Why do Deep Sea Divers use Helium Instead of Nitrogen?

Why do deep sea divers use helium instead of nitrogen?

Helium is used by deep-sea and technical divers to allow them to go so deep without suffering from oxygen toxicity and/or decompression sickness. Below the limit of recreational diving, the percentage of both oxygen and nitrogen in the air becomes problematic, so the concentration of these are reduced with the inert Helium making up the difference.

If it makes our voice squeaky and doesn’t react with anything, why on earth would we want to breathe it, let alone dive with it?

Our planet’s atmosphere is approx. 21% Oxygen (O2), 78% Nitrogen (N2) with the remainder being noble gases like Argon (Ar) and Helium (He).

All animals have evolved to use the 21% of O2 as their primary fuel source, so much so that even a short amount of time without it can cause them and us to literally die.

O2 is highly reactive and we use that to power our cells through aerobic respiration.

But what about N2, He and Ar?

Nitrogen does react, but in its N2 form, (two molecules bound together) far less so, but due to its weight, our bodies absorb N2 in all our tissues to equalize with the air we breathe. The other two are essentially inert and we breathe them in and out with no effect.

I think by now the answer may be obvious?

When deep-sea divers go down, there is a point where too much O2 becomes toxic, so we have to limit it. N2 gets absorbed into our bodies and it’s a slow process for it to come out again, so we have to limit that too.

But if we start limiting what makes up the air, then we have to use something else instead right? Indeed and that something is Helium (He).

Deep-sea divers use it to make up the gas blend they are breathing to reduce both O2 and N2.

The pressure of going deep, and when we say deep we are not talking about recreational diving where 30-40m is the depth limit. No, we are talking 70-100m and much more in some cases…so, kinda deep!

Now it’s also important to know that when technical divers are going this deep, they don’t just start sucking on He at the surface – they have a detailed plan and several cylinders containing mixes of gases: air, O2 and He (called Trimix).

Deep sea diver reading a slate

At different depths, the diver will change the mix of what they are breathing so they can safely keep descending.

Due to the increasing pressure that the ever-deepening water exerts on our bodies, at a certain point, the O2 becomes toxic (remember it’s very reactive and starts to react to all different cells in our body in a bad way), so the O2 needs to be reduced to a percentage we still are able to use it, but not overwhelm our system.

Likewise with N2, if we start absorbing it more and more at pressure and while not toxic, this can mean our ascent rate will be incredibly slow to allow it to safely off-gas from our bodies.

So N2 also needs to also be reduced.

These are called ‘partial pressures’ – where the different gasses we breathe change their effect due to the pressure they are under separate from each other.

Here is where Helium becomes the saviour.

The technical divers will reach certain depths and then change their gas mixture to decrease O2 and N2 and increase He as a percentage they take in with each breath.

The deeper they go, the bigger an increase of He is used, likewise, on the ascent – the He is reduced.

It’s as simple as that, He is inert and doesn’t damage us at all – but it does make for some pretty funny conversations between technical divers at depth on their radios!!

How Deep can a Diver go Using Helium?

Around 300m depth Helium will start to cause inert gas narcosis. Helium is usually used from 70-100m down to 300m. It is not used in recreational diving.

Nothing to do with pressure is simple and all technical divers know this.

Even a gas like Helium that is inert and doesn’t react with other molecules or atoms will still have some effect on us if we keep ramping up the pressure.

Of all the gasses we can use for diving, helium is the least likely to cause us any problems. At extreme depth, however, it can cause inert gas narcosis

Helium will eventually affect our bodies and the result is a narcotic effect that can make us feel ‘drunk’. This starts at around 300m, which is the effective limit for He.

Any deeper, and we are talking a very select few divers here, that need or want to go deeper will require Hydrogen (H) to replace the He at this point. The current world record for SCUBA is 332m.

Why do Divers not use Pure Oxygen?

Pure Oxygen becomes toxic at depths below around 6m. The further a diver decends below this while breathing pure Oxygen, the higher the chance they will suffer from Oxygen toxicity or Central Nervous System Oxygen Toxicity (CNS). These are serious conditions that can lead to convulsions and death.

As we have touched on briefly, O2 is a highly reactive gas, look at how it turns iron into rust quickly, that our bodies use to survive. But too much of a good thing is a bad thing.

We can really only breathe pure O2 down to a depth of 6m, after that it will start to react with many tissues in our body to a point it could kill us. This is known as Oxygen Toxicity.

Is Helium used in Recreational SCUBA Diving?

Helium is not used in recreational SCUBA as the depths of 40m or below don’t require it.

When we go SCUBA diving for fun, we are limited to a maximum of 40m in depth (30m with the deep diver speciality, 18m with just the open water certification). At these depths, we generally use air that has been compressed into our tanks. Sometimes we can change that air mix to increase to O2 and reduce the N2 and this is called Enriched Air or NITROX and is used not to go deeper, but to stay down longer.

Why do Divers Want to go so Deep?

Commercial divers generally go to such depths for work. Work such as underwater construction, engineeering or working on oil rigs etc. Technical recreational divers dive this deep for the same reason Hillary climbed Everest, because they can.

Like everything humanity does, we go places because we can. Usually, this kind of deep diving is done by those who work as a diver at these depths.

There are plenty of other reasons to go deep like exploring shipwrecks, exploring cave systems or recovery of recently sunk watercraft. It takes a lot of experience and training to do this type of diving and usually recreational divers will amass quite a lot of dives before they move into this kind of deep technical diving.

Don’t Forget Your Dive Insurance!

Before you go out on any dive trip or holiday, it is essential to make sure you have insurance that covers you if something goes wrong. Check out our dive insurance article for more information.

Or go straight to these dive insurance company websites:


Diver Alert Network (DAN)

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Helium is not only used for party balloons or party tricks, it has many other uses in our society.

Its use for technical and deep diving has become ubiquitous as part of the Trimix that divers need to use to get their bodies down into the depths.

Technical and deep-diving is not something you will always find at your local dive center.

That being said, there are many technical diving clubs or commercial shops that offer introductions or courses for those who want to change up their dives to get deeper into the ocean.

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Paul Fulbrook

Paul Fulbrook is a writer, scuba diver, ex-science teacher and marine biologist. He has a passion for coral reef biology, diving on coral reefs and writing about diving. He also loves cats and his children (sometimes).

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