Vulnerability of older adults to heatwaves: the Barcelona case

Heatwaves are becoming more frequent, intense, and persistent as a result of global warming, posing a growing public health challenge in Mediterranean cities. In Barcelona, this process is characterized by a pronounced increase in nighttime temperatures, amplified by the urban heat island effect and by a rapidly aging population, which together increase vulnerability to heat-related health impacts. This study aims to assess the effects of high temperatures on mortality in Barcelona, with a particular focus on older adults, and to determine the relative importance of nighttime vs. daytime heat. To this end, the association between temperature and daily mortality was analyzed using time-series epidemiological methods. Health heat waves (HHW) were defined based on epidemiologically derived temperature-mortality relationships and persistence criteria, distinguishing between daytime (DHHW) and nighttime (NHHW) events. Generalized additive models were applied at the city scale to identify health-relevant temperature thresholds and HHW events, while generalized linear models were used at the provincial scale to evaluate temperature-related mortality risks among the older adults. Results show that nighttime temperatures play a more decisive role than daytime temperatures in shaping heat-related mortality in Barcelona. Nighttime health heat waves are more frequent, longer-lasting, and associated with substantially higher mortality than daytime events. Between 2007 and 2018, 721 deaths were attributable to NHHW, compared to 571 deaths attributable to DHHW. Provincial analyses further confirm that both maximum and minimum temperatures significantly increase mortality among older adults, although nighttime effects are likely underestimated due to spatial aggregation and the smoothing of extreme temperatures in reanalysis data. The main contribution of this study is to demonstrate that, in Barcelona, high nighttime temperatures constitute the most harmful thermal factor for human health, particularly among older adults, challenging heat-health frameworks that focus primarily on daytime extremes. These findings highlight the need to revise current heat-health warning systems by explicitly incorporating nighttime temperatures and persistence criteria, in order to improve prevention strategies in aging Mediterranean cities.