Changes in Susceptibility to Heat during the Summer: A Multicountry Analysis

Few studies have examined the variation in mortality risk associated with heat during the summer. Here, we apply flexible statistical models to investigate the issue by using a large multicountry data set. We collected daily time-series data of temperature and mortality from 305 locations in 9 countries, in the period 1985-2012. We first estimated the heat-mortality relationship in each location with time-varying distributed lag non-linear models, using a bivariate spline to model the exposure-lag-response over lag 0-10. Estimates were then pooled by country through multivariate meta-analysis. Results provide strong evidence of a reduction in risk over the season. Relative risks for the 99th percentile versus the minimum mortality temperature were in the range of 1.15-2.03 in early summer. In late summer, the excess was substantially reduced or abated, with relative risks in the range of 0.97-1.41 and indications of wider comfort ranges and higher minimum mortality temperatures. The attenuation is mainly due to shorter lag periods in late summer. In conclusion, this multicountry analysis suggests a reduction of heat-related mortality risk over the summer, which can be attributed to several factors, such as true acclimatization, adaptive behaviors, or harvesting effects. These findings may have implications on public health policies and climate change health impact projections. © 2016 The Author 2016. Published by Oxford University Press on behalf of the Johns Hopkins Bloomberg School of Public Health.This work was supported by the Medical Research Council- United Kingdom (grants MR/M022625/1 and G1002296); the National Institute for Health Research-United Kingdom, Health Protection Research Unit (NIHR HPRU) in Environmental Change and Health; the University of Queensland (Australia); the Ministry of Education (Spain) (grant PRX12/00515); the National Health and Medical Research Council (Australia) (grant 553043); the Australian Research Council (Australia) (grant DP110100651); the Environment Research and Technology Development Fund (Japan) (S-8 & S10); and the National Research Foundation of Korea (grant K21004000001-10A0500-00710).Peer reviewe