Air pollution is a big risk for dementia, study suggests

There are already more than 57 million dementia sufferers worldwide, and by 2050, that figure is predicted to rise to 153 million. It is estimated that risk factors like exposure to air pollution account for up to 40 percent of these instances.

Studies assessed by using ROBINS-E

Weisskopf and his co-authors, Marwa Osman, a doctoral student in the Biological Science in Public Health program, and Elissa Wilker, a researcher in the Harvard Chan-NIEHS Center for Environmental Health, searched more than 2,000 studies and found 51 that examined a connection between ambient air pollution and clinical dementia.

These studies were all published within the last ten years. Sixteen of those papers satisfied the requirements for the meta-analysis after being evaluated for bias using ROBINS-E. The second and third most frequent pollutants considered after PM2.5 were nitrogen dioxide and nitrogen oxide. Nine of the studies incorporated into the meta-analysis did active case ascertainment.

The researchers pointed out that other risk variables like education and smoking have more prominent estimated associations with dementia risk than air pollution. Nonetheless, given how many individuals are exposed to air pollution, there may be significant health effects at the population level.

“Given the massive numbers of dementia cases, identifying actionable modifiable risk factors to reduce the burden of disease would have tremendous personal and societal impact,” Weisskopf said. “Exposure to PM2.5 and other air pollutants is modifiable to some extent by personal behaviors—but more importantly through regulation.”

The full study was published in the British Medical Journal on April 5 and can be found here.

Study abstract:

2080 records identified 51 studies for inclusion. Most studies were at high risk of bias, although in many cases bias was towards the null. 14 studies could be meta-analysed for particulate matter <2.5 µm in diameter (PM2.5). The overall hazard ratio per 2 μg/m3 PM2.5 was 1.04 (95% confidence interval 0.99 to 1.09). The hazard ratio among seven studies that used active case ascertainment was 1.42 (1.00 to 2.02) and among seven studies that used passive case ascertainment was 1.03 (0.98 to 1.07). The overall hazard ratio per 10 μg/m3 nitrogen dioxide was 1.02 ((0.98 to 1.06); nine studies) and per 10 μg/m3 nitrogen oxide was 1.05 ((0.98 to 1.13); five studies). Ozone had no clear association with dementia (hazard ratio per 5 μg/m3 was 1.00 (0.98 to 1.05); four studies). PM2.5 might be a risk factor for dementia, as well as nitrogen dioxide and nitrogen oxide, although with more limited data. The meta-analysed hazard ratios are subject to limitations that require interpretation with caution. Outcome ascertainment approaches differ across studies and each exposure assessment approach likely is only a proxy for causally relevant exposure in relation to clinical dementia outcomes. Studies that evaluate critical periods of exposure and pollutants other than PM2.5, and studies that actively assess all participants for outcomes are needed. Nonetheless, our results can provide current best estimates for use in burden of disease and regulatory setting efforts.