Health risks of hot and cold temperatures in people with pre-existing conditions and social disadvantage

Evidence on the health impacts of ambient temperature relies largely on population-level analyses, providing limited insight into risks among clinically and socially vulnerable individuals, including those with pre-existing disease or socioeconomic disadvantage. Here we show, in 275,763 Finnish adults and an independent UK cohort of 9,681 adults, that health risks during extreme temperatures differed substantially across vulnerability groups. We compared mortality and emergency hospital visits during the hottest and coldest 2.5% of seven-day periods with those during non-extreme temperature periods. Excess heat- and cold-related mortality was more than tenfold higher among individuals with both clinical and social vulnerability than among those with neither or only one vulnerability in both the Finnish and UK cohorts ( P = 0.03 and P = 0.003, respectively). By contrast, excess temperature-related mortality differed little between individuals with neither vulnerability and those with only one vulnerability. Our modelling suggests that future warming could increase temperature-attributable mortality across the population, particularly among individuals with both clinical and social vulnerability. These findings identify individuals with both clinical and social vulnerability as a priority group for climate adaptation strategies.