The colourless, odourless mixture of gases that forms the Earth’s atmosphere. Air consists of 78 per cent nitrogen, 21 per cent oxygen, small quantities of carbon dioxide and other gases, and some water vapour.
Besides these, there are always ozone, minerals and organic matter present in small and variable amounts, and more or less water vapour according to the weather. In the air of towns, sulphurous acid and sulphuretted hydrogen are important impurities derived from combustion. After air has been respired once, the oxygen falls by about 4 per cent and the carbonic acid rises to about 4 per cent, while organic matter and water vapour are greatly increased and the air rises in temperature. The cause of the discomfort felt in badly ventilated rooms and crowded halls is associated with the increase in the temperature and moisture of the air, but a high percentage of carbon dioxide may be present without causing any noticeable discomfort or appreciable quickening of the respiration. A combination of hot weather and emissions from vehicles and fossil-fuel combustion produces pollutants linked to a rise in the incidence of ASTHMA and other cardiorespiratory conditions. Falling levels of ozone in the upper atmosphere are also believed to contribute to global warming because ozone screens the earth from most of the sun’s harmful ultraviolet radiation.
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