An intense heat wave that sent temperatures in Phoenix to 118 degrees Fahrenheit (47.7 degrees Celsius) this weekend has killed four people—and the heat could be worse today.

Those killed so far were all hiking or biking outdoors, but heat waves can kill close to home, too. In 2003, during a major European heat wave, 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" (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. [7 Common Summer Health Concerns]

Part of the insidiousness of heat-related deaths is how quickly they can happen. According to ABC15 News, a mountain biker who died near Phoenix was a fit 28-year-old who had consumed plenty of water and was biking with two doctors. Her pulse stopped at around 9 a.m. on Sunday (June 19). Despite immediate resuscitation efforts, she could not be saved.

Sudden death

The deaths so far in Arizona aren't typical heat deaths, 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 (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 degrees F (40 degrees 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 in the textbook Wilderness Medicine (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. [Roasting? 7 Scientific Ways to Beat the Heat]

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 percent of heat stroke survivors experience permanent damage in brain function, according to Wilderness Medicine.

Far from help

As many as 10 percent of people who experience heat stroke die, according to the American Association of Family Physicians (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 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.

Phoenix, Tucson and other cities hit by the current heat wave are built for extreme 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.

Arizona's Department of Health Services 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.

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