Global warming is substantially increasing the frequency, duration, and intensity of human exposure to high ambient temperatures and has become an increasingly important global public health challenge. As global mean temperatures continue to rise, heatwaves are expected to occur more frequently and persist for longer periods, exposing a growing proportion of the population to potentially harmful thermal stress. A substantial body of evidence has demonstrated that exposure to high temperatures is associated with an increased risk of adverse health outcomes, including cardiovascular morbidity and mortality, heat stroke, cardiac arrest, and other heat-related illnesses. Current public health recommendations, including those issued by the World Health Organization, primarily emphasize minimizing heat exposure, maintaining cool indoor environments, and ensuring adequate hydration. However, complete avoidance of heat exposure is often impractical in daily life because of occupational demands, commuting, recreational activities, and routine physical activity. Therefore, in addition to reducing environmental exposure, strategies that improve physiological resilience and enhance heat tolerance may provide an important complementary approach to reducing heat-related health risks in a warming climate.
Exercise performed in hot environments has long been used to promote heat adaptation and improve heat tolerance, particularly among athletes and military personnel. Repeated exercise in the heat can induce a range of physiological adaptations that reduce cardiovascular and thermoregulatory strain during subsequent heat exposure. Typical adaptations include reductions in resting and exercise core temperature, lower heart rate during exercise, enhanced sweating responses, improved heat dissipation, and better aerobic performance under hot conditions. These adaptations collectively improve the ability of the human body to maintain thermal balance and cardiovascular stability during exercise in the heat.
However, the specific contribution of environmental heat exposure to these adaptations remains unclear. Exercise itself is a potent source of thermal stress because sustained skeletal muscle contraction substantially increases metabolic heat production. Even when exercise is performed under thermoneutral conditions, the resulting increase in internal heat production can elevate body temperature and impose considerable demands on the cardiovascular and thermoregulatory systems. Therefore, some of the adaptations traditionally attributed to exercise in the heat may actually be induced, at least partly, by exercise-generated metabolic heat rather than by environmental heat exposure alone. This raises an important question: Does exercising in a hot environment provide additional physiological benefits beyond those induced by exercise itself, or is metabolic heat production during thermoneutral exercise sufficient to promote meaningful heat-related adaptations?
To address this question, a team of researchers led by Professor Stephen Heung-Sang Wong and Assistant Professor Eric Tsz-Chun Poon from The Chinese University of Hong Kong, China, conducted a systematic review and meta-analysis to directly compare exercise performed in hot conditions with exercise performed under thermoneutral conditions. Three complementary comparisons were conducted: exercise in hot conditions (ExH) versus exercise in thermoneutral conditions (ExN), pre- versus post-intervention responses following ExH, and pre- versus post-intervention responses following ExN. The primary outcomes included aerobic performance in the heat, resting and exercise core temperature, exercise skin temperature, exercise heart rate, and sweat rate. This analytical approach allowed the additional contribution of environmental heat exposure to be evaluated while also examining whether exercise alone could induce heat-related physiological adaptations. The findings of this study were made available online in the Journal of Sports and Health Science on August 24, 2026.
A total of 23 studies were included in the final analysis. Compared with ExN, ExH produced greater improvements in aerobic performance under hot conditions. It also resulted in significantly greater reductions in resting core temperature, exercise core temperature, and exercise heart rate, together with a greater increase in sweat rate. In contrast, no significant between-group difference was observed for exercise skin temperature. "These findings indicate that additional environmental heat exposure during exercise provides a meaningful adaptive stimulus beyond that produced by exercise alone, particularly for cardiovascular and thermoregulatory responses," says Prof. Wong.
Importantly, however, ExN was not physiologically ineffective. Following thermoneutral training, exercise core temperature decreased significantly by approximately 0.14 °C, while exercise heart rate decreased by approximately 5.6 beats/min during subsequent ExH. These findings suggest that exercise-induced metabolic heat production and general exercise training adaptations may themselves promote a certain degree of heat resilience, even in the absence of additional environmental heat exposure. Nevertheless, the magnitude and range of adaptations were generally greater for ExH.
"Overall, these findings support the use of ExH as an effective strategy for enhancing heat tolerance and improving aerobic performance under heat stress. At the same time, they highlight the potential contribution of exercise itself to the development of partial heat-related adaptations," says Dr. Poon. This distinction is particularly important when considering the development of practical, safe, and individualized strategies to improve human resilience to rising environmental temperatures.
Reference
Titles of original papers: Exercise in hot or thermoneutral conditions? Comparative effectiveness on aerobic performance and physiological adaptations in the heat: A meta-analysis
Journal: Journal of Sport and Health Science
DOI: https://doi.org/10.1016/j.jshs.2026.101168
About The Chinese University of Hong Kong, China
The Chinese University of Hong Kong (CUHK) is a leading research university in Hong Kong, China. Founded in 1963, CUHK is known for its strong academic and research environment, international outlook, and distinctive collegiate system. The university offers a wide range of programmes across disciplines, including medicine, science, engineering, social sciences, business, and the humanities.
About Authors
About Assistant Professor Eric Tsz-Chun Poon from The Chinese University of Hong Kong, China
Professor Tsz-Chun Poon is currently an Assistant Professor in the Department of Sports Science and Physical Education at The Chinese University of Hong Kong. His research primarily focuses on high-intensity interval training (HIIT), heat acclimation training, and cardiometabolic health. Several of his research projects have been funded by the Hong Kong Research Grants Council and the Health Bureau. He also serves as an Associate Editor and Editorial Board Member for several international journals in the fields of sports science and physical fitness.
About Professor Stephen Heung-Sang Wong from The Chinese University of Hong Kong, China
Professor Stephen Heung-Sang Wong is currently a Professor in the Department of Sports Science and Physical Education at The Chinese University of Hong Kong and serves as the Head of United College. He is a Fellow of the American College of Sports Medicine and serves as the Asia representative of the Active Healthy Kids Global Alliance (AHKGA). He is also the Editor-in-Chief of the Journal of Exercise Science & Fitness and serves on the editorial boards of several international academic journals in the fields of health and exercise science.
Funding information
This work was supported by the Research Grants Council (RGC) General Research Fund (GRF), Hong Kong Special Administrative Region, China (14618825).
