Heat stress vulnerability and critical environmental limits for older adults

Anthropogenic climate change is increasing global temperatures, while the population is aging rapidly. Individuals aged 65 and older experience disproportionately higher morbidity and mortality during heat waves. While age-related declines in thermoregulation are well-documented (decreased sweating, skin blood flow, increased cardiac strain), the impact on heat vulnerability in apparently healthy older adults remains unclear. Defining specific combinations of ambient temperature and humidity above which heat balance becomes uncompensable (critical environmental limits) is crucial for older adults, as most heat-related deaths occur during daily activities in indoor environments. Previous research has established critical environmental limits for young adults during low-intensity activities, providing a benchmark for comparison with older populations. This study aimed to 1) document increased heat vulnerability in healthy older adults and 2) establish and compare critical environmental limits for older adults at rest and during minimal physical activity simulating activities of daily living (ADL). The hypothesis was that older adults would experience uncompensable heat stress at lower temperature and humidity combinations than young adults, with even lower limits during minimal activity compared to rest. This research is essential for developing evidence-based communication strategies, policies, and interventions to protect older adults from heat-related harm during extreme heat events.

Existing literature confirms age-related physiological changes that impair thermoregulation, including reduced sweating, decreased skin blood flow, and increased cardiac strain. These physiological changes make older adults particularly vulnerable to heat stress. Although it is known that individuals over 65 years of age make up the majority of heat-related deaths during extreme heat events, the specific temperature-humidity combinations above which age-related differences in heat tolerance emerge are poorly understood. Previous studies have established critical environmental limits (combinations of temperature and humidity above which heat balance cannot be maintained) for young adults during various activities, but these limits for older adults, particularly during low-intensity activities resembling those of daily living, are not well defined. The absence of this information is a critical gap in our knowledge, hindering the development of effective strategies to safeguard this vulnerable population.

This study used a progressive heat stress protocol in an environmental chamber to determine critical environmental limits. One hundred and twelve participants were screened, with 51 young adults (23 ± 4 years) and 49 older adults (71 ± 6 years) completing the study. Participants, but not investigators, were blinded to the environmental condition during testing. Young and older adults performed minimal activity (MinAct) trials on a recumbent cycle ergometer at zero resistance, simulating ADL. Older adults also completed seated rest (Rest) trials under the same conditions. During T_crit trials, ambient temperature (T_ab) was held constant (34, 36, 38, or 40 °C), while during P_crit trials, ambient water vapor pressure (P_a) was constant (12 or 16 mmHg). T_ab or P_a was progressively increased in 5-min steps. Gastrointestinal temperature (T_gi), heart rate (HR), oxygen consumption (VO2), and respiratory exchange ratio (RER) were continuously measured. Sweat rate was calculated from changes in body mass. The critical T_ab or P_a was defined as the point where a sustained increase in T_gi occurred, indicating uncompensable heat stress. Data were analyzed using independent-samples t-tests, mixed-effects models, and the Gehan-Breslow-Wilcoxon test for compensability curves. Psychrometric loci were plotted on a standardized psychrometric chart. The rate of change in T_gi was calculated above and below critical environmental limits. All experimental procedures were approved by the Institutional Review Board, and the study is registered on ClinicalTrials.gov (NCT0428439).

The study found significant age-related differences in heat vulnerability. Older adults showed a leftward shift in compensability curves compared to young adults during MinAct, indicating uncompensable heat stress at lower temperature and humidity combinations. During MinAct, critical environmental limits were significantly lower in older adults than in young adults across all conditions. Similarly, older adults had lower critical environmental limits during MinAct compared to Rest, except for one condition (34°C P_crit). Metabolic rates (M_net, VO₂, METs) did not differ significantly between age groups during MinAct, but older adults exhibited lower sweat rates and percent body mass loss than young adults. The slope of the critical environmental limits was steeper in older adults in hot, dry conditions, reflecting lower sweating capacity compared to young adults. This difference was also observed when comparing MinAct to Rest in older adults. The rate of change in T_gi below the critical limit was minimal and consistent across age groups and metabolic rates, but significantly increased above the critical limit. Specific data points include: (a) Significant differences (p < 0.0001) in critical environmental limits between age groups during MinAct across all tested conditions, and (b) Significant differences (p < 0.0001) in critical environmental limits between MinAct and Rest trials in older adults, except for the 34 °C P_crit condition.

The findings confirm increased heat vulnerability in apparently healthy older adults, demonstrating their inability to maintain thermal balance under conditions tolerated by young adults. The lower critical environmental limits observed in older adults during MinAct highlight their increased risk during even minimal physical activity in warm or humid conditions. The differences in critical limits between MinAct and Rest in older adults emphasize that even common daily activities increase heat stress risk in this population. The steeper slope of the critical environmental limit curve in hot, dry conditions for older adults underscores their reduced evaporative cooling capacity. These data are crucial for informing public health interventions, as they provide precise physiological limits to guide the development of effective heat warnings and safety strategies for older adults. The consistency of the rate of change in T_gi above and below the critical limits across age groups and metabolic rates allows for more accurate projections of core temperature increases under different environmental conditions, improving the prediction of heat illness.

This study establishes the heat vulnerability of older adults by defining critical environmental limits during rest and minimal activity. These limits are significantly lower in older adults than in young adults, highlighting their increased susceptibility to heat stress during everyday activities. The data provide a critical physiological basis for evidence-based interventions and strategies to mitigate heat-related harm in older adults. Future research could investigate the influence of comorbidities, medications, and sex differences on critical environmental limits and explore effective personalized heat safety strategies.

The study did not control for heat acclimatization or examine the effects of comorbidities or medications. Participants resided in a specific region, potentially influencing generalizability. The study's indoor setting may limit the extrapolation of findings to outdoor environments. Finally, sample size did not allow definitive analysis of sex differences.