Globally, just over 50 per cent of the world’s population lives in cities; in Australia, it is 90 per cent. In addition, most of the Australian population lives in coastal regions on just 0.22 per cent of the land area (National Sustainability Council 2013). Australia’s population reached 24 million on 16 February 2016. It is expected to double in the next 50 years, with the percentage of the population living in capital cities increasing from 66 per cent in 2011–12 to 74 per cent in 2061. Most capital cities have targets to continue increasing urban population density, with at least half the population growth to be accommodated by urban infill (see the Built environment report). The other half of the population growth is expected to cause the spread of city boundaries, especially in Melbourne and Sydney. This urban spread is increasing the spatial extent of anthropogenic emissions, to occupy a larger portion of the airshed (a region where topography and meteorology limit the movement of air pollutants away from the area). The increasing population is most likely to generate increased emissions, both primary pollutants and precursors that can lead to elevated ozone concentrations. Thus, increases in both population and population density can increase the pressure on ambient air quality. Adverse health outcomes because of increased exposure to air pollutants may be compounded by the greater vulnerability of sensitive populations arising from an ageing demographic.