Indoor temperature is a function of outdoor temperature, the amount of outside solar radiation striking the structure, building insulation and ventilation characteristics. All these factors have influence on the ability of the structure to dissipate stored heat, intentional sources of heat (heating, ventilating, and air-conditioning), and other indoor sources of heat (e.g. artificial lighting, cooking appliances, occupant metabolic heat).

Healthy people can physically adapt to changes in ambient temperature within some limits. However, when temperatures push the upper end of those limits or are combined with other factors – such as high humidity, strenuous activity, or prolonged exposure – human physiologic compensation mechanisms can be overwhelmed. Also when people age, their ability to cope with external environmental stressors decreases.

Research indicates that obesity, hypertension, diabetes, and cardiovascular disease increase susceptibility to the effects of too high heat. Studies also show that controlling indoor temperatures and ensuring adequate ventilation rates were associated with improved work and school performance. Temperate indoor conditions (21–22°C) are associated with higher office and school productivity than colder or warmer environments. Thermal stress has well-documented adverse health effects, and is responsible for excess mortality among exposed persons.