Are Scuba Tanks Filled with Pure Oxygen?

Scuba tanks are not filled with pure oxygen. They are usually filled with compressed purified air, but there are less popular specialized mixes like Nitrox or Trimix that aid diving at greater depths.

While pure oxygen is problematic for breathing, it has its uses. Keep reading to learn how it differs from breathing gasses and what other elements you find in your scuba tanks.

Contents hide

1 Do Divers Breathe Pure Oxygen?

1.1 Uses for Oxygen

1.2 Are Oxygen Tanks Filled with Pure Oxygen?

2 Why Are Scuba Tanks Not Filled with Pure Oxygen?

2.1 Oxygen Toxicity

3 Are Scuba Tanks the Same as Oxygen Tanks?

4 What Is the Air in a Scuba Tank Made Of?

4.1 The Chemical Composition of Air

4.2 Nitrox for Scuba Tanks

4.3 Trimix for Scuba Tanks

4.4 Heliox for Scuba Tanks

4.5 Hydrox for Scuba Tanks

4.6 What Percentage of Oxygen Is in a Scuba Tank?

5 References

Do Divers Breathe Pure Oxygen?

Divers do not breathe pure oxygen. While humans need oxygen for metabolic processes, it makes up a small (but still major) part of the air that we breathe. Most of our atmospheric air is nitrogen, followed by about 21 percent oxygen.

Divers breathe a compressed and purified version of the air that they breathe on land. While there are uses for pure oxygen, oxygen toxicity is a concern and a driving force for specialized training with gas mixes that contain a higher percentage of oxygen.

Uses for Oxygen

You will probably run into pure oxygen tanks on your dive boat, but these are not for use while diving. These tanks are used for emergency medical support, usually if someone comes aboard with symptoms of nitrogen narcosis or DCS.

Hyperbaric oxygen therapy is another common application tied to diving. This type of treatment sets you up in a pressurized chamber or tube, allowing you to get as much as 3 times the oxygen you would normally breathe. Divers use these to treat air or gas embolisms and decompression sickness.

Medical oxygen is tightly regulated. Any pure medical oxygen cylinders must be inspected to ensure there are no contaminants, including other types of gasses, as this can cause adverse reactions during treatment.

Are Oxygen Tanks Filled with Pure Oxygen?

A tank labeled as an oxygen tank is likely filled with pure oxygen, but these are not the same as the scuba diving tanks you take with you into the water.

Why Are Scuba Tanks Not Filled with Pure Oxygen?

Scuba tanks that you take into the water are not filled with pure oxygen because that would be unbreathable air. Our bodies can only handle oxygen at a certain percentage, and anything greater than this amount can lead to issues such as:

Harm to fats, DNA, and protein in the body
Eye problems
Lung damage

Even diving with a tank that has a slight increase in oxygen percentage can cause problems the deeper you dive, notably oxygen toxicity.

Oxygen Toxicity

Oxygen toxicity refers to how increases in oxygen partial pressure affect the protective systems of your body. The increase in partial pressure can cause biochemical reactions that are alarming and sometimes lead to long-term effects.

Pulmonary oxygen toxicity is rarely permanent and feels like a bad case of the flu. Oxygen toxicity of the brain, also referred to as CNS oxygen toxicity, is what puts divers at serious risk for long term complications.

Symptoms include:

Flashing lights in front of your eyes
Tunnel vision
Tinnitus (loud ringing or roaring in your ears)
Confusion
Lethargy
Nausea
Vertio
Numbness
Tingling
Twitching muscles (usually lips)

Grand mal convulsions are one of the most violent of these symptoms. While they are more noticeable, they are also much more difficult to accommodate during a dive.

Are Scuba Tanks the Same as Oxygen Tanks?

Scuba tanks are not the same as oxygen tanks.

Scuba tanks are filled with gasses suitable for human consumption. This is most often compressed air, which contains the same ratio of gasses as atmospheric air, but there are specialized gas mixes that help divers in certain conditions.

Oxygen tanks are filled with pure oxygen, and they’re most commonly used for medical treatment. Because this is hazardous for prolonged breathing, oxygen tanks are not something you would take in the water to dive with.

What Is the Air in a Scuba Tank Made Of?

The composition of the air in a scuba tank depends on what gasses you’re mixing. Compressed air holds the same gasses as atmospheric air, just under greater pressure. Your specialized mixes often limit their fill to two or three gasses for efficiency.

The Chemical Composition of Air

Air is made up of many elements, with nitrogen, oxygen, and argon the most abundant.

Element(s)	Percentage Composition
Nitrogen	78.084%
Oxygen	20.9476%
Argon	0.934%
Carbon Dioxide	0.04%
Neon	0.001818%
Methane, Helium, Krypton, Hydrogen, Xenon, Ozone, Nitrogen Dioxide, Iodine	< 0.001% (in size order)
Carbon Monoxide, Ammonio	Trace

These elements also set the framework for the specialized gas mixes that we’ll talk about below. The air that you use in your scuba tanks focuses on elements with more presence in air, including nitrogen, oxygen, and helium.

Nitrox for Scuba Tanks

Nitrox includes higher levels of oxygen than compressed air, and it’s mostly a mix of nitrogen and about 32 to 36 percent oxygen (often labeled EAN32 or EAN36 accordingly). This allows more oxygen absorption than nitrogen into the bloodstream, reducing your chances and the severity of decompression sickness.

You can stay at depth longer, but you cannot travel as deep as you would with air. Nitrox works well for repetitive diving.

It’s still your responsibility to make sure your tank has a mixture appropriate for your dive and work alongside dive tables and other standards. If you’re working with a dive shop that doesn’t have extensive knowledge working with Nitrox, there’s an extra degree of risk, but taking responsibility for your equipment should keep you safe.

Trimix for Scuba Tanks

In Trimix, your oxygen and nitrogen are partially replaced by helium. This increases your recreational depth limits and decreases your risk of oxygen toxicity, and you can use different blends to suit your dive.

Hyperoxic blends have a higher level of oxygen than air, and you’re restricted to shallower depths compared to air. Normoxic blends have the same level as air, and allow diving to the same depth, while hypoxic blends have less oxygen than air and allow you to dive deeper.

Hypoxic blends cannot be used at the surface, so you’ll use travel gasses until you get to a depth you can switch at.

Heliox for Scuba Tanks

Heliox is similar to Trimix. It has either just as much or less oxygen than regular air, but it completely replaces nitrogen with helium.

Heliox allows you to extend your depth and time limits as the lower oxygen level reduces your risk of toxicity. Like hypoxic blends, you cannot use it at the surface, so you must use a travel gas like air or Nitrox at shallower depths.

Heliox is a poor temperature conductor, and it can increase your loss of body heat while using. Helium is also a much rarer element, so Heliox is more expensive than mixes that blend with nitrogen.

Hydrox for Scuba Tanks

Hydrox has a goal similar to Heliox, but it uses hydrogen instead for cost effectiveness. Hydrox has little to no toxicity issues, and it can be used at all depths.

Hydrogen is much more flammable than other elements, and pairing it with oxygen creates an extremely flammable tank. It’s also relatively new, so it’s mostly used by extreme commercial divers traveling beyond the limits of Trimix and Heliox.

What Percentage of Oxygen Is in a Scuba Tank?

The oxygen percentage in a scuba tank depends on the blend you’re using.

Scuba Tank Mixture	Percentage Oxygen
Compressed Air	20.9476%
Nitrox	22 to 40% (usually 32 to 36%)
Trimix	Same as air (normoxic); More than air (hyperoxic); less than air (~10%) (hypoxic)
Heliox	20% or 30%
Hydrox	4%

Because your specialized blends differ so greatly from compressed air, you need special training to understand how to use them properly and what to look out for.