Eli5: cave diving. Why do you need different gas mixtures at different depths?
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It’s not about cave diving specifically, it’s about all diving at depth.
At the surface, you have the weight of the atmosphere that push down and create a pressure for the air that you breath. Underwater, you have the weight of the atmosphere, but also the weight of the water that create an bigger pressure.
So when you breath air underwater, it’s air that is more pressurized than the one you breath at the surface. The biggest problem is nitrogen. You don’t metabolize it, it’s an inert gas, but under higher pressure it create small bubble that accumulate in the tissues of your body. If later if you go up too fast, they won’t be able to dissipate. Under less pressure, these bubble will go up in size inside your tissues and it care tear your tissues apart from the inside. Now even if those bubble are growing, they are still really small, so it’s not like the tears are big, but there is a lot of them. It can injure your organ and create internal bleeding. We call that decompression sickness or the bends.
The second problem is Nitrogen Narcosis. This happen at depth directly. I’m not totally sure what is the biology of this, but AFAIK your body absorbing more nitrogen would affect some neuro receptor and this create an anesthetic effect to the diver, eventually leading to lost of consciousness. The effect is reversible, but at depth this is obviously very dangerous and will lead to death often.
There is several different type of diving mixes to help against that. With normal air you normally shouldn’t go deeper than 200 feet, with 130 feet being the limit of recreational diving. AKA you shouldn’t go deeper than 130 feet without specific training.
There is several type mix that use helium instead of Nitrogen. You still need decompression to get rid of the helium as you go back up after your dive, but you have less risk of Narcosis. That said, it doesn’t eliminate it, Helium will still cause narcosis, it just happen at bigger depth.
Finally, there is different mixes for oxygen. Because at higher pressure, oxygen can become toxic. So some mixes can be hypoxic, aka there is less oxygen than normal air and those allow you to go deeper. But you can’t use those at the surface, because they don’t have enough oxygen at that depth.
There is also hyperoxic mix, aka that have more oxygen than normal air. You can’t use those at depth at all. You can only use those near the surface. But the higher level of oxygen help flush inert gases out of your system at the end of a big dive or in case of emergency.
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