OSHA term · Glossary
Assigned Protection Factor (APF)
Verified vs OSHA sources · 2026-10-05
The workplace level of respiratory protection that a properly functioning respirator (or class of respirators) is expected to provide to a properly fitted and trained user. OSHA codifies APFs in Table 1 of 29 CFR 1910.134(d)(3)(i). An APF of 10 means the respirator reduces the wearer's exposure to one-tenth of the ambient concentration. Representative values: half-mask air-purifying = 10; full-facepiece air-purifying = 50; loose-fitting PAPR/hood = 25; tight-fitting full-facepiece PAPR = 1000; and full-facepiece pressure-demand SCBA = 10,000. The APF is used to compute the Maximum Use Concentration (MUC = APF × PEL).
Also known as: APF, protection factor
On this page
Assigned Protection Factor (APF) at a glance
- Codified in Table 1, 29 CFR 1910.134(d)(3)(i). Half mask = 10; full facepiece = 50.
- Loose-fitting PAPR/hood = 25; tight-fitting full-facepiece PAPR = 1000.
- Full-facepiece pressure-demand SCBA = 10,000 (the highest APF); demand-mode SCBA/SAR full facepiece = 50.
- Maximum Use Concentration: MUC = APF × PEL — the highest ambient concentration the respirator may be used in.
- An APF only applies within an effective respiratory protection program (fit testing, medical evaluation, training, maintenance).
- A respirator may not be used above its MUC, above the IDLH level, or above cartridge/canister limits.
- Higher APF ≠ automatically better choice — selection matches the APF to the actual exposure via the MUC.
In plain English
A number that tells you how much a respirator cuts down a worker's exposure. An APF of 10 means the air inside the mask is one-tenth as contaminated as the air outside; an APF of 50 means one-fiftieth. You multiply the APF by the exposure limit to find the highest concentration a given respirator can safely be used in — above that, you need a more protective respirator.
What the rule says
“Assigned protection factor (APF) means the workplace level of respiratory protection that a respirator or class of respirators is expected to provide to employees when the employer implements a continuing, effective respiratory protection program as specified by this section.”
In context
An enforcement, recordkeeping, or general-duty concept under the Occupational Safety and Health Act and 29 CFR. These terms define how OSHA inspects, cites, and penalizes employers.
Where this is written in OSHA's rules
- 29 CFR 1910.134(d)(3)(i)Read on eCFR / OSHA.gov
- 29 CFR 1910.134(b)Read on eCFR / OSHA.gov
Example
A contaminant has a PEL of 5 mg/m³ and a worker's measured exposure is 40 mg/m³. Dividing exposure by the PEL gives a required protection factor of 8, so a half-mask respirator (APF 10) is adequate: its Maximum Use Concentration is 10 × 5 = 50 mg/m³, above the 40 mg/m³ exposure. If exposure were instead 400 mg/m³, the half mask's MUC (50 mg/m³) would be far exceeded, so a full facepiece (APF 50, MUC 250 mg/m³) or a PAPR/SCBA would be required.
Why it matters
The APF is the quantitative heart of respirator selection: it converts a respirator class into a concrete usable concentration limit via the MUC (APF × PEL). Choosing a respirator with too low an APF for the actual exposure leaves the worker overexposed even though they are 'wearing a respirator.' Because respirators are the least-preferred control in the hierarchy, getting the APF-to-exposure match right — and staying below the MUC and IDLH — is critical to actual protection.
APF vs. fit factor
These sound similar but measure different things. The ASSIGNED PROTECTION FACTOR (APF) is a policy value from Table 1 — the protection a respirator CLASS is expected to provide across a program (e.g., all half masks get an APF of 10). The FIT FACTOR is an individual measurement from a quantitative fit test — how well THIS respirator seals on THIS person right now (e.g., a fit factor of 150). Fit factors must meet a pass threshold (≥ 100 for a half mask) to qualify the wearer, but the protection you plan around in the workplace is the APF, not the individual fit factor.
OSHA Assigned Protection Factors — Table 1 (29 CFR 1910.134)
| Respirator Type | APF |
|---|---|
| Half mask (air-purifying) | 10 |
| Full facepiece (air-purifying) | 50 |
| Loose-fitting PAPR / supplied-air hood or helmet | 25 |
| Tight-fitting full-facepiece PAPR | 1,000 |
| Demand-mode SCBA / SAR, full facepiece | 50 |
| Pressure-demand SCBA, full facepiece | 10,000 |
Assigned Protection Factor (APF): frequently asked questions
- What does an assigned protection factor of 10 mean?
- An APF of 10 means that a properly selected, fit-tested, and maintained respirator — used within an effective respiratory protection program — is expected to reduce the wearer's exposure to one-tenth (10%) of the airborne concentration outside the mask. A half-mask air-purifying respirator has an APF of 10. To find the highest concentration it may be used in, multiply the APF by the exposure limit (the Maximum Use Concentration).
- How do you calculate the Maximum Use Concentration (MUC)?
- The Maximum Use Concentration is MUC = APF × PEL — the respirator's assigned protection factor multiplied by the relevant exposure limit (typically the PEL). For example, a full facepiece (APF 50) used against a contaminant with a 5 mg/m³ PEL has an MUC of 250 mg/m³. A respirator may never be used above its MUC, and never above the IDLH concentration or the limits of its cartridges/canisters, whichever is lowest.
- Which respirator has the highest assigned protection factor?
- A full-facepiece pressure-demand self-contained breathing apparatus (SCBA) has the highest APF at 10,000. This is the level of protection required for the most hazardous atmospheres, including those that are immediately dangerous to life or health (IDLH). By contrast, a half-mask air-purifying respirator provides an APF of 10.
Related terms
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Area DirectorThe senior OSHA official in charge of a specific OSHA Area Office — the local unit that runs inspections and enforcement in its geographic territory. The Area Director has the authority to issue citations, propose penalties, hold informal conferences with employers, and approve settlement agreements. When a Compliance Safety and Health Officer (CSHO) completes an inspection, it's the Area Director who ultimately issues the citation.
Bureau of Labor Statistics (BLS)The federal statistical agency within the U.S. Department of Labor that collects, analyzes, and publishes data on workplace injuries, illnesses, and fatalities. BLS conducts the annual Survey of Occupational Injuries and Illnesses (SOII) — compiled from employer OSHA 300 logs — and the Census of Fatal Occupational Injuries (CFOI). SOII data drives OSHA's Site-Specific Targeting (SST) program: establishments whose DART rates significantly exceed their BLS industry average are placed on the SST inspection list.
Competent PersonUnder OSHA's construction standards, a person who is capable of identifying existing and predictable hazards in the surroundings or working conditions, and who has the authorization to take prompt corrective measures to eliminate them. The role requires both knowledge and the authority to act, and is mandated by name in many specific standards — excavations, scaffolds, fall protection, asbestos, and more.
CitationThe formal written notice OSHA issues to an employer after an inspection, describing each alleged violation, the standard cited, the proposed penalty, and the date by which the hazard must be corrected (abated). OSHA must issue a citation with reasonable promptness and no later than six months after the violation occurred.
Closing ConferenceThe meeting at the end of an OSHA inspection where the compliance officer confers with the employer and informally advises them of any apparent violations found. Importantly, the closing conference discusses what was observed and the employer's rights — but it does NOT state final citations or penalties, which come later in the written Citation and Notification of Penalty.
Compliance Officer (CSHO)An OSHA Compliance Safety and Health Officer — the inspector who conducts workplace inspections. A CSHO presents credentials (a photo ID and a serial number), holds the opening conference, performs the walkaround, collects evidence, interviews employees, and recommends citations. CSHOs are either safety specialists or industrial hygienists.
ContestAn employer's formal challenge to an OSHA citation, penalty, or abatement date. To contest, the employer must file a written Notice of Contest with the OSHA Area Director within 15 working days of receiving the Citation and Notification of Penalty. Filing sends the case to the independent Occupational Safety and Health Review Commission (OSHRC); missing the deadline makes the citation a final, un-appealable order.
Sources & verification
- 29 CFR 1910.134(d)(3)(i) (eCFR / OSHA.gov)
- 29 CFR 1910.134(b) (eCFR / OSHA.gov)
- OSHA — 29 CFR 1910.134 (Respiratory Protection)
- OSHA — Assigned Protection Factors (Fact Sheet)
Reviewed by HazComFast against eCFR, OSHA.gov, NIOSH, and the Federal Register. Last reviewed 2026-10-05. This glossary is general information, not legal advice; OSHA State-Plan states (e.g. California, Michigan) may adopt stricter requirements.
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