Association between traffic-related air pollution and brain morphology, assessing the olfactory pathway as a mediator

Traffic-related air pollution (TRAP) is a major environmental risk factor with growing evidence linking long-term exposure to adverse brain morphology outcomes. A direct pathway for pollutant translocation to the brain via the olfactory bulb has been proposed, but in vivo its contribution to TRAP-related changes in brain morphology remains inconclusive. We investigated these questions using cranial magnetic resonance imaging (MRI) data. Based on cranial MRIs from the population-based 'Cooperative Health Research in the Region of Augsburg' cohort, we analyzed global and region-specific white matter lesions, cerebral microbleeds, brain volumes, and olfactory bulb signal intensity in 400 participants. Land-use regression models estimated residential long-term exposure to air pollutants, including particle number concentration, particulate matter with different diameters, coarse particulate matter with a diameter between 10 μm and 2.5 μm, absorbance of particulate matter with an aerodynamic diameter ≤2.5 μm, and nitrogen oxides. We used covariate-adjusted regression models to explore the association between TRAP and white matter lesions, brain volumes, or cerebral microbleeds, and investigated whether the olfactory bulb mediates this association. Participants' mean age was 56 ± 9 years, and 42% were female. We found that TRAP was associated with increased odds of prevalent white matter lesions. For global white matter lesions, an interquartile range increase in absorbance of particulate matter was associated with an odds ratio of 1.48 [95% CI: 1.02; 2.14]. Comparable results were found for non-frontal white matter lesions, where nitrogen dioxide, absorbance of particulate matter, and coarse particulate matter were linked to increased odds of white matter lesions. In the mediation analysis we did not find evidence that the association was mediated by the signal intensity of the olfactory bulb. Stratified analyses revealed that women were more susceptible for the detrimental effects of TRAP. Our findings suggest that TRAP exposure was associated with an increased odds for white matter lesions, while the olfactory bulb did not appear to mediate this relationship.