This paper reviews the basic thermoregulatory physiology of healthy people in relation to hazards from external heat stress and internal heat loads generated by physical exercise or radiofrequency (RF) radiation. In addition, members of the population are identified who may be particularly vulnerable to the effects of heat stress. These data are examined in relation to current international guidance on occupational and public exposure to RF radiation. When body temperature rises, heat balance of the body is normally restored by increased blood flow to the skin and by sweating. These responses increase the work of the heart and cause loss of salt and water from the body. They impair working efficiency and can overload the heart and cause haemoconcentration, which can lead to coronary and cerebral thrombosis, particularly in elderly people with atheromatous arteries. These adverse effects of thermoregulatory adjustments occur with even mild heat loads and account for the great majority of heat-related illness and death. They are, therefore, particularly relevant to determination of safe population exposures to additional sources of heat stress. It is concluded that exposure to RF levels currently recommended as safe for the general population, equivalent to heat loads of about one tenth basal metabolic rate, could continue to be regarded as trivial in this context, but that prolonged exposures of the general population to RF levels higher than that could not be regarded as safe in all circumstances.