Understand that the urge to breath is driven by the body’s desire to rid itself of carbon dioxide (CO2)--not (as some assume) your body’s desire to take in oxygen (O2).
Interestingly enough, this isn’t entirely true. If you get a pulse oximeter and a bottle of oxygen you can have some fun with it.
Because of the nonlinearity in the oxygen dissociation curve, oxygen saturation tends to hold pretty steady for a while and then really tank quickly, whereas CO2 discomfort builds more uniformly. In my experience, when I get that really “panicked” feeling and start breathing again, the pulse oximiter on my finger shows my saturation tank shortly after (there’s a bit of a delay, which is useful here for knowing that it’s not the numbers on the display causing the distress).
If it were just CO2 causing the urge to breathe, CO2 contractions and the urge to breathe should come on in the exact same way when breathing pure oxygen, and this is not the case. Instead of coming on at ~2-2.5min and being quite uncomfortable, they didn’t start until four minutes and were very very mild. I’ve broken five minutes when I was training more, and it was psychologically quite difficult. Compartively speaking, 5 minutes on pure O2 was downright trivial, and at 7 minutes it wasn’t any harder. The only reason I stopped the experiment then is that I started feeling narcosis from the CO2 and figured I should do some more research about hypercapnia (too much CO2) before pushing further.
Along those same lines, rebreather divers sometimes drown when they pass out due to hypercapnia, and while you’d think it’d be way too uncomfortable to miss, this doesn’t seem (always) to be the case. In my own experiments, rebreathing a scrubberless bag of oxygen did get uncomfortable quickly, but when they did a blind study on it five out of twenty people failed to notice that there was no CO2 being removed in 5 minutes.
At the same time, a scrubbed bag with no oxygen replacement is completely comfortable even as the lights go out, so low O2 alone isn’t enough to trigger that panic.