We investigated the relationship between residence in the neighbourhood of an aluminium smelter and the prevalence of atopy in schoolchildren (7-13 years of age). Atopy was assessed in 556 of the 620 participants by a skin prick test with eight common aeroallergens. The median exposures to sulphur dioxide and fluoride during the pollen season in the age interval 19-36 months were 24 and 3.1 micrograms/m3 in the spring and 20 and 3.3 micrograms/m3 in the summer, respectively. The odds ratio (OR) of having atopy was 2.0 (95% CI: 1.2-3.3) in those children who had lived in the index area for 7 years or more compared with those who had lived there less than 7 years (cumulative effect). The OR of atopy was 2.5 (1.4-4.4) in those who had lived in the index area during the age interval of 19-36 months compared with rural residence during this age-interval (age-specific effect). When the age-specific effect and the cumulative effect were compared in the same logistic model, the former decreased to 1.1 (0.4-3.0), whereas the latter was 2.2 (0.7-6.6). The results indicate that exposure to these low levels of irritants during early childhood increases allergen sensitization in children.