In an Open Water class, divers learn about some of the hazards of diving – nitrogen narcosis and decompression sickness usually get the most class time. CO2 retention and CO2 narcosis are not usually talked about except maybe in technical diving classes. This has bothered me since learning about it, because CO2 retention is a real hazard to divers at all levels, but can be avoided if you know how. Knowing about CO2 retention doesn’t just make your diving safer. It makes your diving better even if nothing goes wrong.
Your body is always producing CO2 as part of the metabolic process. Normally, breathing keeps the level of CO2 in your body low – breath in oxygen, breath out CO2. CO2 retention is when your body produces more CO2 than it can remove, and CO2 levels start to build up. When CO2 levels get too high, it can cause impaired thinking, increased risk taking, panic, headaches, and eventually unconsciousness.
The first few symptoms should sound familiar – they’re the same symptoms as nitrogen narcosis. In fact, when a diver gets narced at depth, CO2 narcosis can be a bigger factor than nitrogen narcosis. This is good news, because while you can’t do anything to prevent nitrogen narcosis, there are several things you can do to keep CO2 levels low.
So why is CO2 retention a problem in diving but generally isn’t on the surface? The first reason in that divers use their lungs not just for breathing, but for controlling their buoyancy. On the surface, a person’s breathing is controlled unconsciously by the brain stem, mostly based on how much CO2 is in arterial blood. When the brain stem sees too much CO2, it sends the signal to increase breathing. But underwater, that signal to breath can get overridden by the cerebral cortex that’s using breathing consciously to manage buoyancy. (Hopefully I got the parts of the brain right.)
The other reason that divers are more susceptible to CO2 retention is that they breath from a regulator. Every SCUBA regulator on the market today can provide plenty of gas. But it’s not uncommon to find regulators that are out of adjustment and are not easy to breath from. Breathing from one of these is like breathing through a straw – no matter how hard you try, you can only breath slowly. (At greater depths, gas density can also cause work of breathing issues, but that isn’t a problem at recreation depths.)
So what can you do to avoid CO2 retention?
- Make sure you’re weighted correctly. If you don’t have the right amount of weight, it will interfere with your body’s natural breathing instincts. Not enough weight means you won’t be able to take a fully breath without becoming positively buoyant. Too much weight means you’ll have to put more air in your BCD, which means using your lungs to compensate for small depth changes more.
- Make sure your weights are positioned to put you in horizontal trim. When you’re in horizontal trim, you don’t have to work as hard to swim, and don’t produce as much CO2.
- Make sure your regulator is adjusted correctly – it should be about as easy to breath from as breathing on the surface.
If you get narced shallow than your buddies, or feel like you can’t catch your breath underwater, or get headaches after diving, pay attention. CO2 retention might be the problem.