (Z-Tables) & substance-specific standards — verified 2026-07-16.

ACGIH Threshold Limit Values (TLVs) are copyrighted and are deliberately not bundled; enter your own OEL when using a TLV.

Disclaimer

This calculator is a reference and educational aid, not a substitute for a qualified exposure assessment, professional industrial-hygiene judgment, or the written respiratory-protection program required by 29 CFR 1910.134. Confirm every value against the current OSHA and NIOSH sources for your specific substance and workplace before selecting respiratory protection.