noun, the mixture of gases that forms the earth's atmosphere. At sea level dry air has a density of 1.226 kilograms per cubic metre and consists of 78.08 per cent nitrogen, 20.95 per cent oxygen, 0.93 per cent argon, 0.03 per cent carbon dioxide, with smaller quantities of ozone and inert gases; water vapour varies between 0 and 4 per cent and in industrial areas sulphur gases may be present as pollutants

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©HarperCollins 2008. Collins English Dictionary definition of air