O2

Oxygen, or O₂, is a colorless, odorless gas that makes up about 21% of Earth's atmosphere. It is essential for most living organisms because it allows cells to produce energy through a process called respiration. In this process, oxygen is inhaled into the lungs, transferred to the blood, and carried to cells, where it helps convert nutrients into usable energy. Without oxygen, cells and organs can't function properly, leading to life-threatening conditions. Oxygen is also used medically to treat various health issues and is vital for combustion and industrial processes.