Why overheating differs from a fever, even when the thermometer reads the same
A fever and dangerous overheating can show identical temperatures, yet the body is doing opposite things. In fever, the brain deliberately raises its thermostat. In hyperthermia, the thermostat stays put while heat piles up faster than the body can shed it, a failure linked to almost half a million deaths each year.
The distinction hinges on a set point, an optimal temperature maintained by nerve activity in the preoptic region of the anterior hypothalamus, which triggers sweating or shivering as needed. During infection, white blood cells release pyrogens that push this set point higher, producing a fever. In hyperthermia the control centre is unchanged; the body simply absorbs or produces more heat than it loses. Doctors can use this difference: if fever-reducing drugs bring the temperature down, hyperthermia is ruled out.
People cool themselves mainly by sweating, which works through evaporation and falters in humid air, and by widening skin blood vessels, which sheds heat in proportion to the gap between body and surroundings, following Newton's law of cooling. When those systems are overwhelmed, enzymes that drive processes such as cellular respiration lose efficiency and eventually denature, hitting energy-hungry organs such as the heart and brain first. The word combines Greek hyper, above, with thermos, heat.
Heat stroke and drug reactions are the commonest causes. Some anaesthetics trigger a rare inherited reaction called malignant hyperthermia. Protective gear matters too: during the 2014 Ebola epidemic in West Africa, health workers could last only 40 minutes at a stretch in their suits for fear of heat stroke. Heat waves take a heavy toll on older and frail people; Chicago's July 1995 heat wave caused at least 700 heat-related deaths, with living alone and being bedridden the strongest risk factors, and working air conditioning lowering risk.
Temperatures above 40 degrees Celsius can threaten life. The highest recorded in a survivor was 46.5 degrees Celsius, measured on 10 July 1980 in a man hospitalised with severe heat stroke.
Source: Hyperthermia
