One other nasty thing about CO that I didn't notice anybody mention is
that the hemoglobin in your red blood cells with preferentially absorb
CO rather than O2 if CO is present. That's part of the reason it's so
toxic. It locks up your hemoglobin and you suffocate for lack of O2. But
it's also an anesthetic (as is CO2) so you just fall asleep. I've been a
runner forever (well since 1961 - close enough). Back in the 70's when I
used to smoke a pipe, I noticed I just didn't seem to have as much
energy the next day. Turns out it takes ~36 hours for CO to equilibrate
out of your hemoglobin when there is no CO present in the air. One pipe
full of tobacco and I was probably 5% impaired. Pretty scary. That's why
they treat CO poisoning with pure O2, they're trying to get the
hemoglobin to let go of the CO.

Bottom line, be very careful around any source of CO. You could end up
dead and never know it.

Jim