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Bente Oftedal,¹ Wenche Nystad,¹ Bert Brunekreef,^2,3 and Per Nafstad^1,4

¹Division of Epidemiology, Norwegian Institute of Public Health, Oslo, Norway; ²Institute for Risk Assessment Sciences, Utrecht University, the Netherlands; ³Julius Center for Health Sciences and Primary Care, University Medical Center Utrecht, the Netherlands; ⁴Institute of General Practice and Community Medicine, University of Oslo, Norway

Abstract
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 NO₂ 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 NO₂ 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.

Key words: asthma, children, long-term exposure, nitrogen dioxide, NO₂, respiratory symptoms, traffic. Environ Health Perspect 117:839–844 (2009) . doi:10.1289/ehp.11491 available via http://dx.doi.org/ [Online 30 January 2009]

Address correspondence to B. Oftedal, Division of Epidemiology, Norwegian Institute of Public Health, P.O. Box 4404 Nydalen, NO-0403 Oslo, Norway. Telephone: +47 21 07 81 86. Fax: +47 21 07 82 52. E-mail: bente.oftedal@fhi.no

We thank P. Lund-Larsen’s staff at the Norwegian Institute of Public Health for conducting excellent data collection and I. Brandt for excellent data handling of the air pollution files.

The study was financed by grants from the Norwegian Research Council.

The authors declare they have no competing financial interests.

Received 19 March 2008 ; accepted 30 January 2009.