Thermoregulatory Stress and the Ageing Mind: Investigating Environmental High Heat Exposure as a Risk Factor for Dementia

BackgroundExtreme heat exposure is a growing public health threat, particularly for older adults. While air pollution is a recognized modifiable risk factor for dementia, the role of sustained high heat exposure remains underexplored. This study investigates the association between cumulative extreme heat exposure and the prevalence of Alzheimers disease and related dementias (ADRD) and mild cognitive impairment (MCI) in adults. MethodsData were drawn from the All of Us Research Program (n = 286 767). High heat exposure was calculated using the CDCs National Environmental Public Health Tracking Network based on the number of extreme heat days within multi-day events from 2019 to 2023. Exposure was measured using daily maximum temperature and heat index. Diagnoses were identified from electronic health records. Logistic regression models estimated associations between heat exposure and ADRD/MCI, adjusting for demographic, health, behavioral, and socioeconomic variables. Stratified and interaction models assessed differences by age and sex. FindingsEvery 10 extreme heat days were associated with a 4{middle dot}0% increase in ADRD/MCI odds using maximum temperature (OR=1{middle dot}040, 95% CI: 1{middle dot}026-1{middle dot}051) and a 6{middle dot}0% increase using heat index (OR=1{middle dot}058, 95% CI: 1{middle dot}044-1{middle dot}071). Using age-stratified analysis the effect was stronger among adults aged [&ge;]70, with odds increasing by 7{middle dot}8% versus 3{middle dot}9% among those <70 using heat index. A significant interaction by age was observed (p<0{middle dot}001). Odds ratios were slightly higher in females, but sex did not significantly modify the association. Area deprivation accounted for 8{middle dot}6%-9{middle dot}6% of the association, while education and social isolation were not significant mediators. InterpretationCumulative high heat exposure is associated with increased odds of ADRD/MCI, especially among adults aged 70 and older. These findings underscore the need for climate adaptation strategies targeting older and socioeconomically disadvantaged populations. FundingThis research was supported by the All of Us Research Program, and funded by the National Institutes of Health. Research in contextO_ST_ABSEvidence before this studyC_ST_ABSWe searched PubMed and Google Scholar for studies published between January 2005 and April 2025 using combinations of the terms "extreme heat," "heat exposure," "heat waves," "dementia," "Alzheimers disease," "cognitive impairment," and "older adults." While many studies have shown that older adults face increased mortality and morbidity during extreme heat events, few have explored cognitive outcomes. Research specifically linking heat exposure to Alzheimers disease and related dementias (ADRD) is scarce. Most existing studies focus on short-term heat events and general health or mortality outcomes, rather than dementia. Although some evidence suggests temporary cognitive effects from heat, robust studies on diagnosed ADRD are lacking. Only one study has assessed the impact of cumulative extreme heat on cognitive function, and it did not examine clinical diagnoses or adjust for comorbid conditions. Added value of this studyTo our knowledge, this is the first national-level study in the United States to examine the association between cumulative high heat exposure and diagnosed ADRD or mild cognitive impairment (MCI) using detailed individual-level electronic health records. Our study used five years of environmental exposure data (2019-2023) and two distinct metrics--maximum temperature and maximum heat index--linked to a large, diverse cohort from the All of Us Research Program. We adjusted for demographic, clinical, behavioral, and social factors and explored interactions by age and sex. We also assessed mediation of the relationship by area deprivation, education, and social isolation. Our findings reveal that cumulative heat exposure is associated with increased odds of ADRD/MCI and that risk is more pronounced among individuals aged 70 and older. Implications of all the available evidenceCumulative high heat exposure may contribute to neurocognitive risk in later life, particularly among older adults. Our findings suggest that neighborhood-level socioeconomic disadvantage partly explains this association, underscoring the influence of structural and environmental conditions on dementia risk. This research supports the integration of heat-related vulnerability into dementia prevention frameworks and climate adaptation strategies. As climate change intensifies, understanding how environmental stressors such as heat affect cognitive ageing is essential for public health planning and policy. Targeted interventions and protective measures for older adults may help mitigate these long-term cognitive impacts.