Not a constituent of air -
Air is a mechanical mixture of gases. It constitutes nitrogen 78.1%, oxygen 20.93%, carbon dioxide0.03%, argon, neon, krypton, xenon and helium. In addition to these gases, air also contains water vapour, traces of ammonia and suspended matter. Park's Textbook of Preventive and Social Medicine, 25th edition, Page No.791
H2
N2
O2
CO2
Environment and health