Its not just that a large concentration of CO2 dilutes oxygen- in high enough concentrations it can be toxic. 10% of athmospheric volume as CO2 is lethal in min

Its not just that a large concentration of CO2 dilutes oxygen- in high enough concentrations it can be toxic. 10% of athmospheric volume as CO2 is lethal in minutes- concebtrations over 30% are lethal in seconds, which is why in cases of CO2 poisoning at high concentrations victims will not appear to have been distressed- they quite simply did not have time to react. There are approximately 89 cases of CO2 poisoning in the US every year, about 2/3 of whom are themselves emergency rescue personell.