Investigating Syndemic Effects of Air Pollution and Physical Inactivity on Cognitive Decline in Older Adults.

Background: Previous research has independently associated air pollution and physical inactivity with increased mortality and morbidity. There is an ongoing debate about whether those factors interact to cause an even higher burden, suggesting potential syndemics. This study aimed to estimate the interaction between air pollution and physical inactivity on cognitive decline in older adults.

Methods: The study utilized the Northern Ireland Cohort for the Longitudinal Study of Ageing. The outcome was a ≥3 points decline in the Mini-Mental State Examination score between 2 cohort waves. Exposures were annual mean particulate matter smaller than 2.5 μm (PM2.5) in a 1-km buffer around participants' residences estimated based on national monitoring and self-reported recreational moderate to vigorous physical activity (MVPA) minutes per week. Logistic regression models were used to estimate additive and multiplicative interactions between exposures adjusting for confounders.

Results: Among 2836 participants, 137 (4.8%) had cognitive decline between waves. The median PM2.5 was 6.6 μg/m3 (interquartile range: 5.6-7.6), and 50% reported no MVPA in a week (interquartile range: 0-251.2). Models indicated additive (relative excess risk due to interaction = 0.63; 95% CI, -0.98 to 2.24) and multiplicative (synergy factor = 1.76; 95% CI, 0.84 to 3.72) interactions between high PM2.5 and low MVPA on the risk of cognitive decline; however, estimates were not precise.

Conclusions: This study presents a novel quantitative investigation of a potential syndemic focusing on a less-explored outcome of cognitive decline. However, outcome and exposure measurements limited the certainty of our findings. Future studies should include areas with higher variation in air pollution and use more granular exposure and sensitive outcome measures.