Global urbanization is occurring at an unprecedented scale. This brings many environmental issues, not least increasing levels of air pollution in cities. However, air pollution is really just a proxy for the key issue here: population exposure to pollution and the public health effects associated with this. In terms of tackling exposure to air pollution emissions, policies should aim to keep people away from the most polluted areas rather than focusing exclusively on containing the root of the pollution itself. In terms of their scale, the effects of coal smoke in the nineteenth and early twentieth centuries were mainly local and regional. But after the Second World War a number of invisible threats began to emerge - acid rain, photochemical smog, ozone depletion and climate change - that were transnational and global in character. It often required the cooperation of scientific experts across both disciplinary and national borders, as well as computer simulation of the impacts of air pollution, to make these new threats ‘visible’ to the public. Global environmental problems also required collective political and legislative action on the part of nations if solutions were to be found.