A high level of climatic or environmental heat, possibly accompanied by high workload, may result in a variety of bodily heat disorders. These include heat stroke, heat exhaustion (circulatory, water, salt or sweat deficiency), skin disorders (prickly heat), and psychoneurotic disorders (heat fatigue). A tendency to heat disorders may prevent movement and employment of unacclimatized people in regions of great climatic heat.
Heat stress kills more people worldwide than any other weather phenomenon, including cyclones and floods. People can die from heat stress when severe dehydration causes the body to release heat shock proteins. These trigger a cascade of biochemical events that can culminate in a heart attack or stroke. Heat stress can also cause exhaustion, fainting, vomiting and diarrhoea.
Deaths rise by over 50 percent on average during heat waves, and are expected to climb even higher over the coming decades as a result of global warming.
'Heat stress' is the burden or load of heat that must be dissipated if the body is to remain in thermal equilibrium, and is represented by the sum of metabolic rate (minus external work) and gain or loss by convection and radiation. 'Heat strain' is the physiological or pathological change resulting from heat stress, including increase in heart rate and body temperature, sweating, heat syncope, or water and salt imbalance.
Permissible heat exposure threshold limit values have been recommended in terms of the Wet Bulb-Globe Temperature Index which most nearly correlates with the deep body temperature. As an example, the maximum temperatures for light work load are 30 deg C (continuous working), 32.2 deg C (25% work and 75% rest); for a heavy work load they are 25 and 30 deg C respectively.
In Rome, dry tropical air masses increase mortality by up to 25 percent. But the cities hardest hit by heat stress are not the hottest but those with very variable summer weather, like Melbourne and New York.
An analysis of ten Canadian cities found that heat-related mortality could increase in several large cities if temperatures increase as predicted by the greenhouse effect. Affected cities would be those that come under the influence of hot humid air masses -- Toronto, Montreal and Ottawa -- and in Montreal mortality rates could be twenty times that of today.