Not a constituent of air -
H2
N2
O2
CO2
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
Environment and health