The Science of Breathing Underwater

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In this video, we explore the science of breathing under water using different mixes of breathing gasses.

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Out of the Skies, Under the Earth by Chris Zabriskie is licensed under a Creative Commons Attribution license (


Air is composed primarily of nitrogen. There’s also healthy dose of oxygen, and approximately 1% is other gasses.
This gas mixture works great for humans when they are on land, and the atmospheric pressure isn’t too high or low. But when scuba diving, especially deep diving, regular old atmospheric air doesn’t work so well.
You see, as you dive deeper and deeper, the ambient pressure around you starts to increase considerably. And, as you breathe from the scuba equipment, your lungs automatically equalize and become more pressurized to match its surroundings.
But the increased pressure in your lungs can force nitrogen to dissolve into your bloodstream. This is why divers have to surface so slowly. The nitrogen needs to leave body’s tissues and bloodstream in a slow controlled manner.
If a diver tries to surface too quickly, the dissolved nitrogen comes out of solution inside the body in the form of bubbles. These nitrogen bubbles can cause many symptoms; most commonly joint pain and itchy rashsy skin. This is known as Decompression Sickness, or The Bends, and in severe cases it can lead to paralysis or even death.
So, in order to reduce the risk of Decompression Sickness, the air that divers breathe can be replaced with a slightly altered gas mix known as nitrox.
To create this nitrogen and oxygen blend, and a good chunk of the nitrogen is replaced with additional oxygen. Because there is less nitrogen in this mixture, it is able to reduce nitrogen uptake in the body’s tissues.
However, this tactic only works for relatively shallow diving. If one continues to dive deeper, below 30 meters or so, we run into another problem.
At 50 meters, the ambient pressure is about 5 times as great as surface pressure. This means that the lungs would contain approximately five times as many nitrogen and oxygen molecules as they normally would.
Breathing atmospheric air at this depth can lead to Nitrogen Narcosis, which impairs cognitive function, and is similar to that of inhaling nitrous oxide.
Replacing the nitrogen with oxygen, as in the case of nitrox, doesn’t really help either, because, instead of nitrogen narcosis, the extra oxygen can lead to oxygen toxicity; which effects the central nervous system with side effects such as twitching, altered vision, and nausea.
To combat this, both the nitrogen and oxygen must be reduced and replaced with an inert gas, such as helium, which has very little, if any, narcotic effect.
This three gas blend is known as Trimix.
For even deeper dives, the Trimix recipe can be altered by having less nitrogen and oxygen and even more helium.
The nitrogen can even be eliminated altogether, creating a mix known as Heliox. But for especially deep dives, breathing this much Helium presents a new problem: HPNS or high pressure nervous syndrome.
This disorder is still not fully understood, but it has been found that including another gas in the mixture, such as the nitrogen in Trimix, suppresses the effects of HPNS.
The only problem with using Trimix at this depth is that, at this extreme pressure, molecular nitrogen and oxygen, which are somewhat heavier than the helium, become hard to breathe.
Although the oxygen can’t be gotten rid of completely, the nitrogen can be replaced with hydrogen, the lightest element, which is much easier to breathe. This blend is known as Hydreliox.
Even the helium can also be replaced with hydrogen, creating a mixture known as Hydrox.
Both of these blends are used for extremely deep dives, and have been tested to simulated depths in excess of 700 meters.
Although this video has covered a good portion of the breathing gas mixtures utilized by divers, there are still more exotic breathing gasses that were not covered. Regardless, I hope you’ve learned exactly why deep divers can’t rely on atmospheric air alone.


  1. Alex Arias says

    Very interesting video. I watched a video recently about a driver who had was at the blue hole. He had a pure oxygen tank and drowned at the bottom of the ocean floor. The video narrator briefly explained narcosis and oxygen toxicity and I was intrigued but also thoroughly confused. Thank you for this information! The one thing I'm still not getting is why the amount of oxygen and nitrogen increase as we go down deeper in pressure. Do our lungs shrink down there??

  2. IDNeon357 says

    The world record dive is 2300m. Not 300m. Its important to mention this…scuba isnt the only form of Diving

  3. Ranma Saotome says

    Very well explained but i didn't understand a thing, i suck at Science

  4. Christian Grosser says

    Small correction: At 50m below the water surface the pressure is ca. 6 bar (atmospheric pressure + pressure from the water column). (Around 1:32)

  5. Humble Servant says

    After 30m all I hear is “God doesn’t want u swimming here”

  6. Soccer Kid231 says


  7. Demis Farrugia says

    Best video I've ever seen capturing the aspects of Rec and Tec diving straight to the point. Brilliant… well done! <3

  8. AV2 Tech2 says


    I am a project coordinator with Smartbook Media Inc. We produce educational books for the school and library market. We have previously contacted you about some of your videos for a digital learning environment for elementary school children, Corinna Yd Lightbox (

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  9. Jeric Ns says

    Well explained. Thanks for posting.

  10. Temple Jax says

    People have dived to 700 metres with hydrox?

  11. Yo, i'm Alex says

    good one!

  12. HerringHome says

    how far down can you go with natural atmospheric air?

  13. Mikki Hintikka says

    why not pure o2?

  14. Norsie says

    This was a really good video 🙂

  15. Aaron Leroux says

    Great video

  16. Aaron Leroux says


  17. ultimatdestroier says

    Isnt Hydrox explosive? Wouldn’t it explode just because of pressureizimg? I know on Hydreliox the helium prevents the oxigen and hydrogen to react to water.

  18. Σeegma says

    2:56 Having hydrogen and oxygen in the same mixture doesn't sound like a good idea.

  19. Sparryax says

    I’ve been diving for a year. Ask any questions!

  20. Shriya Shukla says


  21. Bill Latibay says

    For more deeper dive use fartox

  22. FAS Ligand says

    So I've got a question. "at 50m the ambient pressure is about 5 times as great as the surface pressure, this means thr lungs will contain five times more oxygen than they normally would"

    Now we're talking about the hydrostatic pressure, right? The pressure acting on the outside of the diver, not on the lungs? At least that's how I imagined it. I thought we have really high pressure air in the tank on our back, that we then convert the air through some clever plumbing and valves into the same gas at lower pressure, which we can then breathe in.

    The hydrostatic pressure can only compress us from outside, making it harder to increase lung volume by inhaling

    I think I'm terribly wrong now

  23. ElMirc says

    So hardcore divers breathe fuel-rich rocket fuel? That's even more hardcore!!

  24. I want to dive but this sounds complicated.

  25. brettj 98 says

    Wanted a video that explained depth vs gas mixes out of curiosity. Its crazy how much the game changes the deeper you go. Gases you consider harmless start to kill you. Nice video and channel man.

  26. subscribed

  27. LEE gizzle says

    But why can't u just breathe pure oxygen

  28. EliteTester says

    So hydrox is basically rocket fuel?

  29. Gearz Pop says

    Is that why you can't smoke when scuba diving?

  30. BuckeyeStorms says

    Very cool video, and the fact that an ad about learning to dive came on before hand really has once again awakened my interest in becoming a SCUBA diver: an idea which pops in my head every couple years.

    At around the 2:30 mark, I couldn't help but feel this had become an ad for some new drug on the market. I guess it was the combination of a disorder, distilled down to an acronym, the tone of your voice, the background music, and the, "we still don't know the cause but (insert brand name sounding word) can help."

    I had to go back a couple times afterwards, and rewatch the end, before I could get past that point without my brain switching into, "ignore the ad," mode. Haha.

  31. Daniel Kitchka says

    awesome video! really good!!

  32. Shankar Ramachandran says

    Fun science project kids : Take a Hydrox tank , open the valve, light it on fire and you have a Rocket!

  33. Stephen Clark says

    That's pretty wild that someone can breath a mix of mostly helium with just a touch of oxygen. I guess the important part is that the necessary percent of oxygen is there.

  34. wumbology says

    The bends? Now I have to listen to some Radiohead!

  35. Nutcake says

    If you were to open a Hydrox container at surface level, wouldn't some of the oxygen and hydrogen combine to form water?
    Does the high pressure when diving prevent this reaction?

  36. Griffin says

    Hydrogen and oxygen under pressure? Better hope nobody lights a cigarette near that tank.

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