Combined effect of heatwaves and residential greenness on the risk of stroke among Chinese adults: A national cohort study

Heatwaves have been associated with an increased risk of stroke, while residential greenness may offer protective benefits. This prospective cohort study examined 22,702 participants aged 35 years or older, with no prior history of cardiovascular disease (CVD), from the China Hypertension Survey (CHS) conducted between 2012 and 2015. Participants were followed up between 2018 and 2019. Heatwaves were defined as daily maximum temperatures exceeding the 92.5th percentile of the warm season for at least three consecutive days. Residential greenness was quantified using the Normalized Difference Vegetation Index (NDVI) within buffers of 300, 500, and 1000 m from participants' residences. Multivariable Cox proportional hazards models evaluated the independent and combined effects of heatwaves and greenness on stroke risk, while restricted cubic spline analyses explored nonlinear relationships. Interaction effects were assessed using both multiplicative and additive Cox regression models. During follow-up, 597 stroke events occurred. Each additional 3-day increase in heatwave days was associated with an increased stroke risk (HR: 1.19, 95 % CI: 1.08-1.31). Interaction analyses demonstrated a synergistic effect between heatwaves and lower residential greenness (NDVI_{300 m}, NDVI_{500 m} and NDVI_{1000 m}) on stroke risk, with significant additive(RERI > 0, P < 0.05) and multiplicative interactions (HR > 1, P < 0.05). The strongest protective effects of greenness were observed within a 500 m buffer zone, particularly for individuals under 60 years, rural residents, and those with higher educational attainment. This study highlights the potential benefits of enhancing greenness for cardiovascular health and provides valuable insights for environmental governance and public health policy in China.