Heatwave Exposure and Body Mass Index among Chinese Children and Young Adults: Modification by Socioeconomic Development and Air Pollution, 1985–2019

Abstract Extreme heat is an increasing environmental health concern, yet its long-term association with body-weight outcomes among young people remains uncertain. We linked eight waves of the Chinese National Survey on Students’ Constitution and Health from 1985 to 2019 with province-level heatwave exposure, PM2.5, and socioeconomic data across 30 provinces, covering 2,310,720 students aged 7–22 years. Heatwave exposure was characterized by intensity, duration, and frequency. Distributed lag nonlinear models estimated cumulative heatwave-related body mass index (BMI) gaps under a P90–P10 exposure contrast, while Bayesian hierarchical models characterized modification by GDP per capita and PM2.5 and between-province heterogeneity in heatwave effects. Pooled cumulative associations were modest, but heatwave-related BMI responses varied substantially across provinces. With increasing GDP, heatwave-related BMI effects shifted toward more positive values, suggesting that socioeconomic development may reshape rather than uniformly buffer these responses; PM2.5 showed a positive modification gradient but greater province-level complexity. The Western Pacific analysis provided external consistency for heterogeneity and the GDP-related pattern, while PM2.5-related patterns varied more across settings. Together, these findings suggest that long-term heatwave-related BMI responses among young people may be systematically shaped by socioeconomic and air-pollution contexts, while pooled averages may conceal important subnational differences for environmental health assessment and climate adaptation.