A method evaluating techniques for carbon neutrality, such as carbon capture and storage (CCS), is important. This study presents a method to evaluate CO₂ emissions from a cement plant based on atmospheric O₂ and CO₂ measurements. The method will also be useful for evaluating CO₂ capture from flue gas at CCS plants, since the plants remove CO₂ from the atmosphere without causing any O₂ changes, just as cement plants do, differing only in the direction of CO₂ exchange with the atmosphere.