Block 2 · More than just air: deco gases & helium

Oxygen is essential for life – but at high partial pressures it can become toxic. For recreational divers, the most relevant risk is CNS toxicity: a seizure underwater is usually fatal. At the same time, it’s important to remember: within accepted limits, the risk is very low. We are working with deliberately conservative safety margins.

From your Nitrox course you’ll remember that paying attention to the MOD – the maximum operating depth – is absolutely essential. Exceeding 1.4 bar pO₂ during the bottom phase and 1.6 bar pO₂ during decompression should be avoided.
That may be most of what stuck. But when it comes to deco gases, it’s worth revisiting oxygen in more detail.
Please refresh the concepts of MOD and best mix, and take another look at CNS clock, CNS toxicity and pulmonary toxicity before continuing with the text here.

Using Nitrox as a deco gas has major advantages. We’ve already seen, when discussing Gradient Factors, that a low GF Low enforces deeper stops. At these deeper stops, medium and slow tissues continue to on-gas, while only the fastest tissues are already off-gassing.
This creates a fundamental problem when choosing the first stop: off-gassing becomes more efficient when the gradient between inert-gas pressure in the tissues and inert-gas pressure in the lungs is higher. With only one gas, the only way to increase that gradient is to ascend further.
At the same time, the risk of bubble formation increases with rising tissue supersaturation. With a single gas, you have to accept that trade-off. With a deco gas, you can decouple the two.
Reducing nitrogen in the breathing gas – and therefore in the lungs – immediately increases the off-gassing gradient. Medium tissues no longer continue to on-gas, or at least much less. Yet you are still at a depth where bubble formation is less likely.
That is the key advantage of a deco gas: it separates inert-gas gradient from ambient pressure.

On deeper dives, using air as a breathing gas causes two problems (which are closely linked physiologically): nitrogen becomes narcotic with increasing depth, and increased gas density makes breathing harder.
While “deep narcosis” itself is still not fully understood, the physiology of increased work of breathing is well established.
As gas density rises, every breath requires more effort, and maximum achievable ventilation decreases. At extreme depths, this could even lead to a situation where you simply cannot breathe enough.
Long before that, another effect may appear: CO₂ levels in the body can rise. Normally, elevated pCO₂ triggers increased breathing to eliminate it. Underwater, however, some divers – so-called “CO₂ retainers” – respond differently: their bodies tolerate higher CO₂ levels to reduce the work of breathing. Since CO₂ is highly narcotic, this effect is suspected to contribute significantly to the unpleasant effects often attributed to deep narcosis.

Limits for gas density and narcosis

What gas density is still acceptable, and which mix is optimal?
Two factors matter when choosing the best mix for a target maximum depth: equivalent narcotic depth (END) and gas density.
To calculate END, you determine at what depth air would have the same pN₂ as the trimix you intend to use at maximum depth.
Gas density at that depth is calculated from the individual components oxygen, nitrogen and helium.

Helium is much lighter than nitrogen and therefore diffuses into tissues much faster. The Bühlmann model assumes that helium half-times are 2.65 times faster than nitrogen half-times. Decompression therefore has to be calculated differently.
Beginners in trimix diving often assume that decompression should be shorter, because the lighter gas leaves the body faster. That assumption is wrong: decompression is usually longer.
The reason lies in deeper stops. Because helium also enters tissues faster, supersaturation during ascent is greater. To off-gas fast tissues, stops have to begin earlier. During those stops, medium and slow tissues continue to on-gas and later have to off-gas at shallower stops.
With the planner you can observe tissue loading in three heatmaps: nitrogen only, helium only, and the combined inert-gas load.
At this point, you should already be using a different, fully validated planner for real dive planning. This tool is for visualisation only.
Still, you can explore different gases and their effect on runtime, and better understand how helium fraction influences decompression.

How do you actually plan a trimix dive? You’ll find discussion of Gradient Factor choices in the block on current debates and research, and practical planning tips in the dive-planning block.