Trichloramine exposure in indoor swimming pools has been suggested to cause asthma in children. We aimed to investigate the risk of asthma onset among children in relation to individual trichloramine exposure.

A longitudinal nested case-control study of 337 children with asthma (cases) and 633 controls aged 16–17 years was performed within a population-based cohort from The Obstructive Lung Disease in Northern Sweden studies (OLIN). Year of asthma onset and exposure time at different ages were obtained in telephone interviews. Trichloramine concentrations in the pool buildings were measured. Skin prick test results for inhalant allergens were available from previous examinations of the cohort. The risk for asthma was analyzed in relation to the cumulative trichloramine exposure before onset of asthma.

The participation rate was high in the original cohort (88 to 96%), and in the case-control study (80%). Trichloramine concentrations ranged from 0.020 to 0.55 mg/m³ (mean 0.15 mg/m³). Swimming pool exposure in early life was associated with a significantly higher risk of pre-school asthma onset. A dose-response relationship between swimming pool exposure and asthma was indicated in children with asthma onset at 1 year of age. Children who were both sensitized and exposed had a particularly high risk.

Early life exposure to chlorinated swimming pool environments was associated with pre-school asthma onset.