Long-Term Traffic-Related Exposures and Asthma Onset in Schoolchildren in Oslo, Norway

BACKGROUND: Whether there is a causal relation between long-term exposure to traffic and asthma development is so far not clear. This may be explained by inaccurate exposure assessment. OBJECTIVE: We investigated the associations of long-term traffic-related exposures with asthma onset assessed retrospectively and respiratory symptoms in 9- to 10-year-old children. METHODS: We collected information on respiratory outcomes and potential confounding variables by parental questionnaire in 2,871 children in Oslo. Nitrogen dioxide exposure was assessed by the EPISODE dispersion model and assigned at updated individual addresses during lifetime. Distance to major road was assigned at birth address and address by date of questionnaire. Cox proportional hazard regression and logistic regression were used. RESULTS: We did not find positive associations between any long-term traffic-related exposure and onset of doctor-diagnosed asthma. An interquartile range (IQR) increase of NO(2) exposure before asthma onset was associated with an adjusted risk ratio of 0.82 [95% confidence interval (CI), 0.67–1.02]. Handling early asthma cases (children < 4 years of age) with recovery during follow-up as noncases gave a less negative association. The associations for late asthma onset (≥ 4 years of age) were positive but not statistically significant. For current symptoms, an IQR increase of previous year’s NO(2) exposure was associated with adjusted odds ratios of 1.01 (95% CI, 0.83–1.23) for wheeze, 1.10 (95% CI, 0.79–1.51) for severe wheeze, and 1.01 (95% CI, 0.84–1.21) for dry cough. CONCLUSIONS: We were not able to find positive associations of long-term traffic-related exposures with asthma onset or with current respiratory symptoms in 9- to 10-year-old children in Oslo.