Respirator APF & Maximum Use Concentration (MUC) Calculator

Q: What is an assigned protection factor?

29 CFR 1910.134(b) defines the APF as the workplace level of respiratory protection that a respirator or class of respirators is expected to provide when the employer runs a continuing, effective respiratory protection program. An APF of 10 means the air inside the facepiece is expected to be one tenth as contaminated as the air outside it.

Q: How do you calculate the maximum use concentration?

Multiply the APF by the exposure limit. A half-mask air-purifying respirator has an APF of 10, so against the 50 µg/m³ silica PEL its MUC is 10 x 50 = 500 µg/m³. Above that concentration, that respirator is not an acceptable selection.

Q: What is the APF of an N95?

10. Footnote 3 to Table 1 puts filtering facepieces and elastomeric half masks in the same air-purifying half-mask row, so a disposable N95 and a reusable half-mask respirator both carry an APF of 10 under 1910.134.

Q: Can I use the MUC in an IDLH atmosphere?

No. 1910.134(d)(3)(i)(B)(2) says employers must not apply MUCs to conditions that are immediately dangerous to life or health. Those conditions require the respirators listed in 1910.134(d)(2) — a full-facepiece pressure-demand SCBA, or a combination full-facepiece pressure-demand supplied-air respirator with an auxiliary self-contained air supply.

Q: Does a high APF alone make the respirator compliant?

No. Footnote 2 to Table 1 states that the assigned protection factors are only effective when the employer implements a continuing, effective respirator program as required by 1910.134, including training, fit testing, maintenance and use requirements. Without medical evaluation and a passing fit test, the number on this page is theoretical.

Q: What if the calculated MUC is very high?

1910.134(d)(3)(i)(B)(3) caps it: when the calculated MUC exceeds the IDLH level for the substance, or the performance limits of the cartridge or canister, the employer must set the maximum MUC at that lower limit. An air-purifying respirator is also only allowed against gases and vapors under the end-of-service-life indicator or change-schedule conditions in 1910.134(d)(3)(iii).

Q: Why is the helmet or hood APF shown as 25 or 1,000?

Footnote 4 to Table 1. A PAPR or supplied-air respirator with a helmet or hood only gets an APF of 1,000 when the manufacturer supplies evidence — a workplace protection factor or simulated workplace protection factor study, or equivalent testing — that it performs at that level. Without that evidence it is treated as a loose-fitting facepiece with an APF of 25.

Q: Does the APF override what a substance-specific standard says?

No. Standards such as the silica rule name the respirator required for a given task in their own tables, and the asbestos, lead and cadmium standards carry their own selection tables. Use this calculator for the arithmetic, then confirm the choice against the standard that covers your substance.

Pick a respirator and this calculator returns its assigned protection factor from the 29 CFR 1910.134 Table 1 list, then multiplies that APF by the exposure limit to give the maximum use concentration. Enter a measured airborne concentration and it also shows the minimum APF the job needs and whether the respirator you picked covers it.

Respirator (29 CFR 1910.134 Table 1)

Table 1 is the full list OSHA publishes. Combination respirators take the APF of the mode they are used in.

Exposure limit used

All five presets are 8-hour TWA permissible exposure limits taken from the substance-specific standard.

Exposure limit, if you chose "another substance"

The PEL, STEL or ceiling limit you are selecting against. Ignored when a preset above is selected.

Unit of that limit

Only used with "another substance". The MUC comes out in the same unit as the limit.

Measured airborne concentration outside the respirator

The 8-hour TWA (or short-term result) from your air sampling, in the same unit as the limit. Enter 0 if you only want the MUC.

Results are estimates for planning purposes and do not replace a competent person's evaluation or an engineer's design. Follow the OSHA standards that apply to your work and your employer's written program.

Table 1 — Assigned Protection Factors (29 CFR 1910.134)

Assigned protection factors by respirator type and facepiece, from 29 CFR 1910.134 Table 1
Type of respirator	Quarter mask	Half mask	Full facepiece	Helmet / hood	Loose-fitting facepiece
1. Air-purifying respirator	5	10 ³	50	—	—
2. Powered air-purifying respirator (PAPR)	—	50	1,000	25 / 1,000 ⁴	25
3. Supplied-air (SAR) or airline respirator — demand mode	—	10	50	—	—
3. SAR — continuous flow mode	—	50	1,000	25 / 1,000 ⁴	25
3. SAR — pressure-demand or other positive-pressure mode	—	50	1,000	—	—
4. Self-contained breathing apparatus (SCBA) — demand mode	—	10	50	50	—
4. SCBA — pressure-demand or other positive-pressure mode	—	—	10,000	10,000	—

Employers may select respirators assigned for use in higher workplace concentrations of a hazardous substance for use at lower concentrations of that substance, or when required respirator use is independent of concentration.
The assigned protection factors are only effective when the employer implements a continuing, effective respirator program as required by 29 CFR 1910.134, including training, fit testing, maintenance and use requirements.
This APF category includes filtering facepieces, and half masks with elastomeric facepieces.
The employer must have evidence provided by the respirator manufacturer that testing of these respirators demonstrates performance at a level of protection of 1,000 or greater to receive an APF of 1,000. Absent such testing, all other PAPRs and SARs with helmets or hoods are treated as loose-fitting facepiece respirators and receive an APF of 25.
These APFs do not apply to respirators used solely for escape.

Where the MUC stops applying

Three limits sit on top of the arithmetic on this page, and all three come from 1910.134(d)(3)(i)(B):

IDLH atmospheres. Employers must not apply MUCs to conditions that are immediately dangerous to life or health. Those conditions require the respirators listed in 1910.134(d)(2) — a full-facepiece pressure-demand SCBA, or a combination full-facepiece pressure-demand supplied-air respirator with an auxiliary self-contained air supply.
The IDLH level and the cartridge. When the calculated MUC exceeds the IDLH level for the substance, or the performance limits of the cartridge or canister, the maximum MUC is set at that lower limit.
Oxygen-deficient air. No air-purifying respirator is acceptable, at any APF, in an atmosphere that does not have enough oxygen. That is a supplied-air question.

Air-purifying respirators used against gases and vapors carry a further condition under 1910.134(d)(3)(iii): an end-of-service-life indicator certified by NIOSH for the contaminant, or a documented cartridge change schedule based on objective information or data.

The APF assumes a real respiratory protection program

Footnote 2 to Table 1 is the one that gets skipped: the assigned protection factors hold only where the employer runs a continuing, effective respirator program — written procedures, medical evaluation before use, fit testing for every tight-fitting facepiece, cartridge change schedules, cleaning, storage and training. A worker in a half mask that has never been fit tested is not covered by an APF of 10; the number describes a program, not a piece of rubber. Facial hair along the sealing surface breaks the seal, and no APF in Table 1 survives that.

How it's calculated

Maximum use concentration (MUC): MUC = APF x exposure limit (PEL, STEL or ceiling limit)
The definition in 1910.134(b): the MUC is determined by multiplying the assigned protection factor specified for a respirator by the required OSHA permissible exposure limit, short-term exposure limit, or ceiling limit. The MUC comes out in the same unit as the limit you entered.
Hazard ratio: hazard ratio = measured concentration outside the respirator / exposure limit
How many times over the limit the air is. This ratio is the minimum protection the respirator has to deliver.
Minimum assigned protection factor needed: minimum APF = hazard ratio, rounded up to the next APF published in Table 1 (5, 10, 25, 50, 1,000, 10,000)
1910.134(d)(3)(i)(A): employers must use the assigned protection factors listed in Table 1 to select a respirator that meets or exceeds the required level of employee protection.
Adequacy check: the selection works when measured concentration <= MUC
1910.134(d)(3)(i)(B)(1): the employer must select a respirator that maintains the employee's exposure to the hazardous substance, when measured outside the respirator, at or below the MUC.
Two limits that cap the MUC: MUC used = the lower of (APF x limit), the IDLH level, and the cartridge or canister performance limit
1910.134(d)(3)(i)(B)(3). And under (B)(2), MUCs are never applied to conditions that are immediately dangerous to life or health: those call for the respirators listed in 1910.134(d)(2).

Frequently asked questions