Associations of Early-Childhood Residential Green Space Exposure with Nail Cortisol Levels and Behavior in Young Children: Insights from the ENVIRONAGE Birth Cohort.

Early childhood is a sensitive developmental period during which environmental conditions may shape stress regulation and behavioral functioning. Green environments have been associated with improved child well-being, yet evidence linking early childhood green space exposure to long-term stress physiology remains limited. We examined associations between residential and school-based green space exposure, chronic stress, and behavioral outcomes in 4- to 6-year-old children from the ENVIRONAGE birth cohort. Green space was quantified at high spatial resolution. Nail cortisol was used as a biomarker of long-term hypothalamic-pituitary-adrenal axis activity, and behavioral functioning was evaluated using the Strengths and Difficulties Questionnaire. Associations were analyzed using adjusted multiple regression models, and mediation by nail cortisol was tested. Residential green space was associated with reduced nail cortisol, with a decrease of 11.8% (95% CI: -21.5%, -0.97%) per IQR increase of low green vegetation within 100 m and of -10.3% (95% CI: -19.2%, -0.47%) within 300 m buffers. Residential green space was also inversely associated with several behavioral domains. No mediation by nail cortisol was observed. These findings suggest that residential green space, particularly in the close residential environment, may support better behavioral development and lower long-term stress in early childhood.