> suffocation (blood acidity induced by excess CO2)
Which, being the budget option, has a failure scenario: When low oxygen occurs without high CO2, people feel fine... until it's too late and they fall over, unconscious.
This can occur in enclosed spaces like caves where some other gas-mix has excluded the oxygen, or when someone hyperventilates before holding their breath.
> This chemoreceptor patent-proposal is kicking my ass. Hundley won't let me down until it's done. Hardly worth filing for, in my opinion. Who wants to smell the difference between xenon and radon?
-- Complaining Versalife employee, Deus Ex (2000)
> This can occur in enclosed spaces like caves where some other gas-mix has excluded the oxygen
Maybe this why some people innate fear of caves, primordial adaptation for suffocation avoidance behavior. I don't know, just fun to wonder about things like that
I would wager a much-bigger feedback loop involves the other creatures that would like to live in those caves and the odds you might get into conflict with them.