Road traffic noise, air pollution, and risk of dementia - results from the Betula project

Background: There is growing evidence for a negative impact of traffic-related air pollution on risk of dementia. However, the contribution of noise exposure to this association has been rarely examined. Objective: We aimed to investigate the individual and combined effect of noise and air pollution on risk of dementia. Methods: Data on dementia incidence over a 15 year period was obtained from the Betula project, a longitudinal study on health and ageing. Estimates of annual mean levels of nitrogen oxides (NO) at the participants' residential address were obtained using a land-use regression model. Modelled data provided road traffic noise levels (Leq. 24 h) at the participants' residential address at baseline. Cox proportional hazard regression was used to calculate hazard ratios (HR). Results: Of 1721 participants at baseline, 302 developed dementia during the follow up period. Exposure to noise levels (Leq. 24 h) > 55 dB had no significant effect on dementia risk (HR 0.95; CI: 0.57, 1.57). Residing in the two highest quartiles of NOx exposure was associated with an increased risk of dementia. The risk associated with NOx was not modified by adjusting for noise. Moreover, we found no significant interaction effects between NO and road traffic noise on dementia risk. Conclusion: We found no evidence that exposure to road traffic noise, either independently or in combination with traffic air pollution, was associated with risk of dementia in our study area. Our results suggest that pollution should be considered the main component in the association between traffic related exposures and dementia.