July 7, 2023 – If you clicked on any major news site this week, there was one consistent headline that would be difficult to miss, let alone ignore: “Hottest Day Ever Recorded on Earth.”

That day was July 3, when average global temperatures reached 62.62 F. But that record was short-lived, as July 4 saw temperatures rise again, to 62.92. Dig a bit deeper, and that seemingly unspectacular average temperature translated into highs of 122 F in Africa. In Antarctica, where it’s currently winter, temperatures reached 47.6 F. And in the U.S., The Washington Post warned that as many as 54 million Americans were in danger of exposure to dangerous (aka “extreme”) heat on that day alone. Scientists are warning that we’re in uncharted territory. And humans are close to reaching the peak of their ability to adapt.

“When we’re talking about evolution, we’re talking about millions of years for [humans] to generate this capacity to thermoregulate,” said Camilo Mora, PhD, a professor of data analytics at the University of Hawaii-Manoa. “So, whenever you increase the temperature outside, it’s going to take millions of years for us to adapt logically to it,” he said.

Mora and his colleagues have spent decades modeling that risk of extreme heat as it relates to global climate change, showing that in the last decade, the planet has warmed by about 1 C (1.8 F), resulting in a greater than 2,300% increase in the loss of human life to heat waves alone.

Multiple Ways Heat Can Kill You

When most people think about the effects of extreme heat, they naturally consider things like fatigue, headaches, or feeling a bit faint or nauseated. But these symptoms are simply the tip of a melting iceberg: Heat exposure is linked to many things that can damage vital organs, sometimes permanently.

It all starts with thermoregulation, a concept that describes how the body maintains a steady internal (or core) temperature of 98.6 F. Thermoregulation is controlled by a gland in the brain called the hypothalamus, which responds to higher air temperature by signaling the blood vessels to expand and redirect blood, salt, and fluids to the skin in order to cool off through the process of evaporation (sweating).

But is there such a thing as “too hot?”

“Some 10 or 12 years ago, a group of meteorologists came up with a concept: the wet-bulb temperature, which is the upper limit for human adaptability or sustainability,” said W. Larry Kenney, PhD, a professor of physiology and kinesiology at Pennsylvania State University in University Park. 

He explained that the term “wet-bulb” came from an experiment in which scientists took a wet cloth, wrapped it around the bulb of a thermometer, and used it as a proxy for human skin. If the moisture from the cloth evaporated, the thermometer reading went down. But if the air was too humid, less or no evaporation occurred. 

“Wet-bulb temperature is a given temperature of the air when it’s 100% saturated. And so, if your skin is 35 C (95 F), and the air temperature is 35 C but completely saturated with humidity, vapor, sweat, can’t evaporate anymore. And so, we lose our primary means of cooling the body,” Kenney said. 

The result isn’t pretty.

In a 2017 review, Mora and his colleagues identified 27 ways that heat exposure can lead to organ failure and death. In simplest terms, when blood gets redirected to the skin surface, blood flow (and oxygen) to other organs (the brain, heart, intestines, liver, and pancreas) is decreased. When the body and cells surpass levels at which they can tolerate heat, cells die and their protective membranes break down. 

Multiple organs then can’t work well. In the heart, loss of cardiac function can lead to a heart attack, and dehydration thickens the blood, increasing the risk of blood clots and stroke. Kidney failure might occur. Injury to the lining of the lungs means that the lungs and bloodstream are eventually deprived of oxygen, leading to respiratory distress.

When cell membranes break down, pathogens and toxins can get into the organs, leading to things like increased inflammation in the pancreas, neurological damage in the brain, and the leakage of bacteria and toxins from the intestines into the bloodstream, which, in turn, can cause sepsis and a systemic inflammatory response that disrupts cellular balance. Combined with injuries to the lining of the veins and other parts of the vascular system, inflammation can trigger clots that cut off the blood supply to vital organs, leading to fatal bleeding. Damaged muscle tissue can release electrolytes and proteins into the blood, causing kidney failure. 

The result is devastating. When the body’s core gets too hot during extreme heat events – such as the one that Texas and other states are experiencing – a vicious cycle of multiorgan breakdown and failure ensues, causing permanent disability and death. 

Heat Perceptions and Risk

On June 23 of this year, a 31-year-old man and his 14-year-old stepson died in Texas’s Big Bend National Park, where air temperatures had reached 119 F. 

What were they thinking when they embarked on their hike that morning?

Nicholas Ravanelli PhD, an assistant professor of kinesiology at Lakehead University in Thunder Bay, Ontario, and a clinical exercise physiologist, has been studying how people perceive, cope, and behave during heat waves – an area that has only recently gained attention in research circles.

“Perception is an important factor among our physiology [in terms of] how well we adapt in response to heat,” he said. “There’s a missing link that we still don’t have enough evidence on; that is, how people are perceiving the environment and making proactive or reactive decisions to cool or protect themselves.” 

“When you get into that [place] where you’re seeing end organ damage, it’s typically when temperatures are above 104 F, 105 F, and the body can’t cool itself any longer to the degree it needs to be. And so, it affects the brain,” said Sara Andrabi MD, an assistant professor of emergency medicine at Baylor College of Medicine and assistant medical director at Ben Taub Hospital Emergency Center in Houston. 

“You see things like dysfunction in how people think. They might not answer questions, they can (have trouble walking). I always tell parents, if you see your kids really irritable and they’re not acting like themselves, that can be a sign [of heat-related illnesses], because they’re not able to verbalize what’s going on,” she said.

When this happens, people may lose the ability to make rational decisions that might save their lives, Penn State’s Kenney said. 

We may never know exactly what was going on that morning in Big Bend, nor the reasons why the man and his stepson embarked on the hike despite heat warnings from park officials and the National Weather Service. 

But not everyone responds to heat the same way. 

Kenney and his colleagues recently discovered that the wet bulb temperature is significantly lower for older adults and other vulnerable populations. 

Older people are also more vulnerable to extreme heat because they can’t get around as well, limiting their ability to move to dissipate heat from their bodies. Some prescription medications also interfere with temperature, including certain drugs for depression and high blood pressure. 

Younger children have smaller surface-to-body ratios, meaning that heat can get to their core much faster, leading to dehydration, electrolyte imbalances, and heat stress. 

Aside from these obvious groups, Kenney said that some people are simply able to tolerate certain heat conditions better than others.

“There are four main drivers,” he said. “The first is genetics. The second is heat acclimation or acclimatization, meaning that the body has gone through a series of adjustments slowly over time, making it better tolerant of conditions of high heat and humidity. The third would be aerobic fitness. And the fourth is hydration status, whether people are or are not well-hydrated.”

Even with a leg-up, all humans are ultimately vulnerable to what the warming planet has in store. 

“This is not something that is happening to people in other parts of the world,” said Mora. “It’s happening to us, and it’s happening everywhere. We’re screwed.”