Frequently Asked Questions

• What defines a pollution episode and when was the first official episode?

  An air pollution episode is the term used for a period of poor air quality, usually lasting up to several days, extending over a large geographical area. Concentrations of all the measured species may increase at the same time, or only one species may be affected. There is no official definition of an episode or requirement for reporting other than when there are exceedances of UK air quality standards during the episode period. They are caused in different ways: During winter months, cold, stable weather conditions can trap pollutants close to their sources and prevent their dispersion. As a result, elevated concentrations of a range of pollutants can build up over several days.

  In the 1950's and 60's the biggest culprit was domestic coal burning, producing large amounts of black smoke and sulphur dioxide. More recently, however, the major pollutant source in much of the UK is the motor vehicle, and in the winter's cold foggy periods, high concentrations of nitrogen oxides, particulates and hydrocarbons are experienced. During the summer, a completely different type of episode occurs during hot and sunny weather. Pollutants emitted within the UK, or from other parts of Europe, can be transported long distances, reacting with each other in sunlight to produce high levels of ozone, and other photochemical pollutants.