Smog and Pollution Episodes   Air pollution in the UK has historically occurred due to the combustion of sulphur-containing fossil fuels such as coal used for domestic and industrial purposes. Today, the major threat to clean air is from traffic emissions. Petrol and diesel-engined motor vehicles emit a wide variety of pollutants including carbon monoxide (CO), oxides of nitrogen (NOx), volatile organic compounds (VOC’s) and particulate matter (PM). These pollutants impact on the immediate vicinity but also can be transported long distances. Photochemical reactions resulting from the action of sunlight on nitrogen dioxide and VOC’s lead to the formation of ozone. This secondary pollutant often impacts areas far from the original emission site. Occasionally weather conditions may lead to higher than normal levels of air pollution. In winter, cold, still conditions can lead to a build up of pollutants at ground level; in the summer, hot weather, without much wind, also leads to raised concentrations of pollutants including ozone.  These raised levels of air pollution can be known as 'smog'.