Associations of greenness surrounding schools with blood pressure and hypertension: A nationwide cross-sectional study of 61,229 children and adolescents in China

Abstract

BackgroundGreenness exposure may lower blood pressure. However, few studies of this relationship have been conducted with children and adolescents, especially in low and middle-income countries. ObjectivesTo evaluate associations between greenness around schools and blood pressure among children and adolescents across China. MethodsWe recruited 61,229 Chinese citizens aged 6–18 years from 94 schools in a nationwide cross-sectional study in seven Chinese provinces/province-level municipalities. Participants' blood pressures and hypertension were assessed with standardized protocols. Greenness levels within 500 m and 1,000 m of each school were estimated with three satellite-based indices: vegetation continuous fields (VCF), normalized difference vegetation index (NDVI), and soil adjusted vegetation index (SAVI). Generalized linear mixed models were used to evaluate associations between greenness and blood pressure, greenness and prevalent hypertension, using coefficient and odds ratio respectively. Stratified analyses and mediation analyses were also performed. ResultsOne interquartile range increase in greenness was associated with a 17%–20% reduced prevalence of hypertension for all measures of greenness (odds ratios for VCF500m = 20% (95% CI:18%, 23%); for NDVI500m = 17% (95% CI:13%, 21%); and for SAVI500m = 17% (95% CI: 13%, 20%). Increases in greenness were also associated with reductions in systolic blood pressure (0.48–0.58 mmHg) and diastolic blood pressure (0.26–0.52 mmHg). Older participants, boys, and urban dwellers showed stronger associations than their counterparts. No evidence of mediation was observed for air pollution (i.e., NO2 and PM2.5) and body mass index. ConclusionHigher greenness around schools may lower blood pressure levels and prevalent hypertension among Chinese children and adolescents, particularly in older subjects, boys, and those living in urban districts. Further studies, preferably longitudinal, are needed to examine causality.