Breathing air grades, and which one your chamber actually needs

1. The four grades, side by side

Reprint of the CGA G-7.1-2011 gaseous air table, published by an accredited testing laboratory. Grade D and Grade E are the two that matter for chambers and respirators; Grade L and Grade N are here because they are the grades most often confused with them.

2. Grade E does not have a dew point limit

This is the single most useful thing on this page, because a lot of specification sheets quietly supply one. In the 2011 reprint above, Grade E's water content and dew point both read not specified. An independent laboratory reprints the same specification the same way: its SCUBA and sport-diving specification table lists CGA Grade E (G-7.1-2011) as total hydrocarbons 25 ppm, CO 10 ppm, CO₂ 1,000 ppm, oxygen 20–22 %, oil 5 mg/m³, odour "none/slight", and water and dew point both N/A.

The same laboratory's kit selection logic depends on it. Its K201 sampling kit is described as "designed for use with routine breathing air specifications which do not have a limit for dew point, such as CGA Grade D1 or E1," and buyers who need moisture results are sent to a different kit. If Grade E carried a dew point limit, that kit would not exist in that form.

What the 2011 reprint does carry is a conditional note, quoted verbatim: "For breathing air used in conjunction with a self-contained breathing apparatus in extreme cold … a dewpoint not to exceed −65 °F (24 ppm v/v) or 10 °F lower than the coldest temperature expected in the area is required" (TRI reprint). That is a cold-weather SCBA provision. It is not the Grade E number.

3. Which authority asks for which grade

Three different tiers of document are involved here, and merging them is how bad copy gets written. A Code requires row is mandatory language in an adopted code or a federal regulation. A Body recommends row is a professional or accrediting organisation's standard of practice, mandatory only if you are seeking that body's accreditation. Nothing on this page is a Vendor asserts row, because a vendor does not get to choose your grade.

4. UHMS moved from Grade D to Grade E, and older articles predate it

The update log in the Fourth Edition of the UHMS accreditation manual records that the required grade was changed from Grade D to Grade E in that edition. So if you are reading an article, a checklist, or a vendor page that says "Grade D for hyperbaric chambers," check its date: it may simply be older than that change rather than wrong at the time it was written.

One more piece of vocabulary hygiene. Grade N shows up in UHMS commentary comparisons and in medical-air discussions, and it is a real grade — it is the tightest of the four in the table above on CO₂ and on oil. It is not the grade Chapter 14 names for compressor air. Where cylinders rather than a compressor supply breathing air, the code points at Medical Air USP instead, which is a different specification again. We cover that on what the code requires.

5. Grade E is a floor, not a world record

It is worth knowing where Grade E sits internationally, because "meets Grade E" is sometimes presented as an achievement rather than a minimum. Burman's comparison table in the UHMS safety article puts CGA Grade E alongside other national breathing-air specifications (UHMS, Table 1), given here as CO₂ / CO / oil:

• EN 12021 — 500 ppm / 15 ppm / 0.5 mg/m³
• Ph. Eur. 1238 — 500 / 5 / 0.1
• CSA CZ275.1 — 500 / 5 / 1
• AS/NZS 2299.1 — 480 / 10 / 0.5
• HSE DVIS 9 — 500 / 3 / 0.5
• ISO 10083 — 300 / 5 / 0.1

Several of those are materially stricter than Grade E on carbon monoxide and on oil. Grade E allows 1,000 ppm CO₂ where most of that list allows 500 or less. None of this makes Grade E inadequate — it is the number the code names, and clearing it is the requirement. It does mean that a system built with margin against Grade E is doing something meaningful, and that a system that only just clears it is not remarkable.

6. Two notes from the older edition that are still worth quoting

A manufacturer reference sheet keyed to CGA G-7.1-1997 gives Grade D and Grade E limits consistent with the 2011 table, adds Grade J as far tighter — CO 1 ppm, CO₂ 0.5 ppm, total hydrocarbons 0.5 ppm — and carries two notes worth reading verbatim (Air Systems International, CGA Air Grade Specifications):

• On odour: "Measurement of odor is impractical. Air may have a slight odor but a pronounced odor renders the air unsatisfactory."
• On oxygen: the "atm" entry in the table denotes oxygen normally present in atmospheric air; the numeric ranges apply to synthesized air.

That sheet's typical-use table assigns Grade D to OSHA breathing air, Grade E to SCUBA, Grade L to SCBA, and Grade N to medical and USP air, and notes that Grades K, G and M were dropped from the table. The odour note is the honest one: it is the only parameter on the list that a laboratory cannot put a number on, and the only one you can check yourself.

7. What to do with this

If you operate a chamber and someone is quoting you air quality, three questions settle most of it. Which grade, naming the edition? Grade E for a Class B chamber means Grade E plus no condensable hydrocarbons — is that in writing? And what dew point are you specifying, and on what basis, given that Grade E does not set one? A vendor who can answer those three is working from the same documents you now are.

What no one can answer is a question about certification, because there is nothing to certify against. Grade E is a measurable air specification verified by a laboratory sample of the air your machine actually produced on the day it was sampled. It is not a product status, it is not permanent, and it is not granted by NFPA or by anyone else. The next page covers how that sample is taken and what it costs.