Oxygen stops being harmless once its partial pressure rises. Above a certain threshold it damages the lungs slowly, and higher still it can cause convulsions with no warning. In professional diving the mix is calculated and continuously monitored to keep the partial pressure within a narrow, safe window.
Anyone starting out in professional diving quickly discovers that oxygen isn't treated as a good where more is better. It's treated like a drug: with a dose, a therapeutic window and known toxicity. The whole architecture of saturation diving mixes revolves around that idea.
What matters isn't the percentage, it's the partial pressure
Breathing air at sea level means breathing oxygen at a partial pressure of around 0.21 bar. That same air at 40 metres depth, where the absolute pressure is five bar, delivers oxygen at 1.05 bar: five times more, with the same percentage in the cylinder. The molecule hasn't changed; what's changed is how many there are per unit of volume and, with that, the load the body receives.
Hence the rule that governs everything: what's controlled is the partial pressure of oxygen, not the fraction. The immediate consequence is that at great depth the mixes contain oxygen percentages that would be asphyxiating at the surface. At 150 metres, keeping a partial pressure around 0.4 bar requires a mix with barely two and a half percent oxygen. That gas would kill anyone at the surface through hypoxia, and it's exactly what the diver needs down there.
The two toxicities
Central nervous system toxicity
This is the acute one, the one that sets the ceiling. It occurs at elevated partial pressures and its most serious manifestation is a tonic-clonic convulsion that often arrives with no clear warning signs. Underwater, with a helmet and an umbilical, a convulsion is a major emergency: there's a risk of losing the gas supply, of drowning and of an uncontrolled ascent.
When there are warning signs, they're usually the ones the English-language mnemonic sums up as VENTID: visual disturbances, ringing in the ears, nausea, muscle twitching —classically in the face and eyelids—, irritability and dizziness. Relying on them as a warning system is a mistake: half of documented cases showed none of them.
The risk depends on partial pressure and exposure time, and increases with exercise, with carbon dioxide retention and with cold. The industry's operating limits leave a wide margin below the experimental threshold precisely because of that variability.
Pulmonary toxicity
This is the chronic one, and the one that shapes life in the chamber. Prolonged exposure to moderately elevated partial pressures irritates the airway lining: dry cough, retrosternal burning, a feeling of tightness and, if it continues, a measurable drop in vital capacity. It's reversible if caught in time, but it builds up over days.
To manage it, the concept of accumulated oxygen dose is used, tracked in units of pulmonary tolerance. The life support technician tracks that tally shift by shift, just as they track the carbon dioxide and humidity balance. On a campaign lasting several weeks, that accounting isn't a formality: it's what allows a team to be kept under pressure without their lung function deteriorating.
The values used in operations
A saturation system runs on two different regimes at once:
- At rest, inside the chamber complex, the partial pressure of oxygen is kept low and stable, around 0.4 bar. The aim is to minimise the accumulated pulmonary dose during rest days, because the diver is going to live there for weeks.
- During work from the bell, that partial pressure is raised somewhat. With more oxygen available, exertion is tolerated better and the excursion's decompression is shortened, and the exposure is a matter of hours, not days.
The adjustment between the two regimes is carried out by the gas panel, which doses oxygen and helium continuously and reads inline analysers. In surface-supplied diving with an air or mixed-gas supply, the same principle applies by limiting the maximum depth according to the mix carried.
Why it's monitored continuously and calculation isn't trusted alone
A mix is calculated on paper, prepared at the cylinder manifold and analysed before it's connected. Even so, it's kept under continuous measurement within the system. The reason is that the diver's consumption, leaks, gas top-up and the dynamics of the circuit itself cause the composition to drift. An analyser with undetected drift, a mislabelled cylinder or a crossed line are failures that have caused real accidents in the industry.
That's why the standard procedure includes redundant analysers, documented calibration, double-checking of connections and logging values at fixed intervals. It's tedious work, and in practice it's the barrier that stops a calculation error reaching someone's lungs. The cross-checking culture described in IMCA guidelines comes from real incidents, not from theory.
Oxygen as treatment
It's worth closing with the other side of the coin: oxygen at elevated pressure is also the main therapeutic tool against decompression sickness and gas embolism. Recompression tables deliberately use high partial pressures, with air breaks interspersed to reduce the risk of convulsion. It's the same substance and the same physics; what changes is that there the dose is administered under medical control, with a benefit that outweighs the risk.
Understanding that duality —a resource and a danger depending on the dose— is one of the first things learned in commercial diver training and one of the things that best explains why professional diving equipment includes so many instruments dedicated to measuring what's being breathed.
The other gases on the panel
Oxygen doesn't travel alone, and controlling it well isn't enough if the rest of the mix is neglected. Two more parameters are watched just as closely.
Carbon dioxide
It's the contaminant that turns dangerous fastest. Excess carbon dioxide causes headache, breathlessness and confusion and, under pressure, markedly increases susceptibility to central nervous system oxygen toxicity. Its source can be a spent scrubber, insufficient ventilation flow in the helmet or, simply, a diver working very hard. It's monitored continuously in the bell and in the chambers, and controlling it is one of the main functions of the life support team.
Hipoxia
The opposite, less-discussed error. A deep mix with two percent oxygen is perfectly breathable at 150 metres and lethal at the surface. That's why procedures for handling deep-mix cylinders include unambiguous labelling, separate storage and verification before connecting: the classic accident isn't breathing too much oxygen down below, it's breathing bottom gas up top.
That's also why ascents and descents aren't simple changes of depth, but changes of gas regime that have to be planned. The correct mix at one depth can be unsuitable twenty metres higher up, and responsibility for that adjustment lies with the gas panel.
Frequently asked questions
What is the partial pressure of oxygen?
It's the fraction of the total pressure that corresponds to oxygen. It's obtained by multiplying the oxygen percentage of the mix by the absolute pressure at which it's breathed. It's this value that determines the physiological effect, not the percentage on its own.
What's the difference between pulmonary and central nervous system toxicity?
Pulmonary toxicity is slow and cumulative: it irritates the airways and reduces vital capacity after prolonged exposure. Central nervous system toxicity is acute and can cause a convulsion with little or no prior warning at high partial pressures.
Why do deep mixes carry so little oxygen?
Because at great depth the absolute pressure multiplies the effect of every molecule. At 150 metres, just two and a half percent oxygen is enough to reach a partial pressure double that of air at the surface.
Can you feel oxygen toxicity coming on?
Warning signs sometimes appear —visual disturbances, ringing, nausea, facial twitching, dizziness— but a significant proportion of documented cases showed none of them. That's why control is based on measuring the mix, not on waiting for symptoms.
What partial pressure is maintained in a saturation chamber?
At rest it's kept low and stable, around 0.4 bar, to limit the accumulated pulmonary dose over the weeks of a campaign. During work from the bell it's raised somewhat, because the exposure only lasts hours.
If oxygen is toxic, why is it used to treat decompression sickness?
Because at elevated pressure it speeds up elimination of dissolved inert gas and improves oxygenation of the affected tissues. The treatment is administered under medical control, with air breaks interspersed to reduce the risk of convulsion.
Published 24 August 2026 · By Arthur More, professional saturation diver
Photo: Royal New Zealand Navy · CC BY-ND via Openverse