2.3 Assigned protection factors and maximum use concentration

1 What an APF actually claims

Paragraph (b) defines it: 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.

Two phrases carry the weight. Class of respirators — the number belongs to the class, not to a brand. And when the employer implements a continuing, effective respiratory protection program — the figure is conditional on everything else in the standard. Note 2 to Table 1 repeats it: the assigned protection factors are only effective when the employer implements a continuing, effective respirator program including training, fit testing, maintenance and use requirements.

In plain terms, an APF of 10 means the air inside should be about one tenth as contaminated as the air outside — provided the wearer was fit tested on that class of respirator, has been trained, seal checks it every time and wears it for the whole exposure. Remove any of those and the number is aspiration.

2 Table 1

Paragraph (d)(3)(i)(A) requires employers to use the assigned protection factors listed in Table 1 to select a respirator that meets or exceeds the required level of employee protection. The table is organized by respirator type down the side and by inlet covering across the top.

Three rows are worth memorizing because they cover most work. A half mask air-purifying respirator is APF 10 — and Note 3 to the table states that this category includes filtering facepieces, and half masks with elastomeric facepieces, so an N95 and a half mask with P100 cartridges share the same assigned protection factor. A full facepiece air-purifying respirator is APF 50. A pressure-demand full facepiece SCBA is APF 10,000, which is why it is the IDLH device.

3 The 25/1,000 footnote

The two “25 / 1,000” entries in the helmet/hood column are the most misread part of the table, and Note 4 explains them.

Note 4: 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. This level of performance can best be demonstrated by performing a WPF or SWPF study or equivalent testing. Absent such testing, all other PAPRs and SARs with helmets/hoods are to be treated as loose-fitting facepiece respirators, and receive an APF of 25.

So a hooded PAPR is APF 25 by default. It becomes APF 1,000 only where the manufacturer has provided the evidence, and where the employer holds that evidence. The difference is a factor of forty, and choosing 1,000 without the documentation is a selection error that will not show up until somebody samples.

4 Maximum use concentration

Paragraph (b) defines maximum use concentration (MUC) as the maximum atmospheric concentration of a hazardous substance from which an employee can be expected to be protected when wearing a respirator, determined by the assigned protection factor of the respirator and the exposure limit of the substance. The same definition gives the arithmetic: the MUC can be determined mathematically by multiplying the assigned protection factor specified for a respirator by the required OSHA permissible exposure limit, short-term exposure limit or ceiling limit.

Paragraph (d)(3)(i)(B) then sets three rules around it. (1) The employer must select a respirator that maintains the employee's exposure, measured outside the respirator, at or below the MUC. (2) Employers must not apply MUCs to IDLH conditions; instead they must use the respirators listed for IDLH conditions in (d)(2). (3) When the calculated MUC exceeds the IDLH level for a substance, or the performance limits of the cartridge or canister, the employer must set the maximum use concentration at the lower of those.

A worked example. Respirable crystalline silica has a permissible exposure limit of 50 µg/m³ as an 8-hour TWA under 1926.1153(d)(1). A half mask air-purifying respirator is APF 10. The maximum use concentration is therefore 10 × 50 = 500 µg/m³. If the measured exposure is above that, the half mask is the wrong class of respirator, and you move up the table.

The respirator APF calculator does that arithmetic and compares it to a measured exposure.

5 Where Table 1 does not apply

Note 5 to the table sets the boundary: these APFs do not apply to respirators used solely for escape. For escape respirators used with specific substances covered by 29 CFR 1910 subpart Z, the employer must refer to the appropriate substance-specific standard; escape respirators for other IDLH atmospheres are specified by 1910.134(d)(2)(ii).

Two more boundaries are worth naming. Substance-specific standards may set their own respirator requirements that override a table lookup — Table 1 of 1926.1153 specifies a minimum APF for each silica task directly, and 1926.62 specifies respirators for lead by exposure band. And Note 1 to Table 1 confirms a common-sense permission: employers may select respirators assigned for use in higher workplace concentrations for use at lower concentrations, or when required respirator use is independent of concentration. Going higher up the table is always allowed.

6 Selecting a respirator, end to end

1. 1
   Is it IDLH, oxygen deficient or unknown? If yes, stop — (d)(2)(i) gives you two options and neither is air-purifying.
2. 2
   What is the exposure limit — the PEL from 1910.1000 or 1926.55, or from a substance-specific standard?
3. 3
   What is the measured or estimated exposure? Divide it by the limit to get the protection factor you need.
4. 4
   Pick a class from Table 1 whose APF meets or exceeds it, and check the MUC does not exceed the IDLH level or the cartridge's performance limits.
5. 5
   Pick the element for the physical form — filter class, or cartridge with an ESLI or a change schedule.
6. 6
   Check the wearer can use it: medical evaluation, fit test on that class, and no facial hair at the sealing surface for a tight-fitting facepiece.