| This relies on several common misconceptions. Earth's atmosphere is only about 0.03% carbon dioxide. CO2 levels high enough to cause catastrophic global warming still wouldn't displace enough oxygen to matter to humans - it would be a smaller change than a few thousand feet of elevation, which is entirely safe. The second issue: CO2 is not inert in the human body. It's not used for any crucial reactions, but it's more relevant to us than N2 (largely inert at STP, not while diving) and vastly more reactive than He. That's why divers commonly use Trimix (O2, N2, He), and deep divers use Heliox (O2, He). CO2 is present in solution in your bloodstream at all times, partly as an output of chemical reactions and partly absorbed from the air. Among other roles, CO2 levels are detected in human lungs - high CO2, not low O2, is what makes you feel the burning "need to breath" sensation. That's why hyperventilating is a common cause of shallow-water blackouts - it lowers blood CO2 levels more than it increases O2 levels, allowing you to run out of air without feeling it. There's some data suggesting that elevated CO2 levels cause cognitive impairment, which can become quite serious. There's other data (e.g. from nuclear submarines) showing that humans can tolerate CO2 with minimal effect. No one has reconciled this neatly, but it's definitely not true that O2 displacement is the problem. |