This type of episode seems to occur most frequently in December. The major historic smog of the 1950's occurred on the four days between 4 and 8 December 1952, when smoke measurements taken at the National Gallery in London suggest that the concentration reached 14 mg/m³ - which was 56 times levels normally experienced at the time.

More recently, an episode occurred between 21 and 23 December 1994. During this period, the stable weather conditions causing the high concentrations moved across the country and it was possible to track the progress of the centre of the episode. Winter weather conditions also tend to cause a more specifically sourced episode of elevated pollutant concentrations. Celebrations involving bonfires, for example Guy Fawkes night on November 5th, can cause significant amounts of particles to be emitted into the atmosphere. The most severe winter episode in recent years also occurred in December, in London on 12-15 December 1991. The meteorological conditions were cold and foggy for several days, and the resulting build-up of vehicle emissions led to record breaking levels of nitrogen dioxide. The maximum hourly concentration was over 400ppb, the highest seen in the UK since automatic monitoring began in the early 1970's.

The degree to which these events increase the concentrations of particles in the area is highly dependent on the weather conditions that are prevailing at the time. Figure 10 shows the 24 hour running mean particle concentrations for Guy Fawkes night for 1994 - 1998. Very high pollution levels were recorded in 1995 (when the National Air Quality Standard was breached for 40 hours), whilst in 1996 concentrations remained virtually unchanged from normal levels. 1994, 1997 and 1998 had pollution levels between these two extremes. In Northern Ireland, the major celebration involving bonfires takes place on July 11th. The summer weather conditions prevailing during this period allow efficient dispersal of pollutants, hence particle concentrations remain around normal levels.

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