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

Area of Science:

• Environmental epidemiology
• Public health
• Climate change adaptation

Background:

• Limited research exists on the seasonal variation of heat-associated mortality risk.
• Understanding these variations is crucial for effective public health interventions and climate change impact assessments.

Purpose of the Study:

• To investigate the temporal variation in heat-mortality relationships throughout the summer season.
• To quantify the reduction in mortality risk from early to late summer across diverse geographical locations.

Main Methods:

• Utilized a large multicountry dataset (305 locations, 9 countries, 1985-2012) with daily temperature and mortality time-series data.
• Employed time-varying distributed lag non-linear models (DLNMs) with bivariate splines to assess exposure-lag-response functions.
• Pooled location-specific estimates using multivariate meta-analysis to derive country-level and overall seasonal trends.

Main Results:

• Significant reduction in heat-related mortality risk observed from early to late summer.
• Relative risks (99th percentile vs. minimum mortality temperature) decreased from 1.15-2.03 in early summer to 0.97-1.41 in late summer.
• Late summer showed wider thermal comfort ranges and higher minimum mortality temperatures, primarily due to shorter exposure lag periods.

Conclusions:

• Heat-related mortality risk diminishes over the course of the summer season.
• Observed attenuation is likely due to physiological acclimatization, behavioral adaptations, and potential harvesting effects.
• Findings inform public health policy development and refine projections of climate change-related health impacts.