Resting the green dragon/the dangers of radical environmentalism.

[Tellos Athenaios]

CO is toxic because the body can't unbind it properly from hemoglobin, unlike O₂ and CO₂. Pure oxygen is used to treat CO poisoning, by flushing out the CO faster.
That conversion thing is bull.

That sentence of mine about CO is a trainwreck, yes. But there are quite clearly two things at work here. One of those is that CO will react with O2 if given the chance, which is not good for your survival prospects (at it lowers the concentration of O2 still further and increase the concentration of CO2). The other is the issue with CO not getting "released" from hemoglobin in a timely fashion.

Now I don't have the numbers to hand, but there are clearly at least two ways for CO2 to kill you: that it binds to hemoglobin and does so more efficiently than O2; and that it will convert to H2CO3 in aqueous environments which in sufficient concentrations will upset the pH buffer of blood. Mind you, the buffer effect is precisely why humans can tolerate acidic conditions a bit better than some other organisms. I am assuming a "normal" habitat here, not the effects you get when you are under water, or subject to low air pressure environments.