Oxygen, the breath of life or oxygen, the first toxic oxidizing air pollutant

Abstract The element oxygen (chemical symbol 0) exists in air as a ‘ double’ molecule, two atoms being joined together to give O2 (dioxygen). Oxygen was first isolated and characterized between 1772 and 1774 by the individual skills of three great European scientists— Scheele, Priestley, and Lavoisier. Oxygen appeared in significant amounts in the Earth’s atmosphere some 2.5 × 109 years ago, and geological evidence suggests that this was due to the photosynthetic activity of certain microorganisms, the blue-green algae. As they split water to obtain their essential requirement for hydrogen atoms, blue-greens released tonnes of oxygen into the atmosphere, creating perhaps the worst case of environmental pollution ever recorded on this planet.