Early Life Exposure to Green Space and Childhood Cognition: A mediation analysis of physical activity and air pollution

Abstract

Background: Natural vegetation, or “greenness,” may influence cognition by increasing physical activity or buffering air pollution exposure. Early life influences of greenness on cognition, and the mediating roles of physical activity and air pollution, are not well understood. Aim: We examined associations of early childhood neighborhood greenness with mid-childhood cognition. We quantified the extent to which physical activity and air pollution explained this association.Methods: We studied 857 mother–child pairs in Project Viva, a pre-birth cohort study in Massachusetts. In early childhood (median 3.2y), we estimated residential greenness using 30m resolution Landsat satellite imagery [Normalized Difference Vegetation Index], residential particulate matter using spatiotemporal models, and number of hours/day spent in active play, reported by parents, for physical activity. In mid-childhood (7.7y), we administered standardized assessments of verbal and nonverbal intelligence, visual motor abilities (VMA), and visual memory (VM) as outcomes. Using linear regression, we examined associations of early childhood greenness with mid-childhood cognition, adjusting for demographic and socioeconomic factors. We used a causal mediation framework to determine the mediation effect through physical activity and air pollution. Results: The mean(SD) for verbal IQ was 114(14), non-verbal IQ was 107(17), VMA was 93(17), and VM was 17(4). Children living in the highest 3 greenness quartiles vs. lowest in early childhood had higher verbal intelligence (2.0 points, 95%CI: -0.06, 4.1), and higher VM (1.0 point; 95% CI: 0.3, 1.8), but lower VMA (-2.6 points, 95%CI: -5.5, 0.2). We did not observe associations of greenness with non-verbal intelligence. The associations of greenness on verbal intelligence, VM or VMA were not mediated by air pollution or physical activity (e.g.indirect effect on verbal intelligence through air pollution:-0.5, 95%CI:-1.2,0.2). Conclusions: Early childhood greenness may affect verbal intelligence, VM and VMA through other mechanisms different than air pollution or physical activity, perhaps through lowering stress levels.