Examining associations of air pollution and green space with depressive symptoms in adults: A LongITools cross-cohort analysis

Objectives:
Evidence suggests that high levels of air pollution and less green space increase depressive symptoms in adults. However, results are mixed and cross-cohort comparisons are scarce, largely due to heterogeneity in exposure assessment. Also, the impact of these exposures on the trajectory of depressive symptoms over time has been less studied. We investigated the association of air pollution and green space with depressive symptoms in adulthood and whether these exposures modify the trajectory of depressive symptoms leveraging harmonized data from four population-based cohorts across the Netherlands and United Kingdom (UK).

Methods:
We analyzed data from the Dutch Famine Birth Cohort (DFBC) (n=840, baseline ages: 56-61), and the Rotterdam Study (RS) (RS-I n=4,049, baseline ages: 61-101 and RS-II n=2,861, baseline ages: 55-99), in the Netherlands, and the Avon Longitudinal Study of Parents and Children (ALSPAC) (n=17,100, baseline ages: 18-71) in the UK, each using a different validated instrument for depressive symptoms, with 3 to 11 repeated measures. European-wide environmental data was linked to participants’ addresses at baseline. Linear mixed-models were used to estimate associations of air pollution and green space with standardized cohort-specific depressive symptoms, and whether these exposures modify the trajectory of depressive symptoms.

Results:
Long-term exposure to fine particulate matter (PM2.5) was positively associated with overall higher standardized depressive symptom scores in ALSPAC and RS-I (β per 10 µg/m3 increase in PM2.5: 0.07 SD, 95%CI 0.02, 0.11 and 0.13 SD, 95%CI 0.02, 0.24, respectively). Exposure to higher normalized difference vegetation index (NDVI) at 300 meter buffer was associated with lower depressive symptoms in DFBC (β per 0.1 increase in NDVI: -0.08, 95%CI -0.14, -0.01). In RS-II, the positive effect of higher NDVI at 300-meter buffer on depressive symptoms decreased over time, but this effect was very small (β per 0.1 increase in NDVI: 0.01 SD per year, 95%CI 0.00, 0.01).

Conclusion:
Air pollution in the form of particulate matter as well as green space were associated with depressive symptoms across multiple cohorts. In the majority of cohorts, depressive symptoms increased with age, but we found little evidence that trajectories of depressive symptoms are influenced by exposure to environmental variables.