Purpose: Air pollution is an established lung carcinogen, and there is increasing evidence that air pollution also negatively affects the brain. We have previously reported an association between air pollution and risk of brain tumors in a cohort study based on only 95 cases. We set out to replicate that finding in a large nationwide case-control study.

Methods: We identified all 4,183 adult brain tumor cases in Denmark in the years 2000-2009 and 8,018 risk set sampled population controls matched on gender and year of birth. We extracted residential address histories and estimated mean residential nitrogen oxides (NO x ) concentrations since 1971 with a validated dispersion model. Categorical and linear odds ratios (OR) and confidence intervals (CI) were calculated with conditional logistic regression models.

Results: The highest risk estimates for any brain cancer were observed among subjects with the highest average exposure levels (80-99 µg/m(3): OR 1.27, 95% CI 0.82-1.96; ≥100 µg/m(3): 1.40, 95 % CI 0.87-2.26 as compared to <20 µg/m(3) NO x ), but there was no increased OR at NO x levels below 80 µg/m(3) and when modeled linearly there was no significant association with risk of brain cancer (OR 1.11, 95% CI 0.84-1.46 per 100 µg/m(3) NO x ). In sub-analysis the OR associated with exposures ≥100 µg/m(3) was 2.30 (95% CI 1.15-4.59) for non-glioma and 0.89 (95% CI 0.44-1.77) for glioma.

Conclusions: This study did not support the relatively strong linear association between air pollution and risk of brain tumors which was found in our previous study. The suggestion of an increased brain tumor risk at high exposures merits further attention as does the differing results according to tumor morphology.