The number of capillary blood eosinophils and the prevalence of bronchial hyperresponsiveness (BHR) were compared between schoolchildren living in a polluted area (Ardal) and unpolluted area (Laerdal). In Ardal there is an aluminium smelter emitting sulphur dioxide and fluoride to the environment. Daily measurements of these pollutants in ambient air were available. The mean number of eosinophils in Ardal was 220 x 10(6)/l compared with 106 x 10(6)/l in Laerdal. The prevalence of BHR was 15.9% in Ardal and 11.8% in Laerdal. The odds ratio of having BHR in relation to these pollutants during the last 24 hours were: 1.12 (95% confidence interval (CI) 1.01 to 1.24) by increasing sulphur dioxide with 10 micrograms/m3, and 1.31 (95% CI 1.07 to 1.60) when fluoride exposure increased with 1 micrograms/m3. Similarly, these exposures were associated with a decrease in eosinophils of -21 x 10(6)/l (95% CI -36 to -6) and -52 x 10(6)/l (95% CI -98 to -8), respectively, in atopics. It is hypothesised that recent exposure to irritants induces changes in the airways leading to BHR in addition to recruitment of eosinophils to the airways in atopic subjects.