Effect of heatwaves and greenness on mortality among Chinese elderly people

Abstract

BACKGROUND AND AIM: Both heatwaves and greenness have been shown to affect health, but the evidence on their joint effects is needed to be better elucidated. We aim to assess the associations of the combined exposure to greenness and heatwaves and whether the effect varies between different subgroups. METHODS: We utilized the Chinese Longitudinal Healthy Longevity Survey (CLHLS), a prospective cohort of older adults aged 65. We used the number of heatwave days in one year before death to estimate individual long-term heatwaves exposure and cumulative Normalized Difference Vegetation Index (NDVI) during follow-up to assess individual greenness exposure. Cox proportional hazards models were used to assess the effects of greenness, heatwaves, and their interaction on mortality, adjusted for a range of covariates. We conducted subgroup analyses by residence, gender, and age. RESULTS:There were 20,758 participants in our study, totaling 67,312 person-years of follow-up. The mean NDVI was 0·41 (SD 0.13), and the mean number of heatwave days was 8.92 (2.04). In the adjusted model, the mortality hazard ratio (HR) for each 3-day increase in heatwave days was 1.04 (95% CI 1.04,1.05), each 0.1-unit decrease in cumulative NDVI was 1.06 (1.05,1.07). In the adjusted model with an interaction term, the HR for the interaction term was 1.01 (1.01,1.02) with a p-value less than 0.001. In our subgroup analyses, the HR for each 3-day increase in heatwave days was higher in urban areas than in rural areas (1.06 vs. 1.03), and the HR for 0.1-unit decrease in NDVI was higher in urban areas than in rural areas (1.08 vs. 1.04). CONCLUSIONS:Greenness can protect against the effect of heatwaves on mortality, and the number of heatwave days will affect the health effects of greenness. Urban dwellers have a higher response to the detrimental effect of heatwaves and have a higher marginal benefit from greenness exposure. KEYWORDS: Green space, Temperature, Environmental epidemiology