The main greenhouse gas causing climate change is carbon dioxide - a colourless, odourless gas that is otherwise harmless to our health. But emissions of CO2 are often accompanied by other, toxic gases. So can we tackle air pollution - which kills 7 million people a year around the world - at the same time as we fight climate change? We hear from three cities - Buenos Aires, London and Singapore, that are doing just that by managing road transport to cut emissions.