Method & sources
How this calculator works
Every number this tool produces is traceable to a public OSHA or NIOSH source. Here is exactly how it is calculated, and where the data comes from.
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.
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.
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:
Air-Purifying Respirator (APR)
| Facepiece | Mode | APF |
|---|---|---|
| Quarter mask | — | 5 |
| Half mask | — | 10 |
| Full facepiece | — | 50 |
Powered Air-Purifying Respirator (PAPR)
| Facepiece | Mode | APF |
|---|---|---|
| Half mask | — | 50 |
| Full facepiece | — | 1,000 |
| Helmet / hood | — | 25 |
| Loose-fitting facepiece | — | 25 |
Supplied-Air Respirator / Airline (SAR)
| Facepiece | Mode | APF |
|---|---|---|
| Half mask | Demand | 10 |
| Full facepiece | Demand | 50 |
| Half mask | Continuous flow | 50 |
| Full facepiece | Continuous flow | 1,000 |
| Helmet / hood | Continuous flow | 25 |
| Loose-fitting facepiece | Continuous flow | 25 |
| Half mask | Pressure-demand / positive-pressure | 50 |
| Full facepiece | Pressure-demand / positive-pressure | 1,000 |
Self-Contained Breathing Apparatus (SCBA)
| Facepiece | Mode | APF |
|---|---|---|
| Half mask | Demand | 10 |
| Full facepiece | Demand | 50 |
| Full facepiece | Pressure-demand / positive-pressure | 10,000 |
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.