Importance Evidence on the association of exposure to ambient particulate matter with a diameter less than 2.5 µm (PM 2.5 ) and nitrogen dioxide ( NO 2 ) and the later development of dementia with Lewy bodies (DLB) or Parkinson disease–related dementia (PDD) is limited. Objective To determine whether exposures to ambient PM 2.5 or NO 2 are associated with risk of incident DLB or PDD. Design, Setting, and Participants This population-based case-control study used linked Danish nationwide registry data to identify individuals with DLB and PDD diagnosed between January 1, 2001, through December 31, 2021. Each individual with dementia was matched with 10 controls on age, sex, and time. Data were analyzed from June 2025 through February 2026. Exposures A 10-year time-weighted mean concentration of PM 2.5 or NO 2 prior to DLB or PDD diagnosis. Main Outcomes and Measures A conditional logistic regression model was used to examine the associations of exposure to PM 2.5 or NO 2 with risk of DLB or PDD. The models were sequentially adjusted for age, sex, and calendar period, then individual-level socioeconomic characteristics, then medical and psychiatric comorbidity, and finally area-level socioeconomic factors. Results Among 2 184 847 Danish citizens aged between 65 and 95 years, there were 3024 individuals with DLB (mean SD age, 78 6, 1894 62.6% male) and 3808 with PDD (mean SD age, 78 6, 2400 63.0% male) who were matched with 30 240 and 38 080 controls, respectively. PM 2.5 and NO 2 exposure were each associated with increased risk of DLB or PDD after controlling for age, sex, and calendar period. In fully adjusted models, each 5 µg/m 3 increase in PM 2.5 exposure was associated with nearly 4-fold greater risk of DLB (adjusted odds ratio aOR, 3.70; 95% CI, 2.69-5.10) and more than 2-fold greater risk of PDD (aOR, 2.41; 95% CI, 1.88-3.08), while each 10 µg/m 3 increase in NO 2 exposure was associated with 95% greater risk of DLB (aOR, 1.95; 95% CI, 1.69-2.27) and 14% greater risk of PDD (aOR, 1.14; 95% CI, 1.01-1.29). Conclusions and Relevance In this case-control study, exposures to ambient PM 2.5 or NO 2 were each associated with increased risks of DLB and PDD, with a greater magnitude of association for risk of DLB. These findings warrant concern about the impact of air pollution on brain health.
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