A correlation between air pollution and its effects on health was first observed in 1952 in London, England, when 4,000 persons died from respiratory diseases aggravated by a high concentration of acid smog. The term "smog," which has for some 30 years been used to mean chemical air pollution, is a contraction of the words "smoke" and "fog." Smog is formed mainly above urban centres by concentrated human activities that include the combustion of fossil fuels and the smelting of ores. Smog is composed mainly of tropospheric ozone, produced by a photochemical reaction between volatile organic compounds and nitrogen oxides; primary particulate matter such as pollen and dust; and secondary particulate matter such as sulphur oxides, volatile organic compounds, and ammonia gas. The severity of smog is usually assessed by measuring ground-level ozone.