Air pollution may increase risk for dementia

Key points:

• This meta-analysis, which includes the most recent studies evaluating the link between air pollution and dementia, is the first to include studies based on active case ascertainment and to evaluate studies using a new, more powerful bias assessment tool.
• The findings support the public health importance of a proposal, currently under consideration by the Environmental Protection Agency, to strengthen regulations on PM2.5

Boston, MA—Exposure to fine particulate air pollutants (PM2.5) may increase the risk of developing dementia, according to a new meta-analysis from Harvard T.H. Chan School of Public Health.

“This is a big step in providing actionable data for regulatory agencies and clinicians in terms of making sense of the state of the literature on this hugely important health topic. The results can be used by organizations like the Environmental Protection Agency, which is currently considering strengthening limits on PM2.5 exposure,” said lead author Marc Weisskopf, Cecil K. and Philip Drinker Professor of Environmental Epidemiology and Physiology. “Our findings support the public health importance of such a measure.”

The study is the first systematic review and meta-analysis to use the new Risk of Bias In Non-Randomized Studies of Exposure (ROBINS-E) tool, which addresses bias in environmental studies in greater detail than other assessment approaches. It also is the first to include newer studies that used “active case ascertainment,” a method that involved screening of entire study populations followed by in-person evaluation for dementia among individuals who did not have dementia at baseline.

The study will be published online April 5, 2023, in The BMJ.

More than 57 million people worldwide are currently living with dementia, and estimates suggest that number will increase to 153 million by 2050. Up to 40% of these cases are thought to be linked to potentially modifiable risk factors, such as exposure to air pollutants.

Weisskopf and his co-authors, Elissa Wilker, researcher in the Harvard Chan-NIEHS Center for Environmental Health, and Marwa Osman, a doctoral student in the Biological Science in Public Health program, scanned more than 2,000 studies and identified 51 that evaluated an association between ambient air pollution and clinical dementia, all published within the last 10 years. Those studies were assessed for bias using ROBINS-E, and 16 of them met the criteria for the meta-analysis. The majority of the research was about PM2.5, with nitrogen dioxide and nitrogen oxide being the next most common pollutants studied. Of the studies used in the meta-analysis, nine used active case ascertainment.

The researchers found consistent evidence of an association between PM2.5 and dementia, even when annual exposure was less than the current EPA annual standard of 12 micrograms per cubic meter of air (μg/m3) In particular, among the studies using active case ascertainment, the researchers found a 17% increase in risk for developing dementia for every 2 μg/m3 increase in average annual exposure to PM2.5. They also found evidence suggesting associations between dementia and nitrogen oxide (5% increase in risk for every 10 μg/m3 increase in annual exposure) and nitrogen dioxide (2% increase in risk for every 10 μg/m3 increase in annual exposure), though the data was more limited.

The researchers noted that air pollution’s estimated association with risk of dementia is smaller than that of other risk factors, such as education and smoking. However, because of the number of people exposed to air pollution, the population-level health implications could be substantial.

“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.”

Funding for the study came from Biogen and from National Institutes of Health grants P30 ES000002 and T32 HL007118.

“Ambient air pollution and clinical dementia: systematic review and meta-analysis,” Elissa H. Wilker, Marwa Osman, Marc G. Weisskopf, The BMJ, online April 5, 2023, doi: 10.1136/bmj-2022-071620

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Harvard T.H. Chan School of Public Health brings together dedicated experts from many disciplines to educate new generations of global health leaders and produce powerful ideas that improve the lives and health of people everywhere. As a community of leading scientists, educators, and students, we work together to take innovative ideas from the laboratory to people’s lives—not only making scientific breakthroughs, but also working to change individual behaviors, public policies, and health care practices. Each year, more than 400 faculty members at Harvard Chan School teach 1,000-plus full-time students from around the world and train thousands more through online and executive education courses. Founded in 1913 as the Harvard-MIT School of Health Officers, the School is recognized as America’s oldest professional training program in public health.