Air pollution, social engagement, and depression in older adults: Results from a Swedish population-based cohort study.

Although emerging research has investigated the relationship between outdoor air pollution and depression risk in older adults, the results remain inconclusive. We aimed to determine the relationship between long-term exposure to ambient air pollution and depression among older adults and explore whether active social engagement may modify this association. At baseline (2001-2004), 2812 depression-free older adults from Swedish National Study on Aging and Care in Kungsholmen (SNAC-K) were included. SNAC-K is a longitudinal population-based cohort in Stockholm, Sweden. Incident depression cases occurred during 2004-2013 were ascertained using the Diagnostic and Statistical Manual of Mental Disorders 4th Edition. Air pollution [particulate matter (PM) and nitrogen oxides (NOx)] at the residency were estimated using dispersion models. Social engagement was measured as active participation in social activities (at least twice/week) or inactive (less than twice/week) in the last 12 months. The hazard ratios (HR) and 95% confidence intervals of depression from air pollution exposure of 3-year moving average before diagnosis (1-μg/m difference in PM and PM, and 10-μg/m difference in NOx) were obtained from Cox models considering greenspace and noise. A product term of air pollutant and social activity was added to test the multiplicative interaction and attributable proportion due to interaction was calculated for assessing additive interaction. We identified 137 (4.9%) incident depression cases. Participants exposed to higher concentrations of PM NOx, and PM had 53% (HR:1.53 [1.22, 1.93]), 26% (HR:1.26 [1.01, 1.58]), and 7% (HR:1.07 [0.98, 1.18]) increased hazard of depression, respectively. These associations were largely attenuated in people with active social engagement (HR for PM: 1.04 [0.70, 1.55]; HR for PM: 0.98 [0.81, 1.18]; and HR for NOx: 1.09 [0.71, 1.66]). Our findings suggest long-term exposure to air pollution may be a risk factor for depression among older adults. An active social engagement might however decrease this risk.