A long-term female nurse study suggests a link between long-term exposure to certain air pollutants, but not road traffic noise, and the risk of developing dementia [1].
Air pollution as a risk factor
Air pollution is a known risk factor for many conditions, such as cardiovascular diseases, respiratory diseases, Type 2 diabetes, and lung cancer, which we have discussed previously. Similarly, road traffic noise is associated with sleep disturbance and stress, leading to an increased risk of “cardiovascular disease and possibly psychiatric disorders” [2, 3]. Both risk factors share sources, such as road traffic, along with the biological pathways they impact, such as neuroinflammation and brain damage [4], which can lead to dementia.
Studies that examined air pollution (but not road traffic noise) have suggested a link between “long-term exposure to air pollution and the risk of developing dementia.” Specifically, the authors acknowledge the well-researched and established link between dementia and particulate matter with a diameter of ≤2.5 μm (PM2.5) [5]. However, the literature about other air pollutants and traffic noise is inconsistent.
Since previous research on the impact of the combination of air pollution and road traffic noise on dementia risk is limited, in this study, the authors analyzed the data from 25,233 female nurses in Denmark (Danish Nurse Cohort) to establish whether there is a connection “between long-term exposure to both air pollution and road traffic noise and dementia incidence.”
Air pollution and dementia
The data analysis indicated “strong associations between long-term exposure to major air pollutants and incidence of dementia.” The authors elaborate in more detail on which specific pollutants show the association.
The data show that PM2.5 and NO2 have a strong positive association with dementia incidence, while ozone has shown a negative association. Those results are consistent with several previous studies [5]. They also identified an association between black carbon and the incidence of dementia. They report that these associations are linear, meaning that they persist below European Union and World Health Organization standards set for PM2.5 and NO2 air pollution.
Regarding the observed negative association between ozone and the incidence of dementia, the authors hypothesize that it is most likely a result of ozone having lower concentrations near NO2 and black carbon sources. Therefore, the researchers explain, it is not that ozone has a protective effect but that it is more likely to be around where the more dangerous pollutants are not.
While the authors found an association between air pollution and the incidence of dementia, they didn’t identify such a strong association “between road traffic noise and incidence of dementia after adjustment for air pollution.” This observation also agrees with previous research.
Positive effects of exercise
Modifying air pollution exposure is not an easy task and would require major changes in life, such as moving from a big city to a small town or village, which is something not everyone can do. Therefore, the authors looked for something that could mitigate the risk of dementia despite the exposure to air pollution. According to their analysis, physical activity was the one identified factor that modified the association between air pollution and the incidence of dementia.
In this study, they observed that despite being exposed to the same level of PM2.5 pollutants, physically active nurses had a lower risk of developing dementia compared to those with low physical activity levels.
The authors believe that their observation needs to be further assessed and confirmed but that they provide a good starting point for the development of preventive measures against dementia.
Strengths and limitations
As with every study, this one has strong and weak points. One of the main strengths of this study is the cohort the researchers used. This cohort had information on risk factors and behavior, data from nationwide registers, and a 20-year follow-up period. The researchers also used high-quality data regarding air pollution and road traffic noise, to which they had access dating from 14 years before any given nurse was included in the study. Adding the follow-up period, they analyzed around 35 years of exposure.
The cohort they used had many strengths, but it included only females, who have higher risks of developing Alzheimer’s disease. Therefore, there is still a need to investigate whether sex differences play a role in air pollution and dementia association.
The authors believe that there is a need “to identify groups that are most susceptible to harmful air pollution effects on dementia, for which preventive strategies can be designed.”
We show that long-term exposure to air pollution can lead to the development of dementia, even after adjustment for road traffic noise. This brings strong new evidence that supports existing findings in the current literature, suggesting that air pollution is an important risk factor for dementia. Moreover, we present a novel finding that physical activity may help mitigate the adverse effects of air pollution on dementia.
Literature
[1] Tuffier, S., Zhang, J., Bergmann, M., So, R., Napolitano, G. M., Cole-Hunter, T., Maric, M., Antic, S., Brandt, J., Ketzel, M., Loft, S., Lim, Y. H., & Andersen, Z. J. (2024). Long-term exposure to air pollution and road traffic noise and incidence of dementia in the Danish Nurse Cohort. Alzheimer’s & dementia : the journal of the Alzheimer’s Association, 10.1002/alz.13814. Advance online publication.
[2] Münzel, T., Molitor, M., Kuntic, M., Hahad, O., Röösli, M., Engelmann, N., Basner, M., Daiber, A., & Sørensen, M. (2024). Transportation Noise Pollution and Cardiovascular Health. Circulation research, 134(9), 1113–1135.
[3] Clark, C., Crumpler, C., & Notley, A. H. (2020). Evidence for Environmental noise Effects on Health for the United Kingdom Policy Context: A systematic review of the effects of environmental noise on mental health, wellbeing, quality of life, cancer, dementia, birth, reproductive outcomes, and cognition. International Journal of Environmental Research and Public Health/International Journal of Environmental Research and Public Health, 17(2), 393.
[4] Jafari, Z., Kolb, B. E., & Mohajerani, M. H. (2018). Chronic traffic noise stress accelerates brain impairment and cognitive decline in mice. Experimental neurology, 308, 1–12.
[5] Wilker, E. H., Osman, M., & Weisskopf, M. G. (2023). Ambient air pollution and clinical dementia: systematic review and meta-analysis. BMJ (Clinical research ed.), 381, e071620.
