system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why does it hurt to hold your breath but you can die in your sleep from carbon monoxide poisoning?

Wow folks, thanks for all the solid answers! I went to workout and came back to more information than I could have ever asked for!
gpt
Your body doesn't actually know how much oxygen is in your blood stream! That's plentiful in our normal environment and not something worth the expense of monitoring, evolutionary speaking. It actually measures the CO2 your body is trying to get rid of and preventing blood toxicity caused by excess CO2 which is an actual daily threat.

CO binds like O2 in your blood, but it's hundreds of times "stickier" and doesn't let go so O2 can't stick. It slowly decreases the amount of O2 your blood can grab until it can't grab any O2, all the while your body thinks everything is great because your CO2 levels are not exceeding dangerous levels. By this point you pass out/die without realizing what is happening.