Effect of health status on physiological strain during hot/humid heat exposure in older adults - The Human Heat Stress Project

Introduction: Greater heat-related health risks are a direct consequence of global warming. These risks are precipitated by physiological heat strain, specifically an increase in core temperature and heart rate, and sweating-induced dehydration. Older adults face greater heat-related heath risks, especially amongst those living with chronic health conditions. To our knowledge, limited research has explored the effect of health status on physiological strain of older adults during environmental heat exposure. Objectives: The objective of this study is to compare physiological heat strain between healthy older adults, older adults with risk factors for chronic diseases, and older adults living with chronic disease during exposure to a hot and humid environment with short bouts of light exercise to simulate activities of daily living. We hypothesized that physiological heat strain would be greatest for adults living with chronic disease, followed by those living with risk factors compared to healthy older adults. Methods: Seven healthy older adults (60-74 years, 5 females), 23 adults living with ≥1 risk factor according to the harmonized definition of metabolic syndrome (61-88 years, 6 females), and 10 older adults living with ≥1 non-communicable chronic disease (61-84 years, 2 females) were exposed to a 38°C environment with 60% humidity in an environmental chamber for 4 hours. To induce energy expenditure associated with activities of daily living (~3.5 METs), participants performed 10 minutes of light exercise (treadmill walking or cycling) every hour. Participants could drink water served at 15°C ad libitum. Rectal temperature and heart rate were measured continuously. Dehydration was calculated as the difference between pre- and post-exposure nude body mass (post-void) relative to pre-exposure nude body mass. Data are presented as mean ± standard deviation. Results: During the 4 hours of heat exposure, rectal temperature increased by 0.4 ± 0.3°C in healthy older adults, 0.5 ± 0.3°C in those living with at least 1 risk factor, and by 0.5 ± 0.4°C in older adults living with chronic disease. Heart rate increased by 11 ± 6 bpm in healthy older adults, 15 ± 7 bpm in those living with at least 1 risk factor, and by 9 ± 4 bpm in older adults living with chronic disease. Dehydration was 0.3 ± 1.0% in healthy older adults, 0.7 ± 0.8% in those living with at least 1 risk factor, and 0.8 ± 0.7% in older adults living with chronic disease. Conclusion: During exposure to hot/humid heat that integrates light exercise to simulate activities of daily living, these preliminary results suggest that thermal and cardiac strain do not differ markedly as a function of health status amongst older adults. However, older adults living with at last 1 risk factor or those living with chronic disease seem to experience greater dehydration. The clinical consequences of these differences remains to be determined. Canadian Institutes of Health Research This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.