Heat illness

Heat illness describes a spectrum of medical disorders caused by an increase in body temperature. These conditions result from either environmental heat exposure or intense physical exertion. The severity of the illness varies significantly across the spectrum. Minor conditions include heat rash, heat cramps, and heat edema. More serious issues include heat syncope and heat exhaustion. The most critical condition is heat stroke, which is a medical emergency. Heat illness can affect any or all anatomical systems within the human body.

Different heat illnesses involve specific biological mechanisms. Heat edema causes swelling in the hands, feet, and ankles. This happens because the body increases aldosterone secretion, which leads to water retention. This process, combined with peripheral vasodilation, causes fluid to accumulate in the extremities. Heat rash, or prickly heat, occurs when sweat ducts become blocked. This can cause the ducts to dilate or even rupture, creating small itchy vesicles. Heat cramps often result from hyponatremia, which is a lack of electrolytes. This typically happens when people perform heavy exercise and drink only water without replenishing salts.

Medical professionals classify these conditions by their specific symptoms and severity. Heat stroke is defined by a body temperature greater than 104°F (40°C) caused by environmental heat. It involves a failure of thermoregulation, which is the body's ability to control its own temperature. Symptoms include dry skin, dizziness, and a rapid, strong pulse. Heat exhaustion is often a precursor to heat stroke. It is marked by heavy sweating, rapid breathing, and a fast, weak pulse. Unlike heat stroke, a person with heat exhaustion maintains intact neurologic function. Heat syncope causes fainting or dizziness due to orthostatic hypotension, or a drop in blood pressure.

History shows that humans have struggled with heat for millennia. The Greek physician Hippocrates noted illnesses caused by excessive heat in early records. In the past, people sometimes blamed heat illnesses on a tropical fever called calenture. Military history also provides examples of heat's impact on large groups. When Alexander the Great led his army across the Gedrosian desert, he lost two-thirds of his soldiers. These men suffered from dehydration and heat fatigue. These historical accounts demonstrate that heat has always been a significant biological challenge.

Global statistics highlight the massive scale of heat-related mortality. Between 2000 and 2019, approximately 489,000 heat-related deaths occurred annually. Of these, 45% occurred in Asia and 36% in Europe. In the summer of 2022, Europe saw an estimated 61,672 excess heat-related deaths. In the United States, heat illness is common in emergency departments. An average of 5,946 people are treated annually in US emergency rooms. Most of these patients are male, accounting for 72.5% of cases. Additionally, 35.6% of these patients are between 15 and 19 years of age.

Certain activities and professions carry much higher risks for heat illness. In American sports, football has the highest rates of exertional heat illness. Football is 11.4 times more likely to involve heat illness than all other sports combined. August is particularly dangerous, accounting for 66.3% of exertion-related heat illness time-loss events. Outdoor workers are also at high risk. Between 1992 and 2006, 423 workers died from heat illness in the United States. The transportation and material moving sector was the most dangerous, representing 25.4% of nonfatal occupational injuries. In 2015, Kansas had the highest rate of work-related heat injury.

Prevention and climate trends are vital areas of modern study. Experts recommend gradual adjustment to heat and consuming sufficient fluids and electrolytes. Avoiding certain medications, such as diuretics or anticholinergics, can also reduce risk. These medications can interfere with the body's ability to dissipate heat. Climate change is expected to increase these risks significantly. A 2016 U.S. government report suggests climate change could cause tens of thousands of additional deaths annually by the end of this century. For example, heat exposure deaths tripled in Arizona between 2014 and 2017.