The extinguishing efficiency of CO2 is primarily due to the reduction of the oxygen content in the air to a value at which the fire is no longer self-sustaining. The cooling effect of CO2 is minimal when compared to this smothering effect. Once a fire has been extinguished it may be necessary, in order to prevent reignition, to maintain the CO2 concentration until hot objects have had time to cool down.
To ensure a fire can be extinguished in a designated area, a calculation of how much carbon dioxide needs to be released to displace the oxygen level below 15% concentration and extinguish the fire.