The constituent of air that acts as a diluent is nitrogen. Diluent refers to a substance that is added to a gas or liquid to reduce its concentration. In the case of air, nitrogen is the most abundant gas and makes up about 78% of the air we breathe. The other gases present in air such as oxygen, carbon dioxide, and noble gases make up the remaining 22%. Nitrogen, being an inert gas, does not react with other elements, which makes it an ideal diluent in air. It helps to lower the concentration of reactive gases, making it easier for us to breathe and for combustion reactions to occur safely.