New Data Indicate Link Between Air Pollution and Dementia Risk

Elissa Wilker, ScD

Credit: Harvard Chan-NIEHS Center for Environmental Health

A new study provides evidence of a possible association between air pollution and dementia impacting adults in the US.¹

The team of investigators used US Environmental Protection Agency criteria air pollutants and proxies of traffic pollution, averaged exposure over a year or more, and reported associations between ambient pollutants and clinical dementia. The objective was to investigate the role of air pollutants in the risk of dementia, considering differences by study factors that could influence findings.

Air pollution appears in various areas of health-related research, and is known to cause respiratory and cardiovascular diseases.² The investigators of this study aimed to contribute to recent data that implied it may also be linked to cognitive decline and an increased risk of dementia.

Assessing the Impacts of Air Pollution

Elissa H Wilker, Research Affiliate, Department of Epidemiology, Harvard TH Chan School of Public Health, Boston, and investigators conducted a systematic review of several databases, including EMBASE, PubMed, Web of Science, Psycinfo, and OVID Medline. The search timeline extended from database inception through July 2022.

Studies with adult populations (≥18 years) and a longitudinal follow-up, were included. A predefined data extraction form was implemented to extract data and assess the risk of bias using the Risk of Bias In Non-randomised Studies of Exposures (ROBINS-E) tool.

Out of 2080 records identified, 51 studies were reviewed for analysis. Investigators acknowledged that most studies were at high risk of bias, although in many cases bias was towards the null.

A meta-analysis could be performed on 14 studies for particulate matter <2.5 µm in diameter (PM_2.5), one of the most harmful air pollutants. According to the results, the overall hazard ratio per 2 μg/m3 PM_2.5 was 1.04 (95% CI 0.99-1.09), indicating a possible association between PM_2.5 and dementia.

However, the hazard ratio among 7 studies that used active case ascertainment was 1.42 (1.00 to 2.02), which suggests a stronger association. Among 7 studies that used passive case ascertainment, the hazard ratio was 1.03 (0.98-1.07).

Additionally, results showed that nitrogen dioxide and nitrogen oxide may also be risk factors for dementia, although it was noted that these data were more limited. The overall hazard ratio per 10 μg/m3 nitrogen dioxide was 1.02 ([0.98 to 1.06]; 9 studies) and per 10 μg/m3 nitrogen oxide was 1.05 ([0.98 to 1.13]; 5 studies).

No clear association between ozone and dementia (HR per 5 μg/m3:1.00 [0.98 to 1.05]; 4 studies).

Limitations of the Meta-Analysis

Investigators concluded that PM_2.5, nitrogen dioxide, and nitrogen oxide have the potential to be risk factors for dementia, although the meta-analyzed hazard ratios are subject to limitations that require interpretation with caution.

It was addressed that outcome ascertainment approaches differ across studies, and each exposure assessment approach likely only serves as a proxy for causally relevant exposure in relation to clinical dementia outcomes. However, the findings highlighted the need for further research into this area, along with the importance of policies that reduce air pollution levels to protect public health.

“Studies that evaluate critical periods of exposure and pollutants other than PM_2.5, and studies that actively assess all participants for outcomes are needed,” investigators wrote. “Nonetheless, our results can provide current best estimates for use in burden of disease and regulatory setting efforts.”

References:

1. Wilker EH, Osman M, Weisskopf MG. Ambient air pollution and clinical dementia: systematic review and meta-analysis. BMJ. 2023;381:e071620. doi: https://doi.org/10.1136/bmj-2022-071620
2. Grossi G. The American Lung Association will Publish 2023 "State of the Air" Report Next Week. HCPLive. April 12, 2023. https://www.hcplive.com/view/american-lung-association-2023-state-of-the-air-report