Parts of Wales have some of the highest levels of air pollution in the United Kingdom with communities up and down the country blighted by dirty air. Thousands of people are being exposed to often illegal levels of gases and fine particles, with potentially fatal consequences. Now the Welsh Government is promising a new law that campaigners believe could put Wales at the forefront of the fight to put an end to dirty air. Oliver Hides asks what a Clean Air Act might mean for us all, the sacrifices we may have to make and the benefits they could bring