2.1 Two families: air-purifying and atmosphere-supplying
Every respirator in the world belongs to one of two families, and the difference is not a detail of design — it decides what the device can and cannot do. One cleans the air you are standing in. The other brings you air from somewhere else. Once that distinction is clear, the rest of respirator selection becomes a short list of questions.
1 The two families
Paragraph (b) defines both. An air-purifying respirator means a respirator with an air-purifying filter, cartridge or canister that removes specific air contaminants by passing ambient air through the air-purifying element. An atmosphere-supplying respirator means a respirator that supplies the user with breathing air from a source independent of the ambient atmosphere.
Read the first definition closely and two limits fall out of it immediately. It removes specific contaminants — the filter or cartridge has to match what is in the air. And it passes ambient air — whatever oxygen the room has is what you get. That is why an air-purifying respirator can never be used in an oxygen-deficient atmosphere or an IDLH atmosphere, no matter how good the filter is.
Within the atmosphere-supplying family, paragraph (b) separates two types. A supplied-air respirator (SAR) or airline respirator is one for which the source of breathing air is not designed to be carried by the user — a hose to a compressor or cylinder bank. A self-contained breathing apparatus (SCBA) carries its own air supply.
2 Negative and positive pressure
The second distinction that matters is what the pressure inside the facepiece does when you breathe in.
In a negative pressure respirator, inhalation lowers the pressure inside the facepiece below the outside air. Any gap in the seal — stubble, a scar, a badly fitted strap — becomes a path for contaminated air to be drawn in. Every ordinary filtering facepiece and every elastomeric half or full mask on a filter is a negative pressure respirator.
In a positive pressure respirator, the pressure inside the facepiece stays above ambient. A leak then tends to push clean air out rather than draw contaminated air in. That is why powered air-purifying respirators and pressure-demand supplied-air respirators carry much higher assigned protection factors, and it is why paragraph (d)(2)(i) allows only pressure-demand devices in IDLH atmospheres.
One useful consequence: paragraph (f)(8) requires fit testing of tight-fitting atmosphere-supplying respirators and tight-fitting powered air-purifying respirators to be done in the negative pressure mode, regardless of the mode used for protection. The test measures the seal, and the seal is only challenged when the pressure inside is lower.
3 Facepieces and inlet coverings
Paragraph (b) also defines the coverings, and the vocabulary matters because Table 1 of the standard is organized by them.
A tight-fitting facepiece is a respiratory inlet covering that forms a complete seal with the face. A loose-fitting facepiece does not. A hood completely covers the head and neck and may cover portions of the shoulders and torso. A helmet is a rigid respiratory inlet covering that also provides head protection against impact and penetration.
Tight-fitting facepieces come in quarter mask, half mask and full facepiece. Only tight-fitting facepieces need fit testing — paragraph (f)(1) applies to employees using a tight-fitting facepiece respirator. Only tight-fitting facepieces are defeated by facial hair, which is the subject of (g)(1)(i)(A) and of Part 4 of this course.
The trade-off runs the way you would expect: a hood or loose-fitting facepiece is comfortable, works with beards and glasses, and needs no fit test — but it must be powered or supplied to work, because there is no seal to hold anything out.
4 NIOSH certification and 42 CFR part 84
Paragraph (d)(1)(ii) is short and absolute: the employer shall select a NIOSH-certified respirator, and the respirator shall be used in compliance with the conditions of its certification.
That second clause is not decoration. A NIOSH approval covers a complete assembly — a particular facepiece with particular cartridges and particular parts. Fitting a different manufacturer's cartridge to a facepiece, or replacing a valve with a hardware-store part, takes the assembly outside its approval. Paragraph (h)(4)(i) reinforces it from the maintenance side: repairs shall use only the respirator manufacturer's NIOSH-approved parts designed for the respirator.
Paragraph (j) makes the approval visible: the employer shall ensure that all filters, cartridges and canisters used in the workplace are labeled and color coded with the NIOSH approval label, and that the label is not removed and remains legible. A cartridge whose label has worn off cannot be shown to be the right cartridge, and is therefore not usable.
5 Which family for which problem
| Atmosphere | Air-purifying? | Why |
|---|---|---|
| Particulate below the maximum use concentration, adequate oxygen | Yes, with the right filter | The element removes the specific contaminant |
| Gas or vapor, adequate oxygen, known concentration | Yes, with an ESLI or a documented change schedule | (d)(3)(iii) |
| Oxygen below 19.5% | No | A filter cannot make oxygen; treated as IDLH — (d)(2)(iii) |
| IDLH | No | Only pressure-demand full facepiece SCBA, or SAR with auxiliary SCBA — (d)(2)(i) |
| Unknown | No | Considered IDLH — (d)(1)(iii) |
| Escape from an IDLH atmosphere | Only if NIOSH-certified for escape from that atmosphere | (d)(2)(ii) |
Two of those rows are the ones that get people killed, and both are about not knowing. The standard resolves both the same way: when in doubt, it is IDLH, and IDLH means supplied air.
6 Combination respirators
Some devices operate in more than one mode — an airline respirator with an air-purifying filter for escape, for instance, or a combination cartridge with both a sorbent and a particulate filter.
The note to paragraph (d)(3)(i)(A) is the rule to remember: when using a combination respirator, employers must ensure that the assigned protection factor is appropriate to the mode of operation in which the respirator is being used. An airline respirator worn on airline gives you the airline protection factor; the same device on its filter, during an escape, gives you the air-purifying one.
The same idea applies to cartridges. A combination cartridge protects against a gas and a particulate, and its two elements have different lifetimes: the particulate side loads gradually and increases breathing resistance, while the sorbent side reaches breakthrough silently. Whichever runs out first sets the change schedule.
- Air-purifying respirators clean ambient air and remove specific contaminants; atmosphere-supplying respirators bring air from an independent source.
- No air-purifying respirator is ever acceptable in an oxygen-deficient, IDLH or unknown atmosphere.
- Negative pressure draws leaks inward; positive pressure pushes outward, which is why it earns higher protection factors.
- Only tight-fitting facepieces need fit testing, and only they are defeated by facial hair.
- Respirators must be NIOSH-certified and used within the conditions of the approval; cartridge labels must stay legible.
- With a combination respirator, the protection factor is the one for the mode actually in use.
Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.