Why is air called the "breath of life"?

Air is called the "breath of life" because it contains oxygen, which is essential for cellular respiration in most living organisms. Animals, plants (during non-photosynthetic periods), fungi, and many microorganisms require oxygen to convert nutrients into usable energy through aerobic respiration. This process is far more efficient than anaerobic metabolism, producing approximately 18 times more energy from glucose. Without a continuous supply of oxygen from the air, complex multicellular life as we know it could not exist. Additionally, air contains carbon dioxide needed for photosynthesis, nitrogen that supports protein synthesis (after bacterial fixation), and provides the medium for many essential ecological processes including weather patterns, sound transmission, and temperature regulation. The phrase captures the fundamental dependency of life on Earth's atmosphere.