When extreme heat overwhelms the body beyond its ability to regulate temperature, a cascade of heat-related illnesses ensues: heat cramps, heat exhaustion, heat stroke, and hyperthermia, in order of increasing severity.
In fact, the exacerbation of chronic conditions and not heat stroke itself is the main cause of death during a heat wave. Beyond its direct effects on human physiology, exposure to extreme heat indirectly compromises human health by restricting people’s capacity to work and exercise and by negatively impacting mental wellness.
These include the elderly, children, and pregnant and breastfeeding women. According to Lancet Countdown, the annual heat-related mortality of people aged 65 and older increased by 68% between 2000–2004 and 2017–2021. Children are at higher risk because their bodies adjust more slowly to changes in environmental temperature. Children also have less agency to change their behaviors, lifestyles and surroundings to better adapt to extreme heat.
The links between extreme heat, human health, and climate change are clear – research has shown that 37% of heat-related deaths are attributable to human-induced climate change.
The optimal ‘set point’ for human body temperature is 37°C (98.6°F). When body temperature rises above this set point, blood thickens and the heart pumps harder to circulate blood to organs. If blood circulation is compromised, organ damage occurs.
When the body is overwhelmed by large increases in surrounding temperature and can no longer sufficiently cool itself by sweating, dehydration and mild symptoms of heat exhaustion set in. If not treated, heat exhaustion quickly escalates to more serious and deadly heat illnesses such as heat stroke.
A wet bulb temperature of 35°C (95°F) marks the upper limit to the human body’s ability to shed heat. Wet bulb temperature is a technical indicator that combines heat and humidity to gauge heat stress, and is defined as the temperature read by a moistened thermometer bulb exposed to air flow. In other words, a wet bulb temperature of 35°C represents a literally uninhabitable environment in which humans cannot survive.
Some coastal subtropical locations may have already reached this level for very brief periods of time, namely in South Asia, the coastal Middle East, and coastal southwest North America. The health consequences of extreme heat are already significant at temperatures well below the 35°C wet bulb threshold—for instance, the European heatwave of 2003 and the Russian heatwave of 2010 together caused over 125,000 deaths, and they corresponded to wet bulb temperatures of 28°C or less.
While this list is by no means exhaustive, here are some of the most common effects that extreme heat has on the human body.
Heat cramps are the mildest form of heat illness and consist of painful muscle cramps and spasms that occur during or after intense exercise in high heat. When the body is dehydrated, heat cramps progress to heat exhaustion with headache and fatigue, which when left untreated escalates to heat stroke. Heat stroke is a life-threatening medical emergency
Breathing in hot and humid air can trigger asthma attacks. Air pollution, pollen, and other allergens tend to concentrate in hot and humid conditions, and these air particles further irritate sensitive airways. People with compromised lungs, such as individuals with chronic obstructive pulmonary disease (COPD), experience extra difficulty breathing in the midst of heat.
High temperatures place a burden on the heart to pump blood through dilated vessels in order for the body to dissipate heat. This in turn increases the risk of heart attacks and heart failure. Heat-induced dehydration also causes electrolyte imbalances in the blood, which can lead to heart arrhythmias.
High temperatures are associated with premature birth (studies have shown a 16% increase in preterm births during heat waves) and low birthweight in newborns. Mothers are affected, too – maternal exposure to heat waves during pregnancy correlates with increased risk of high blood pressure, eclampsia, and uterine bleeding.
“Summer diarrhea” is a real thing, y’all. A research study found an increased risk of infectious gastroenteritis outbreaks and inflammatory bowel disease (IBD) flares during heat waves. Individuals with sensitive guts and irritable bowel syndrome (IBS) are also more susceptible to diarrhea when their bodies go into stress mode from hot weather. More broadly, diarrhea is one of the symptoms of heat exhaustion.
Your kidneys are in charge of regulating the volumes and electrolyte compositions of the fluids in your body. This means that they’re also particularly sensitive to dehydration. Occupational heat exposure has been linked to the development of chronic kidney disease due to repetitive kidney damage over time. Heat stroke is associated with acute kidney injury, and heat stress predisposes individuals to the development of kidney stones.
Your thermal environment affects your sleep quality because your body’s temperature regulation is closely tied to your body’s sleep regulation mechanism. Your core body temperature is supposed to drop during sleep, so when hot surroundings suppress this decrease in core body temperature, your sleep is disrupted. Furthermore, hot surroundings prevent you from entering the deep-sleep phase, which means that you may wake up feeling poorly rested.