[en] Belgium has five major urban areas or conurbations with a total population of 2 million people, representing 20% of the total population. Using data from three nationwide monitoring networks, the past and present sulphur dioxide, lead, nitrogen dioxide and ozone situation in the conurbations of Antwerp, Brussels, Charleroi, Gent and Liege are summarised and analysed over a period of 15 to 25 years. Major conclusions are marked downward trends in the ambient levels of sulphur dioxide, nitrogen dioxide and lead in all five conurbations reducing the present levels under all legal national and European limit values, and bringing them in general, beneath the guidelines values for the protection of human health recommended by the World Health Organisation. For ozone the trend over the past 10 to 15 years has been stationary. Ozone guideline values, as well as recommended threshold values are still frequently violated. (Author)