Intraseasonal variation of the association between heat exposure and mortality risk in Shandong province, China

Abstract

Exposure to ambient high temperature has been associated with numerous health outcomes. It remains largely unclear whether the heat effect may vary in the course of the warm season. This study aimed to explore the intraseasonal variation of the association between heat exposure and mortality risk during the warm season in Shandong province between 2013 and 2018. Daily data on mortality and meteorological conditions were collected from each of the 1822 sub-districts in Shandong Province during the 2013–2018 warm seasons (June–September). Conditional logistic regression with a distributed lag non-linear model was used to fit the heat-mortality association over lag 0–10 days from early (June–July) to late (August–September). The extreme heat effect was defined as the cumulative odds ratio (OR) at the 97.5th percentile of the temperature range compared to the minimum mortality temperature (MMT). Stratified analysis was conducted by sex, age-group, educational level, and cause of mortality. A J-shaped association between ambient heat and mortality risk was consistently observed across the warm season, with the MMT located at 22.5 °C. The extreme heat effect (OR) decreased significantly from 3.52 (95%CI: 3.10, 4.00) in the early warm season to 2.85 (95%CI: 2.48, 3.28) in the late warm season (P = 0.028). Females, elderly aged over 75 years, individuals with junior high school education or lower, and cardiorespiratory patients were more vulnerable to heat exposure compared to other population subgroups, especially in the early warm season. In conclusion, this study observed a J-shaped association between ambient warm temperature and mortality risk, with the effect size of extreme heat decreasing substantially from the early to the late warm season. The findings highlight the importance of developing health promotion strategies to offset the adverse impact of early heat.