Right. Humans don't have a blood oxygen sensing system; you can be trained to recognise the signs of hypoxia (as divers, military pilots, and astronauts are), b

Right. Humans don't have a blood oxygen sensing system; you can be trained to recognise the signs of hypoxia (as divers, military pilots, and astronauts are), but there's no physiological alarm for it. This has resulted in many accidental recreational and commercial deaths, where e.g. divers hyperventilate beforehand to extend their time underwater (purging more CO2, so they can stave off that chest-crushing feeling of needing to breathe right now) which leads to blackout and drowning because you ran out of oxygen before the high blood CO2 sensing kicked in, or someone walking into an inert gas storage tank to perform maintenance (mistakenly believing that it had been purged), and just dropping on the spot and asphyxiating with no warning.