CO2 is toxic for the same reason HCN is (albeit that HCN is far, far more toxic): it binds to hemoglobin, and it does so significantly more readily than O2 does. Therefore prolonged exposure to high concentrations of the stuff, especially in environments which are relatively low on O2 will result in asphyxiation.
CO is particularly toxic because it will readily react with O2 (2CO + O2 -> 2CO2). That means that O2 bound to hemoglobin may wind up being converted to CO2 due to reaction with CO even before it reaches the muscles which were supposed to use it. However, CO2 does most definitely bind to hemoglobin; and this property of hemoglobin facilitates transportation back to the lungs.
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