Why heat waves kill so quickly

A male construction worker wipes sweat off his forehead.
When core body temperature gets too high, everything breaks down. (Image credit: FG Trade via Getty Images)

It's been a sweltering week for many in the northern hemisphere. Temperatures in parts of England rose past 104 degrees Fahrenheit (40 degrees Celsius) on Tuesday (July 19), a record never before seen in the country, while more than 100 million Americans were under excessive heat warnings as of Tuesday evening. The heat is not just uncomfortable. It can be deadly. 

In Spain and Portugal, the broiling temperatures of the last two weeks have contributed to 1,169 deaths, according to ABC News (opens in new tab). The fatalities harken back to the devastating 2003 European heat wave, in which 14,802 people died of hyperthermia in France alone. Most were elderly people living alone in apartment buildings without air conditioning, according to Richard Keller, a University of Wisconsin-Madison professor of medical history and bioethics and author of "Fatal Isolation: The Devastating Paris Heat Wave of 2003 (opens in new tab)" (University of Chicago Press, 2015).

So how does heat kill? When core body temperature rises too high, everything breaks down: The gut leaks toxins into the body, cells begin to die, and a devastating inflammatory response can occur. 

Part of the insidiousness of heat-related deaths is how quickly they can happen. Older individuals are more at risk, often because their cardiovascular systems are less resilient to the strain caused by excess heat, according to a 2014 article in the journal Medicine & Science in Sports & Exercise (opens in new tab). But in extreme enough temperatures, even young, physically fit people can succumb quickly. One victim of a heat wave in Phoenix in 2017 was a personal trainer who was mountain biking with friends on a day when temperatures would soar to 118 F (47.7 C). Despite drinking plenty of water and biking with two doctors who immediately attempted to resuscitate her, the woman died, according to ABC15 News (opens in new tab). And in 2021, Philip Kreycik, an expert trail runner in California succumbed to heat stroke, on a day when temperatures approached the triple-digits, Outside reported (opens in new tab).

And in Northern California that same summer, a family was found dead in Sierra National Forest for similar reasons. The story made national news because it was not clear at first what had killed the family. But an investigation showed that temperatures reached up to 109  F (42.7 C) that August day, and the family ran out of water, according to NPR (opens in new tab). Jonathan Gerrish, Ellen Chung, their 1-year-old daughter, Miju, and their dog, Oski, all died. 

Sudden death

These kinds of tragedies aren't typical heat deaths, however, Keller told Live Science. Rather, they're "like shots across the bow telling you that something is coming," he said. Outdoorsy types and outdoor workers like roofers might suffer first, but it's the elderly and the mentally ill who make up the majority of deaths.

The medical term for excessive body heat is hyperthermia. The first phase is heat exhaustion, a condition marked by heavy sweat, nausea, vomiting and even fainting. The pulse races, and the skin goes clammy. Muscle cramping can be an early sign of heat exhaustion, according to the Centers for Disease Control and Prevention (opens in new tab) (CDC).

Heat exhaustion can be reversed by moving to a cool location, loosening clothing and applying cool, wet washcloths to the body. But when people with heat exhaustion can't find relief, they can quickly advance to heat stroke. This condition happens when a person's core body temperature rises above 104 F (40 C). (This number is something of an estimate; there are a few degrees' variability among people as to how much internal heat they can tolerate.)

In heat stroke, sweating stops and the skin becomes dry and flushed. The pulse is rapid. The person becomes delirious and may pass out. When trying to compensate for extreme heat, the body dilates the blood vessels in the skin in an attempt to cool the blood. To do this, the body has to constrict the blood vessels in the gut. The reduced blood flow to the gut increases the permeability between the cells that normally keep gut contents in, and toxins can leak into the blood, according to a book chapter (opens in new tab) in the textbook "Wilderness Medicine (opens in new tab)" (Mosby, 2011).

These leaky toxins trigger a massive inflammatory response in the body, so massive that the attempt to fight off the toxins damages the body's own tissues and organs. It can be hard to tell what damage is caused directly by heat and what is caused by the secondary effects of toxins, according to Wilderness Medicine. Muscle cells break down, spilling their contents into the bloodstream and overloading the kidneys, which in turn start to fail, a condition called rhabdomyolysis. 

Proteins in the spleen start to clump as a direct result of heat; they're essentially cooked. The blood-brain barrier that normally keeps pathogens out of the brain becomes more permeable, allowing dangerous substances into the brain. Autopsies of people killed by heat stroke often reveal microhemorrhages (tiny strokes) and swelling, and 30% of heat stroke survivors experience permanent damage in brain function, according to Wilderness Medicine.

Far from help

As many as 10% of people who experience heat stroke die, according to the American Association of Family Physicians (opens in new tab) (AAFP). Heat exhaustion requires immediate medical treatment and rapid cooling.

In the case of a hiker on a trail, there may not be time to get to a spot that's cool enough to reverse the damage. Similarly, people who live in urban areas and lack air conditioning may end up disabled in their own homes, unable to get help before they die from heat stroke.

The elderly and those with chronic medical conditions have more difficulty regulating their body temperatures than those in midlife, Keller said, and medications for some chronic diseases can make the problem worse. Likewise, the signals between body and brain that make people feel thirsty may not function as well in old age. (Babies and young children also have more difficulty regulating their temperature than people in the prime of life.)

The elderly, neurologically disabled and mentally ill also tend to be more socially isolated than their younger, healthier counterparts.

"They tend to find themselves socially isolated," Keller said. "And that's really, far and away, the biggest risk factor for dying during a heat wave."

In France in 2003, the heat hit in August, when many Europeans go on vacation. Elderly people found themselves in mostly empty apartment buildings when the heat crisis reached them. Some were found dead with their doors ajar, Keller said, suggesting that they were trying to get out and get help when they collapsed.

Others were functionally trapped, he said. An 80-year-old in a seventh-floor walkup who recently had hip surgery can't get down the stairs by themselves.

"They had no way to seek help," Keller said.

Finally, some may not have realized the severity of the situation. A 2013 analysis by the New York Department of Health and Mental Hygiene found that people who died of heat stroke in that city were not necessarily more likely to live alone (opens in new tab) than people who survived, in contrast to the 2003 European heat wave. However, the people who died in New York might not have been aware of the warning signs of heat stroke, the researchers wrote. Some people during the European heat wave probably thought they were going through an uncomfortable time and didn't recognize how precarious their survival was, Keller said.

Regions used to high heat are built to keep people comfortable despite soaring temperatures, Keller said, so they're unlikely to see high levels of mortality. Most at-risk are low-income people or those living in marginal housing, such as mobile homes, he said. The United Kingdom is not accustomed to dealing with heat; a 2021 government report estimated that only 5% of homes in England have air conditioning.

Arizona's Department of Health Services (opens in new tab) has shared the following tips for preventing heat illness:

  • Drink at least 2 liters (about a half-gallon) of water per day if you are mostly indoors and 1 to 2 additional liters for every hour of outdoor time. Drink before you feel thirsty, and avoid alcohol and caffeine.
  • Wear lightweight, light-colored clothing and use a sun hat or an umbrella to deflect the sun's rays.
  • Eat smaller, more frequent meals instead of large ones.
  • Avoid strenuous activity.
  • Stay indoors as much as possible.
  • Take regular breaks if you must exert yourself on warm days.

Original article published on Live Science on June 20, 2016 and updated on July 20, 2022.

Stephanie Pappas
Live Science Contributor

Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.