Abstract Epidemiologic studies have shown that hay fever and asthma in children have increased over the last 50 years. Experimental data have suggested that exposure to environmental substances such as diesel exhaust particles can induce production of allergen specific IgE antibodies, while exposure to other substances such as endotoxins may rather induce non-allergic immune responses and suppress IgE production. The end of the East–West division of Europe provided the opportunity to assess the effect of different environmental conditions on the development of allergies. Differences in exposure to possible risk factors and their effect on allergic sensitisation in early life, a period considered critical for the development of persistent IgE reactions, are particularly important to analyse. Cars, NO 2 and NO exposure were more prevalent in Western Europe, while pollution by heavy industry and coal heating were present in Eastern cities. The results of population based comparisons have produced little evidence of air pollution causing new cases of allergy in children, and it seems rather unlikely than an excess in external toxic exposure is responsible for the world wide increase in pediatric atopy. However, some studies analysing local traffic exposure (in particular truck traffic) have found an increased allergy prevalence at least in subgroups of children. While a link between the inception of new allergies and air pollutants has to date not been convincingly shown, the increased respiratory morbidity associated with such exposure in children with established asthma as well as healthy children does result in considerable health costs.