Long-term heatwave exposure and mortality: a prospective cohort study in the UK

Background Heatwaves are becoming increasingly frequent, prolonged, and intense due to climate change. While most epidemiological research has focused on acute health impacts, evidence from large-scale longitudinal studies on the chronic effects of long-term heatwave exposure remains scarce. Methods This prospective cohort study included 332,175 participants enrolled in the UK Biobank between 2006 and 2010. Heatwave exposure was conceptualized as a cumulative, time-varying stressor, quantified by three metrics: heatwave events (HWE), heatwave days (HWD), and heatwave intensity (HWI). Mortality data were obtained from the National Death Registry. Cox regression models with time-varying exposures estimated hazard ratios (HRs) per interquartile range (IQR) increase in heatwave exposures. Results Over a median follow-up of 12.8 years, 23,836 deaths occurred. For heatwaves lasting ≥ three consecutive days, each IQR increase in HWE, HWD, and HWI increased risks of all-cause mortality by 14% [95% Confidence Interval (CI): 12%, 17%], 21% (95% CI: 19%, 23%), and 10% (95% CI: 8%, 12%), respectively. Associations with cause-specific mortality varied by heatwave metrics and death causes. Higher risks were observed in those with lower socioeconomic status and those living in northern areas. The modifying effects of urbanicity differed across heatwave metrics. Conclusion Recurrent, prolonged, and intensified heatwave exposure is associated with elevated risks of all-cause and cause-specific mortality among UK adults. These associations differed according to heatwave characteristics and population context, including socioeconomic status, geographic region, and urbanicity. Our findings highlight the need for targeted heat-health adaptation strategies that address both the nature of heatwave exposure and population-specific vulnerabilities, thereby reducing the health burden of chronic heat exposure and strengthening resilience to climate change.