CO and CO2
 

Our body produces CO2 through respiration. Well, CO2 does no harm to our body but when we inhale CO, we'll die (that's if it's already saturated in our body).

Both CO and CO2 is just one oxygen atom difference, but why it can do so much difference??

CO has more affinity for hemoglobin then CO2 does.

that I know, but how can you explain in further details??