The air a commercial diver breathes is an industrial supply with defined requirements, not simply compressed air. The risks — carbon monoxide, oil, moisture and particulates — are controlled through correct intake placement, a maintained filtration chain and documented periodic analysis. Under pressure, any contaminant's effect is multiplied.
There's a part of commercial diving equipment that's almost never discussed and that matters more than many others: the air. Not the equipment that delivers it, but the air itself — its source, its treatment and its quality.
The idea to take on board is that diving breathing air is an industrial supply with specifications, not simply compressed air. And that under pressure, any defect in that specification is multiplied.
Why pressure changes everything
The physical reason is simple. As ambient pressure increases, so does the partial pressure of every component of the gas being breathed, contaminants included. A concentration that would be tolerable at the surface can produce serious effects at thirty metres.
On top of that there's the operational circumstance: the diver can't take off their helmet and breathe something else. They're in a closed system fed from the surface, and if that supply is contaminated, it stays contaminated for the whole dive and for everyone breathing from it.
That's why the quality criteria for diving breathing air are stricter than those applied to general industrial compressed air, and why the same equipment can't be used for both.
The four contaminants that matter
Carbon monoxide
This is the danger that tops every list, and it earns first place for one reason: it can't be detected by the senses. It's odourless and colourless, and its effects — headache, confusion, loss of coordination — appear with no clear warning and are especially treacherous in someone already exerting themselves underwater.
The important thing is that its usual source isn't a compressor failure, but the location of the air intake. Exhaust from an engine, a generator or a vehicle near the intake is enough. On a quay or on a vessel's deck, where there are engines everywhere, that risk is an everyday one.
Aceite
Lubricated compressors can carry oil into the air stream, as vapour or as an aerosol. Breathing it affects the respiratory system and, in the equipment itself, degrades elastomers and fouls the filtration chain.
Compressors designed for breathing air solve this by design — oil-free in the air stream, or with dedicated separation and filtration — and with maintenance that leaves no room for improvising on the type of lubricant used.
Humidity
Water seems like a minor problem, and it isn't. It corrodes the inside of the circuits, saturates the filter media and stops them retaining other contaminants, and can freeze at expansion points. A system with poorly controlled moisture ends up with general air-quality problems, not just a water problem.
Particulates
Rust, gasket debris, drawn-in dust. As well as the direct effect on the airways, they damage valves and regulators, which are precisely the components the diver depends on for flow when it's needed.
The air intake: the cheapest decision, and the most forgotten
If there were only one preventive measure to keep, it would be this: where the compressor's intake is sited.
The intake should be positioned upwind of any source of exhaust gases, raised above deck or ground level, away from engines, generators, traffic and welding work, and at a point where the wind can't change the situation without anyone noticing. That last point is key: a location that's correct first thing can stop being so when the wind shifts or when someone parks a truck next to it.
That's why, on a well-run operation, the intake's position is a checkpoint in every day's pre-dive check, not a decision made once when the equipment is set up. And that's why deck personnel need to know why that hose is where it is.
The treatment chain
Between the compressor and the diver there's a series of components whose job is to deliver air within specification: water and oil separation, particulate filtration, drying and vapour retention. The specific detail depends on the equipment, but the logic is common and has one very clear practical consequence.
Filter media wear out. They don't fail all at once: they saturate, and past a certain point they let through what they should be retaining, with nothing showing from outside. That's why they're replaced on the manufacturer's criteria — running hours or a time interval — it's logged, and it's never put off because "it's still working". Nobody can tell whether it's working just by looking at it.
The record of those replacements is part of the equipment's maintenance system, alongside the compressor checks, and it's documentation a demanding client can ask for. The general logic is covered in the article on inspection and maintenance of diving equipment.
Analysis: the proof that the system works
All of the above is prevention. The check is air analysis, carried out by a laboratory using recognised methods, which verifies that what comes out of the system meets the applicable specification.
How often it's done is set by the country's regulations and the company's management system, and it varies. What doesn't vary are three practical rules: the analysis is documented and the certificate is kept; it's repeated after any significant work on the compressor or a change of location; and on site, the current certificate is documentation you can be asked to produce, in the same way as the certifications personnel's certificates.
Alongside periodic analysis, many systems include continuous monitoring of certain parameters. It's a useful addition, not a substitute: monitoring flags a deviation as it happens; analysis certifies the quality of the whole.
Reserves and back-up supply
Air quality isn't determined by the compressor alone. Every surface-supplied diving system has a gas reserve capable of sustaining the diver if the main supply fails, and that reserve has to meet exactly the same quality requirements.
That means the reserve cylinders are filled with certified air from the same controlled source, and their contents and condition form part of the pre-dive checks. Distribution and control of the supply are managed from the gas panel, and handling it is the responsibility of the deck personnel throughout the dive.
The daily check
All of the above translates, day to day, into a short routine that deck personnel carry out before the first dive, and it shouldn't take more than a few minutes.
The intake location is checked against the day's wind and whatever is around it at that moment: a generator that wasn't there yesterday, a parked truck, welding in progress. The compressor's condition is checked — levels, leaks, abnormal noises — and the separators are purged. The filters' condition and validity are verified against the log, not against the impression of whoever's looking. The reserves' pressure and contents are confirmed, along with their correct connection to the panel. And it's checked that the current analysis certificate is available with the operation's documentation.
That routine has a value that goes beyond immediate safety: it forces someone to look at the system every day. Most air-quality problems don't appear all at once; they build up gradually, with a filter that runs past its hours, an intake that stayed where it was when the wind changed, or a separator nobody purges. The system doesn't give a warning, so the routine has to.
When something doesn't add up, the rule is the same as for any critical item: the equipment isn't used until the doubt is resolved. Nobody has ever regretted delaying a dive to check the air.
When the gas isn't air
In mixed-gas work — and systematically in saturation— the problem widens: as well as the gas's cleanliness, its composition has to be controlled, with the precision the depth demands. The article on heliox and breathing mixtures covers that ground, and helium recycling explains why, in saturation, gas is also treated as an expensive resource.
What doesn't change between disciplines is the principle: whoever breathes from the surface depends entirely on someone on deck having done a job well that nobody sees. Air quality is probably the best example of that dependence, and one of the reasons why, in this profession, the word trust has a very literal meaning.
Frequently asked questions
Why is a contaminant more dangerous underwater?
Because the partial pressure of every gas increases with depth. A concentration of a contaminant that would be tolerable at the surface can produce serious effects under pressure, and on top of that the diver can't simply take off their helmet and go breathe clean air. That's why the limits for diving breathing air are stricter than those for generic industrial compressed air.
What's the most feared contaminant?
Carbon monoxide, because it's odourless, colourless and its effects appear with no clear warning. Its usual source isn't a compressor failure but the location of the air intake: exhaust from engines, generators or vehicles near the intake. It's a siting problem before it's a machine problem.
How often is the air analysed?
How often is set by the applicable regulations and the company's management system, and it varies between countries. What's universal is that the analysis is done by a laboratory using recognised methods, it's documented and kept, and it's repeated after any significant work on the compressor or a change of location. The current certificate is documentation you can be asked to produce on site.
Will any compressor do?
No. A breathing-air compressor is a specific machine, oil-free in the air stream or fitted with adequate separation and filtration, with the corresponding treatment chain and documented maintenance. A generic industrial compressor isn't fit for purpose no matter how much pressure it reaches.
What role does moisture play?
More than it seems. Water in the system corrodes internal components, degrades the filters, can freeze at expansion points and shortens the working life of the whole system. It also ruins the filter media's ability to retain other contaminants, so a moisture problem ends up as a general air-quality problem.
Published 8 September 2026 · By Arthur More, professional saturation diver
Photo: w_lemay · CC BY-SA via Openverse