The range and sources of air pollutants are almost limitless – as are the public’s concerns about them. It is only necessary to examine press reports over the past two decades to see how concern over different air pollutants has risen in the public view and then declined. There have been the large global and regional concerns such as those of acid rain. However, even this has changed in character. Initially, concern was voiced on sulphur dioxide emissions from power stations and the acidification effects that these were having. However, this soon included concern over nitrogen oxide emissions from power stations and motor vehicles. The deposition of nitrogen also adds nutrients to ecosystems, so attention has also turned to the emissions of ammonia from agriculture. Nitrogen oxides also contribute to ozone formation in the lower atmosphere when they react with volatile organic compounds. Thus, quite quickly, the ‘simple’ story is complicated by a wide range of pollutants with many sources and impacts.