Particulate matter smaller than 2·5 µm is a common useful indicator of the risk associated with exposure to a mixture of pollutants from diverse sources and in different environments, including ambient particulate matter pollution from transportation, wind-blown dust, burning of bio mass, and industrial sources; second-hand smoke; burning of biomass and coal for household energy; and active smoking.132,133 However, existing studies cover only small concentration ranges—for example, ambient particulate matter pollution studies have been restricted to yearly average concentrations of particulate matter smaller than 2·5 µm of roughly 5 µg/m3 to 30 µg/m3,134–137 but much higher concentrations of ambient particulate matter have been recorded in polluted cities in Asia and elsewhere. The relation between concentration of small particulate matter and risk of disease is probably non-linear.132,133
