Abstract
Rising global temperatures and increasingly frequent heatwaves pose a growing threat to human health. Understanding how humans tolerate environmental heat has therefore become a central challenge in environmental physiology. Building upon the scientific legacy of John R. Sutton, this narrative review examines the physiological determinants of human heat tolerance and their implications for health in a warming climate. We first consider approaches used to characterize thermal strain, highlighting evidence that body temperature measurements provide only a partial representation of whole-body heat storage and physiological burden. We then review advances in whole-body calorimetry that have improved understanding of thermoregulatory capacity and revealed substantial variability in heat dissipation associated with aging, chronic disease, biological sex, fitness, acclimation, and other modifying factors. Foundational concepts of compensable and uncompensable heat stress are discussed alongside recent efforts to define critical environmental limits and human survivability thresholds. Emerging evidence indicates that heat tolerance reflects dynamic interactions between thermal demand and heat dissipation capacity rather than a single physiological or environmental threshold. We further examine findings from prolonged heat exposure and heatwave simulation studies demonstrating that physiological strain may develop under thermally compensable conditions and accumulate across multiple systems even when the body is able to maintain thermal equilibrium. These findings highlight an important distinction between human survivability and liveability during extreme heat events. Collectively, the evidence supports an integrative framework in which human heat tolerance is viewed as a dynamic, multisystem process shaped by environmental conditions, exposure duration, recovery opportunities, mitigation strategies, and individual physiological capacity.