There is a difference between high and low concentrations of CO2 and reactions. In high concentrations you get similar effect to lack of oxygen (as it completely replaces oxygen in the air), in low concentrations it prevents the HbCO2 from unbinding (only 7% of CO2 in air is enough to do that IIRC) and causes silent death. There are stories of miners sitting down for a lunch and never standing up again because of that.