A recent systematic review and meta-analysis revealed a connection between ambient air pollution and the risk of clinical dementia, particularly in relation to exposure to fine particulate matter (PM2.5). The study also suggests potential associations with nitrogen dioxide (NO2) and nitrogen oxide (NOx) but with more limited data. The findings, published in the bmj, highlight the need for further research and regulatory action to address air pollution as a potential modifiable risk factor for dementia.
The study analysed data from 51 studies, including 14 that could be meta-analysed for PM2.5, and identified an overall hazard ratio per 2 μg/m3 PM2.5 of 1.04. The hazard ratio was higher, at 1.42, among seven studies that used active case ascertainment, and 1.03 among seven studies that used passive case ascertainment. The hazard ratios per 10 μg/m3 were 1.02 for nitrogen dioxide and 1.05 for nitrogen oxide. Ozone, however, demonstrated no clear association with dementia.
These findings are significant as more than 57 million people worldwide are living with dementia, and the global burden continues to increase. While interventions to delay or prevent the onset of dementia are scarce, long-term ambient air pollution has been acknowledged as a potentially modifiable risk factor. This is due to the association between exposure to air pollution and cardiovascular disease, stroke, and cognitive impairment. Additionally, studies have shown that reductions in air pollution concentrations lead to reduced mortality.
The results from the systematic review and meta-analysis suggest consistent evidence of an association between ambient air pollution and clinical dementia, particularly for PM2.5. This is even below the current US Environmental Protection Agency (EPA) annual standard of 12 μg/m3 and well below the limits of the UK (20 μg/m3) and the European Union (25 μg/m3). Evidence for an association between NO2 and NOx is more limited, and data for other pollutants are even more scarce.
The authors of the study caution that the meta-analysed hazard ratios are subject to limitations that require careful interpretation. Outcome ascertainment approaches differ across studies, and each exposure assessment approach is likely only a proxy for causally relevant exposure in relation to clinical dementia outcomes. The researchers call for further studies that evaluate critical periods of exposure and pollutants other than PM2.5, as well as studies that actively assess all participants for outcomes.
Despite these limitations, the study’s results provide current best estimates for use in the burden of disease and regulatory setting efforts. The researchers developed the research question based on discussions with community members and people involved in environmental policy, though not by patients.
The researchers also plan to share their findings with specific interested parties involved in environmental policy, including the National Institutes of Health, the National Institute of Environmental Health Sciences, the EPA, and relevant European Union committees.