Adults juggling both chronic illness and neighborhood deprivation face a mortality risk more than ten times higher during extreme temperature swings than those with neither vulnerability. Published in Nature Communications, a large-scale study tracking more than 285,000 adults across Finland and the UK details how environmental extremes disproportionately threaten vulnerable populations as the climate warms.
In Plain English: The Clinical Takeaway
- Double Vulnerability: Patients managing pre-existing medical conditions alongside socioeconomic disadvantage face a sharply magnified risk of death during severe heatwaves or cold snaps.
- Physiological Strain: Extreme temperatures act as acute stressors, worsening pre-existing or subclinical illnesses.
- Preventive Action: Public health strategies are needed to protect vulnerable people from adverse health outcomes during extreme weather conditions.
Tracking Mortality Rates Across Extreme Weather Thresholds
Researchers analyzed individual-level health records from 275,763 Finnish adults and 9,681 UK adults to evaluate how environmental temperatures alter acute health outcomes. Clinical vulnerability in the cohort was defined by 17 chronic diseases or disease groups, including diabetes, cancer, and cardiovascular and kidney conditions. Social vulnerability was determined from neighborhood-level deprivation scores rather than individuals’ incomes or housing conditions.
The investigators defined extreme temperatures as the hottest and coldest 2.5% of seven-day periods in each cohort. In the Finnish cohort, seven-day average ambient temperatures above 20.4 °C or below –11.4 °C were associated with 1.4-fold and 1.1-fold higher mortality rates, respectively, than moderate temperature exposure. The estimated increase in cold-related mortality in Finland was not statistically significant in the main analysis.
When researchers isolated individuals carrying both clinical and socioeconomic burdens—numbering 15,326 participants in Finland and 354 in the UK—the data revealed a striking divergence. Excess mortality associated with hot and cold periods combined was more than tenfold higher within this dual-vulnerability subgroup compared to participants with neither or only one vulnerability. Excess temperature-associated mortality differed little between individuals with neither vulnerability and those with only one vulnerability.
| Cohort Group | Participant Count | Key Vulnerability Criteria | Relative Mortality Risk (Extreme Temperatures) |
|---|---|---|---|
| Finnish Cohort | 275,763 | 17 chronic diseases or disease groups / neighborhood deprivation | 1.4-fold (Heat >20.4 °C); 1.1-fold (Cold <-11.4 °C) |
| UK Cohort | 9,681 | Neighborhood deprivation metrics | Similar patterns to Finnish cohort |
Pathophysiological Mechanisms and Non-Fatal Emergency Visits
During heat and cold exposures, the human body maintains core temperature through sweating and vasodilation (widening of blood vessels) and through vasoconstriction (narrowing of blood vessels) and thermogenesis, respectively. These regulatory processes can be impaired by preexisting health conditions, further increasing the risk of temperature exposure-related morbidity and mortality.
Beyond mortality, the Finnish dataset tracked non-fatal emergency hospital visits. Cold periods were associated with a 9% lower rate of emergency hospital visits, whereas hot periods were associated with a 5% higher rate of such visits. The observed associations were stronger among individuals with both clinical and social vulnerability than those with neither vulnerability or only one vulnerability.
The study associated higher temperatures with increased rates of emergency hospital visits for infectious and digestive diseases. Emergency hospital visits for skin, neurological, and mental disorders, and for external causes, also increased with higher temperatures, while respiratory-related visits were less frequent.
Projecting Future Climate Pressures on Regional Healthcare Systems
Climate modeling suggested that an increase from approximately 1°C to 4°C could raise the proportion of deaths attributable to extreme temperatures from about 1% to more than 5%, roughly a fivefold increase in both Finland and the UK. Climate projections suggest that as warming progresses, increases in heat-related mortality could outweigh reductions in cold-related mortality.
Because unemployed people and those with work disabilities were excluded at baseline, the participants are generally healthier than the overall population. These factors may have led to an underestimation of the adverse health effects of extreme temperatures. As the study was observational, the reported associations cannot establish that temperature exposure directly caused the estimated excess deaths.
References
- Kivimäki, M., Pentti, J., Frank, P. et al. Health risks of hot and cold temperatures in people with pre-existing conditions and social disadvantage. Nature Communications, 2026. DOI: 10.1038/s41467-026-79203-3.
