Dry air is mostly nitrogen and oxygen, with argon in third place and carbon dioxide present in a much smaller proportion. Water vapour is variable, which is why humidity changes so much.
Nitrogen is relatively unreactive under ordinary conditions. Oxygen supports respiration and many oxidation reactions. Argon reacts very little. Carbon dioxide is a small fraction of air but is crucial to photosynthesis and Earth’s energy balance.
Air chemistry also matters indoors. Cooking, heating, ventilation, cleaning products and outdoor pollution can all change the gases and particles we breathe.