Once you move beyond basic recreational diving, you quickly discover that divers can breathe a surprisingly wide range of gas mixtures. Nitrox, Triox, Trimix, Heliox, decompression gases — and then there is argon, which you absolutely do not want to breathe.
But why do divers use different gases in the first place? The answer is simple: no single breathing gas is ideal for every depth, workload or stage of a dive.
As pressure increases with depth, the gases we breathe affect the body differently. Oxygen can become toxic at high partial pressures, nitrogen can have a narcotic effect, and the density of the breathing gas increases, making it harder to breathe.
By changing the proportions of oxygen, nitrogen and helium, divers can choose a gas that better suits the dive. This can mean extending no-decompression time, reducing nitrogen narcosis, reducing breathing resistance at depth, managing oxygen exposure or making decompression more efficient.
This is why technical divers may carry several cylinders containing completely different gases on the same dive.
Let's break down what these gases actually are — and, more importantly, why a diver would choose each one.
Important: Gas selection is not something to improvise underwater. The appropriate mixture, maximum operating depth, exposure limits and gas-switching procedures depend on the dive plan, training, decompression methodology, equipment and environmental conditions. This article explains the principles; it is not a substitute for appropriate training or dive planning.
First: What are we actually changing?
Before looking at individual gases, it helps to understand the three main components we are changing: oxygen, nitrogen and helium.
Oxygen
Oxygen is the part of the breathing gas our metabolism needs. But more oxygen is not automatically better. As depth increases, the partial pressure of oxygen (PO₂) increases even though the percentage of oxygen in the cylinder remains unchanged. At sufficiently high partial pressures, oxygen can become toxic and, in severe cases, cause a central nervous system seizure (CNS Toxicity).
For the working or bottom phase of an open-circuit recreational dive, a commonly used planning value is a PO₂ of around 1.4 ATA, while values around 1.2 ATA are used by technical divers, and 1.6 ATA are commonly used for resting decompression exposures. Exact limits can vary depending on the training agency and operational context. This means that a high-oxygen gas can be extremely useful at shallow depths but potentially dangerous at greater depths.
Nitrogen
Nitrogen is the largest component of normal air. Under increased pressure, it also has a narcotic effect, which can become more significant as depth increases.
Nitrogen is an inert gas from a metabolic perspective, meaning that the body absorbs it during a dive and later eliminates it. Managing how much inert gas the body takes up is one of the foundations of decompression planning.
Helium
Helium is also physiologically inert, but unlike nitrogen, it is considered essentially non-narcotic for diving purposes and is much less dense. This makes helium particularly useful as depth increases. A helium-containing mixture can reduce the narcotic effect of the breathing gas and reduce breathing resistance caused by increasing gas density. These benefits become increasingly important when a diver is deep, dealing with a high workload, or using a rebreather.
The downside? Helium is expensive, can be difficult to obtain in some locations, increases heat loss and introduces additional considerations into gas and decompression planning.
Air
Air is the simplest breathing gas, the one we breathe on the surface. Compressed scuba air is approximately:
- 21% oxygen
- 78% nitrogen
- about 1% other gases, including argon and trace gases
For most ordinary recreational diving, air works just fine. It is readily available, inexpensive and suitable for a huge range of dives. Air also provides a useful balance between oxygen exposure and inert-gas loading.
At 30 metres, for example, the PO₂ of air is approximately 0.84 ATA, which is well below the commonly used 1.4 ATA working limit. As depth and workload increase, however, nitrogen narcosis and gas density become increasingly important. Deeper dives also bring more complex decompression requirements. → This is where changing the breathing gas can start to make sense.
Nitrox
Nitrox is a breathing mixture of oxygen and nitrogen with a higher oxygen percentage than normal air. In other words, compared with air, some nitrogen has been replaced with oxygen.
The term nitrox covers a broad range of oxygen-enriched mixtures. In recreational diving, common examples include EAN32 and EAN36, while higher-oxygen nitrox mixtures are also widely used for decompression.
One of the most common recreational mixtures is:
EAN32 = 32% oxygen / 68% nitrogen
Why use Nitrox?
Nitrox has many benefits. The main reasons are to reduce nitrogen uptake or extend the no-decompression limit (NDL). Because nitrox contains less nitrogen than air, the diver breathes less nitrogen and more oxygen at the same depth. This reduces inert-gas uptake compared with breathing air on the same profile.
There are several reasons why Nitrox is popular for recreational diving:
1. Extended bottom time — At a given depth, Nitrox can provide longer no-decompression limits than air.
2. Reduced nitrogen loading — A diver using Nitrox can take up less nitrogen than they would on the same profile using air.
3. Repetitive diving — Lower inert-gas loading can be useful when performing multiple dives over the same day, although the actual surface interval and next dive still need to be planned correctly.
4. Liveaboards and dive holidays — Nitrox can be particularly convenient for divers doing several dives per day.
5. A potential decompression buffer — Using Nitrox while following appropriate air-based limits can reduce inert-gas loading compared with breathing air on the same profile. However, Nitrox does not make decompression sickness impossible and should not be treated as a guarantee against DCS.
Some divers also report feeling less tired after Nitrox dives, particularly during repetitive diving, although individual experiences vary and this should not be considered a guaranteed physiological benefit.
Why not use more oxygen?
Because oxygen itself becomes the limiting factor.
Take EAN32 as an example. At 30 metres, the absolute pressure is about 4 ATA: 0.32 × 4 = 1.28 ATA PO₂, which is below the commonly used 1.4 ATA working limit. At greater depth, however, the PO₂ rises. At approximately 34 metres, EAN32 reaches about 1.4 ATA. → This is why increasing the oxygen percentage does not make a gas suitable for deeper diving. In fact, the opposite happens: the higher the oxygen fraction, the shallower the maximum operating depth for the same PO₂ limit.
So Nitrox gives us less nitrogen → less inert-gas uptake, but also more oxygen → a shallower oxygen limit. That balance is the entire point of gas selection.
Triox
Triox is a three-gas mixture containing oxygen, nitrogen and helium. This makes it closely related to what is generally called Trimix. The terminology is not completely universal, but in many technical diving contexts, Triox refers to helium-containing mixtures with a relatively high oxygen content (more than 21% of O₂), while Trimix is more commonly associated with deeper diving and lower oxygen fractions.
Triox: O₂ > 21% + He
Trimix: O₂ £ 21% + He
One particularly well-known configuration is Triox 30/30 (containing 30% oxygen + 30% helium + 40% nitrogen).
(GUE uses Triox 30/30 as a standard helium-based gas for dives to 30 metres/100 feet in relevant recreational programs.)
So why would someone use helium at a depth where ordinary Nitrox is still available? Because depth is not only about decompression limits. As depth increases, divers must consider factors like nitrogen narcosis, increasing gas density, work of breathing, CO₂ production and retention, cognitive workload and physical exertion. Adding helium addresses several of these factors.
Helium is much less narcotic than nitrogen and significantly less dense. This can reduce the narcotic effect and breathing resistance of the mixture. These benefits can become particularly useful when a diver is swimming against a current, performing physical work or dealing with a high task load.
This is one reason helium-containing mixtures are not exclusively a deep-diving tool. Triox can be used at recreational depths while providing some of the benefits of helium without moving into a hypoxic trimix. Triox is useful for more demanding diving where maintaining mental capacity and managing gas density are important.
Trimix
Trimix is a blend of three gases: oxygen + nitrogen + helium. Exactly like Triox. The difference is mainly in how the mixture is formulated and what it is intended to accomplish.
Once diving gets deeper, the oxygen fraction is reduced to keep the PO₂ within an acceptable range, while helium replaces a larger proportion of nitrogen. A trimix is normally written using its oxygen (first number) and helium (second number) percentages. For example:
Trimix 21/35
- 21% oxygen
- 35% helium
- 44% nitrogen
Or:
Trimix 12/65
- 12% oxygen
- 65% helium
- 23% nitrogen
These gases are dramatically different from air even though they all contain oxygen, nitrogen and/or helium.
Why use Trimix?
Simply, to make deeper diving more physiologically manageable. At depth, breathing regular air can produce significant nitrogen narcosis, while gas density also increases. Helium allows the diver to replace some of the nitrogen while also reducing the density of the breathing gas.
But there is another problem. At great depth, you cannot simply keep the oxygen percentage high enough for comfortable surface breathing. The deeper you go, the more the oxygen fraction may need to be reduced to keep the PO₂ within the planned limit. That is where normoxic and hypoxic Trimix come in.
Normoxic Trimix
“Normoxic” generally refers to a trimix with enough oxygen to support breathing at the surface or near the surface.
A commonly used practical boundary is around 18% oxygen or higher, although exact definitions vary between training organizations. Normoxic mixture might therefore contain around 18–21% oxygen. (Some agencies define Normoxic Trimix between 16–21% oxygen.)
Normoxic Trimix is typically used for dives in the 30–60 metre range and contains enough oxygen to be breathable from the surface. It allows divers to benefit from helium at greater depths while still maintaining a sufficient oxygen content for surface and shallow-water breathing.
Hypoxic Trimix
Hypoxic means that the mixture contains too little oxygen to be safely relied upon as a normal surface breathing gas. The exact boundary used to define “hypoxic”, again, varies between diving organizations and contexts. For practical technical diving, mixtures below approximately 18% oxygen are often treated as hypoxic, although some sources use different thresholds. The important point is not the exact number, but whether the gas provides enough oxygen at the pressure where it is being breathed.
For example, a Trimix containing only 10% oxygen produces a very low PO₂ at the surface and is not suitable as a normal surface breathing gas. At depth, however, ambient pressure increases the PO₂. At 90 metres, for example, the ambient pressure is roughly 10 ATA:
0.10 × 10 = 1.0 PO₂
Suddenly, the same gas that would be hypoxic at the surface has a perfectly usable oxygen partial pressure at depth.
Hypoxic Trimix is used because deep diving requires low oxygen fractions. The deeper you go, the lower the oxygen percentage can be while still maintaining an appropriate PO₂. At the same time, helium can replace large amounts of nitrogen, helping to reduce the narcotic and density effects of the breathing gas.
But hypoxic gases create an important additional requirement:
You need a way to get to the depth where the gas becomes breathable.
Hypoxic Trimix is typically used for dives below 50–60 metres and doesn’t contain enough oxygen to be breathable from the surface on open circuit. This requires a separate travel gas with a higher oxygen content. The diver descends on the travel gas, reaches the depth where the hypoxic bottom gas has an appropriate PO₂, and then performs the planned gas switch. This is one reason gas management becomes increasingly complex as technical diving gets deeper. Rebreathers make this process simpler (no gas switches, only managing the PO₂).
Heliox
Heliox is exactly what the name suggests: helium + oxygen. There is no nitrogen in the mixture. Because nitrogen is removed completely, Heliox can be useful for very deep diving where nitrogen narcosis and gas density become major concerns.
Helium is essentially non-narcotic and much less dense than nitrogen, making it suitable for extremely deep operations. The oxygen percentage is then selected according to the pressure and exposure requirements. There is no single oxygen percentage that defines Heliox. A very deep heliox mixtures indeed contain way less than 10% oxygen, but other heliox mixtures (in commercial and saturation diving) can contain significantly more.
Helium has disadvantages too. It is expensive, it increases heat loss and it requires careful planning and logistical support. And at very great depths, heliox can contribute to high-pressure nervous syndrome (HPNS), one of the reasons nitrogen may deliberately be retained in some extremely deep mixed-gas operations.
So, once again, there is no universally “best” gas.
Decompression Gases
Technical divers don't necessarily use the same gas all the way to the surface. The gas that is appropriate at depth can become less efficient for decompression, while a gas with a high oxygen percentage that is useful during decompression could be unsafe to breathe at depth.
This is where decompression gases come in.
During ascent, the diver moves into shallower water, where a higher-oxygen gas can produce a higher oxygen partial pressure without exceeding the planned oxygen exposure. The goal is to use the higher oxygen fraction to support more efficient elimination of inert gases during decompression. Technical divers may carry one or more decompression gases depending on the dive. Two common examples are:
Nitrox 50
(50% oxygen / 50% nitrogen)
A classic use is to switch to 50% oxygen at around 21 metres / 70 feet, where the PO₂ is approximately 1.55 ATA. This is within the commonly used 1.6 ATA decompression PO₂ limit. The exact gas-switch depth depends on the planned PO₂ limit, gas composition, dive plan and decompression methodology.
100% Oxygen
Pure oxygen is commonly used during the shallowest part of decompression. A suitable gas-switch depth for this gas is at 6 metres / 20 feet. At this depth, breathing 100% oxygen produces a PO₂ of about 1.6 ATA.
At shallow depths, a high oxygen fraction can create a high oxygen partial pressure without exceeding the planned decompression limit. At the same time, the diver is breathing little or no nitrogen. That means the gradient driving inert gases out of the body can be increased, making decompression more efficient. This is the basic principle behind accelerated decompression.
But why not breathe oxygen at 30 metres?
Because the same gas that is useful at 6 metres can become dangerously toxic at depth.
At 30 metres: 100% O₂ × 4 ATA = 4.0 PO₂
That is vastly beyond accepted diving exposure limits. This is one of the most important principles in mixed-gas diving: A gas can be exactly right at one depth and dangerously wrong at another. That is why technical divers analyze their cylinders, label them, plan their gas switches and maintain strict gas discipline. Breathing the wrong gas at the wrong depth can be fatal.
Argon — the gas you don't breathe
Not every cylinder attached to a technical diver contains a breathing gas. Some divers carry a small cylinder of argon. Argon is an inert gas and is not a breathing gas. It is used by divers to inflate a drysuit (or even a wing). Laboratory and field studies have investigated argon as a drysuit insulation gas, and its physical properties make it attractive as an insulating gas, particularly for cold-water and helium-based diving. However, the real-world thermal benefit of argon depends on many factors, including the drysuit, undergarments, water temperature, exposure time and how the suit is used. Using helium to inflate a drysuit would be particularly undesirable from a thermal perspective. Helium transfers heat away from the body efficiently.
So why are there so many gases?
Because there are many problems divers experience underwater.
→ Too much nitrogen,
→ Too strong narcosis,
→ Too dense gas,
→ Too high PO₂,
→ Innapropriate deco gas,
→ Inapropriate inflation gas…
Every gas comes with a compromise.
Increasing oxygen can reduce nitrogen loading — but reduces maximum operating depth. Adding helium can reduce narcosis and gas density — but increases cost. Reducing oxygen allows deeper diving — but may make the gas hypoxic at the surface. Increasing oxygen during decompression can accelerate inert-gas elimination — but can be dangerous if breathed at excessive depth or for extended duration.
“The best” gas depends on the problem. Gas is a tool. And like every other piece of diving equipment, the tool only makes sense when you understand the problem it is solving.
Want to go deeper?
Understanding breathing gases is only one part of technical diving. Our approach to technical diving combines gas planning, equipment configuration, team awareness, decompression strategy, and human performance.
Explore our technical and GUE training programs to see how these concepts are applied in real diving — from nitrox-based recreational diving to trimix CCR diving and decompression diving.
