4.4 Breathing air quality for supplied-air systems
If a respirator brings its own air, that air becomes part of the respirator. Paragraph (i) is the only place in the standard where the protective device can actively deliver a hazard to the wearer — and it is the reason a compressor, a set of couplings and a bed of sorbent are treated with as much care as the facepiece itself.
1 Grade D breathing air
Paragraph (i)(1) requires that compressed air, compressed oxygen, liquid air and liquid oxygen used for respiration accord with two specifications. Compressed and liquid oxygen shall meet the United States Pharmacopoeia requirements for medical or breathing oxygen ((i)(1)(i)). And compressed breathing air shall meet at least the requirements for Grade D breathing air described in the ANSI/Compressed Gas Association Commodity Specification for Air, G-7.1-1989 ((i)(1)(ii)).
That ANSI/CGA specification is a private standard, but the standard itself then prints what Grade D includes, at (i)(1)(ii)(A) through (E):
| ¶ | Parameter | Requirement |
|---|---|---|
| (A) | Oxygen content (v/v) | 19.5 – 23.5% |
| (B) | Hydrocarbon (condensed) content | 5 milligrams per cubic meter of air or less |
| (C) | Carbon monoxide | 10 ppm or less |
| (D) | Carbon dioxide | 1,000 ppm or less |
| (E) | Odor | Lack of noticeable odor |
Note the oxygen range: the bottom of it, 19.5%, is the same figure that defines an oxygen-deficient atmosphere in paragraph (b). Supplied air is not allowed to be worse than the air a room would have to be for the standard to call it deficient.
2 Oxygen is not breathing air
Two short paragraphs prevent a dangerous substitution.
Paragraph (i)(2): the employer shall ensure that compressed oxygen is not used in atmosphere-supplying respirators that have previously used compressed air. Residual oil and hydrocarbon in equipment that has carried compressed air becomes a fire and explosion risk in an oxygen stream.
Paragraph (i)(3): the employer shall ensure that oxygen concentrations greater than 23.5% are used only in equipment designed for oxygen service or distribution. Oxygen-enriched atmospheres make ordinary materials burn violently, which is the same physics behind the prohibition in 1926.353(a)(6) on using oxygen for ventilation, comfort cooling or blowing dust from clothing.
3 Cylinders
Paragraph (i)(4) sets three requirements for cylinders used to supply breathing air:
- 1(i) Cylinders are tested and maintained as prescribed in the Shipping Container Specification Regulations of the Department of Transportation, 49 CFR part 180.
- 2(ii) Cylinders of purchased breathing air have a certificate of analysis from the supplier that the breathing air meets the requirements for Grade D breathing air.
- 3(iii) The moisture content in the cylinder does not exceed a dew point of −50 °F (−45.6 °C) at 1 atmosphere pressure.
The certificate of analysis at (ii) is the practical control on purchased air, and it is a document to keep with the program. The dew point requirement is about ice: moisture in a high-pressure cylinder can freeze at a regulator and block flow at exactly the wrong moment.
4 Compressors
Paragraph (i)(5) requires compressors used to supply breathing air to be constructed and situated so as to:
(i) Prevent entry of contaminated air into the air-supply system — which is a siting requirement as much as a design one. An intake downwind of an engine exhaust, a generator, a loading dock or a paint booth is the classic failure.
(ii) Minimize moisture content so that the dew point at 1 atmosphere pressure is 10 degrees F (5.56 °C) below the ambient temperature.
(iii) Have suitable in-line air-purifying sorbent beds and filters to further ensure breathing air quality, maintained and replaced or refurbished periodically following the manufacturer's instructions.
(iv) Have a tag containing the most recent change date and the signature of the person authorized by the employer to perform the change, maintained at the compressor.
That tag is the visible evidence of the whole system. An untagged filter housing means nobody can say when the sorbent was last changed — and sorbent that is spent is doing nothing at all.
5 Carbon monoxide, the hazard the system can deliver
Two paragraphs handle the specific risk of a compressor producing carbon monoxide.
Paragraph (i)(6): for compressors that are not oil-lubricated, the employer shall ensure that carbon monoxide levels in the breathing air do not exceed 10 ppm.
Paragraph (i)(7): for oil-lubricated compressors, the employer shall use a high-temperature or carbon monoxide alarm, or both, to monitor carbon monoxide levels. If only high-temperature alarms are used, the air supply shall be monitored at intervals sufficient to prevent carbon monoxide in the breathing air from exceeding 10 ppm.
The mechanism is worth explaining rather than asserting: an oil-lubricated compressor that overheats can break down lubricating oil and generate carbon monoxide in the delivered air. A high-temperature alarm is therefore a proxy for the CO risk — and if you use only the proxy, you owe periodic monitoring to confirm it.
Note that 10 ppm figure. It is one fifth of the 50 ppm permissible exposure limit for carbon monoxide in Table Z-1 of 1910.1000 and Table 1 of 1926.55. Air that is delivered to a facepiece is held to a far tighter standard than ambient workplace air.
6 Couplings, containers, and the last two rules
Paragraph (i)(8) prevents the most direct failure mode of all: the employer shall ensure that breathing air couplings are incompatible with outlets for nonrespirable worksite air or other gas systems. And: no asphyxiating substance shall be introduced into breathing air lines.
The incompatible-coupling rule exists because shop air, nitrogen and argon lines are physically present in the same buildings as breathing air, and a fitting that mates is an invitation. Fittings must be different by design, so that a wrong connection cannot be made.
Paragraph (i)(9) closes the section: the employer shall use only the respirator manufacturer's NIOSH-approved breathing-gas containers, marked and maintained in accordance with the Quality Assurance provisions of the NIOSH approval for the SCBA, as issued under the NIOSH respirator-certification standard at 42 CFR part 84.
That is the same principle as (d)(1)(ii) and (h)(4)(i), applied to air: the approval covers an assembly, and substituting a component takes you outside it.
- Grade D breathing air: oxygen 19.5–23.5%, condensed hydrocarbons 5 mg/m³ or less, CO 10 ppm or less, CO₂ 1,000 ppm or less, and no noticeable odor.
- Compressed oxygen is never used in equipment that has previously carried compressed air; above 23.5% oxygen needs oxygen-service equipment.
- Purchased breathing air cylinders carry a certificate of analysis, and cylinder moisture must not exceed a −50 °F dew point.
- Compressor intakes must be sited to prevent contaminated air entering, with in-line sorbent beds and a change tag at the compressor.
- Oil-lubricated compressors need a high-temperature or CO alarm; CO in breathing air stays at 10 ppm or less — one fifth of the workplace PEL.
- Breathing air couplings must be incompatible with other gas outlets, and no asphyxiating substance may be introduced into the lines.
Free educational content — not OSHA-authorized training, no certificate or card issued. Follow your employer's program and the standards cited.