It’s the June 25th, and I’m trembling in my laboratory-supplied underwear in Fort Worth, Texas. Libby Cowgill, an anthropologist clad in a furry parka, has transported me and my cot into a metal-walled room chilled to 40 °F. A raucous fan blasts me from above, extracting the remnants of my body heat through the cot’s mesh from below. A sizable respirator sits tightly over my nose and mouth. The machine monitors the carbon dioxide in my exhalations—a surrogate for how my metabolism fluctuates during the experiment. Sooner or later, Cowgill will take off my respirator to insert a slender metal temperature probe several sharp inches into my nostril.
Cowgill and a grad student quietly watch me from their designated “climate chamber.” Just hours before, I had been seated next to them to observe another volunteer, a 24-year-old personal trainer, as he withstood the cold. Every few minutes, they recorded his skin temperature using a thermal camera, his core temperature via a wireless pill, and his blood pressure along with other readings that indicated how his body copes with extreme cold. He lasted nearly an hour without shivering; when it’s my turn, I shake uncontrollably on the cot for almost an hour straight.
I’m in Texas to understand this experiment on how diverse bodies react to extreme climates. “What’s the record for the quickest to shiver so far?” I jokingly inquire of Cowgill as she affixes biosensing devices to my chest and legs. After I leave the cold, she catches me off guard: “You, believe it or not, were not the worst subject we’ve encountered.”
Climate change compels us to confront the intricate science of how our bodies relate to the environment.
Cowgill is a 40-something anthropologist at the University of Missouri who powerlifts and instructs CrossFit during her free time. She’s petite yet robust, sporting dark bangs and geometric tattoos. Since 2022, she’s spent summers at the University of North Texas Health Science Center conducting these uncomfortable trials. Her team aspires to innovate the science of thermoregulation.
While we grasp the generalities of how humans thermoregulate, the science behind staying warm or cool is riddled with gaps. “We have the overarching image. We don’t have many specifics for vulnerable populations,” comments Kristie Ebi, an epidemiologist at the University of Washington who has researched heat and health for over three decades. “How does thermoregulation function if there’s heart disease involved?”
“Epidemiologists possess specific tools for addressing this question,” Ebi proceeds. “However, we require more insights from various fields.”
Climate change is exposing vulnerable individuals to temperatures that challenge their thresholds. In 2023, approximately 47,000 heat-related fatalities are thought to have occurred in Europe. Researchers forecast that climate change could add another 2.3 million heat-related deaths in Europe this century. This elevates the urgency for uncovering exactly what transpires to bodies under extreme conditions.
Severe temperatures already jeopardize vast areas of the globe. Communities throughout the Middle East, Asia, and sub-Saharan Africa frequently encounter highs surpassing widely accepted human heat tolerance. Expanses of the southern US, northern Europe, and Asia now also experience unprecedented lows: The 2021 freeze in Texas claimed at least 246 lives, and a 2023 polar vortex plunged temperatures in China’s northernmost city to a record hypothermic –63.4 °F.
This transformation is present, and more is imminent. Climate scientists warn that curtailing emissions can avert lethal extremes from advancing elsewhere. However, if emissions continue unchanged, severe heat and even cold will penetrate deeper into every continent. Around 2.5 billion people in the hottest regions of the world lack air-conditioning. When people do have it, it can exacerbate outdoor temperatures, intensifying the heat island effect in congested cities. And neither AC nor heating systems are very effective when heat waves and cold snaps overwhelm the power infrastructure.
“You, believe it or not, were not the worst subject we’ve ever encountered,” the author was told after experiencing Cowgill’s “climate chamber.”
Through experiments like Cowgill’s, researchers globally are redefining the parameters of when extremes transition from uncomfortable to lethal. Their discoveries reshape our understanding of the boundaries of hot and cold—and how to endure in an evolving world.
Embodied change
Archaeologists have understood for some time that we previously endured colder temperatures than had been thought. Humans ventured into Eurasia and North America long before the last glacial epoch concluded approximately 11,700 years ago. We were the only hominins to survive this period. Neanderthals, Denisovans, and Homo floresiensis all faced extinction. The exact cause of their demise remains uncertain. However, we do know that humans thrived due to protection from clothing, extensive social networks, and physiological adaptability. Our resilience to extreme temperatures is ingrained in our bodies, behaviors, and genetic makeup. We wouldn’t exist without it.
“Our bodies are in constant communication with the environment,” states Cara Ocobock, an anthropologist at the University of Notre Dame studying our energy expenditure in extreme conditions. She has collaborated with Finnish reindeer herders and Wyoming mountaineers.
Yet the dynamics between bodies and temperature remain surprisingly enigmatic to scientists. In 1847, anatomist Carl Bergmann noted that animal species grow larger in frigid climates. Zoologist Joel Asaph Allen observed in 1877 that cold-adapted animals had shorter appendages. Then there’s the nasal aspect: In the 1920s, British anthropologist Arthur Thomson speculated that people in colder regions have relatively long, narrow noses, which better heat and moisten the air they inhale. These theories originated from observations of animals like bears and foxes, and subsequent ones stemmed from studies contrasting the physiques of cold-adapted Indigenous groups with white male control groups. Some theories, particularly concerning surface area optimization, hold logic: It seems plausible that a tall, slender physique amplifies the skin’s capacity to dissipate excess heat. The challenge lies in that scientists have never actually subjected these ideas to human testing.
“Our bodies are in constant communication with the environment.”
Cara Ocobock, anthropologist, University of Notre Dame
Much of our current knowledge on temperature tolerance is derived from centuries-old racial theories or the assumption that anatomical features govern everything. However, science has progressed. Biology has evolved. Childhood experiences, lifestyles, fat composition, and peculiar biochemical feedback mechanisms can construct a more malleable view of the body than previously envisioned. This shift is encouraging researchers to adapt their methodologies.
“If you take someone who’s extremely tall, lean, and elongated and place them in a cold environment, will they expend more calories to maintain warmth than a shorter, stockier individual?” Ocobock questions. “No one has investigated that.”
Ocobock and Cowgill have collaborated with Scott Maddux and Elizabeth Cho at the Center for Anatomical Sciences at the University of North Texas Health Fort Worth. All four are biological anthropologists who have pondered whether the principles proposed by Bergmann, Allen, and Thomson hold true.
Over the past four years, the group has been examining how aspects like metabolism, fat, perspiration, blood flow, and individual history govern thermoregulation.
Your local climate, for example, might affect how you respond to temperature extremes. In a unique analysis of mortality data from 1980s Milan, residents from warm southern Italy were more likely to survive heat waves in the northern regions of the country.
Similar patterns have been noted in cold regions. Researchers commonly assess cold tolerance by measuring an individual’s “brown fat,” a specialized type of fat that aids in heat generation (contrary to white fat, which mainly stores energy). Brown fat represents a cold adaptation as it generates warmth without relying on shivering. Research has linked it to habitation in colder climates, especially during early life. Wouter van Marken Lichtenbelt, the physiologist at Maastricht University who, alongside colleagues, identified brown fat in adults, has demonstrated that this tissue can become more active with cold exposure and even assist in regulating blood glucose and shaping how the body metabolizes other fats.
This adaptability served as a preliminary clue for the Texas research team. They aim to uncover how a person’s reactions to extreme heat and cold relate to height, weight, and body shape. What distinguishes, asks Maddux, “a male standing 6 foot 6 and weighing 240 pounds” from another individual “who is 4 foot 10 and weighs 89 pounds” in the same conditions? Yet, the team also contemplated if shape was merely a fraction of the entire narrative.
Their expansive experiment employs instruments that anthropologists of a century ago could not have envisioned—devices that monitor metabolism in real time and assess genetics. Each participant undergoes a CT scan (for body shape), a DEXA scan (to evaluate fat and muscle ratios), high-definition 3D imaging, and DNA analysis from saliva to investigate genetic ancestry.
Volunteers recline on a cot in underwear, as I did, for around 45 minutes in varying climatic conditions, each on distinct days. There’s dry cold, about 40 °F, akin to enduring a stroll through a walk-in refrigerator. Then comes dry heat and humid heat: 112 °F with 15% humidity and 98 °F at 85% humidity. They refer to this as “going to Vegas” and “going to Houston,” Cowgill notes. The chamber session is lengthy enough to note an effect, yet brief enough to ensure safety.
Prior to my trip to Texas, Cowgill indicated that she suspected the old principles might falter. Studies linking temperature tolerance with race and ethnicity appeared weak due to contemporary biological anthropologists dismissing the notion of distinct races. It’s a flawed premise, she remarked: “No biological anthropologist would dispute that humans vary globally—that’s evident to anyone observing. [However] clear demarcations around populations cannot be drawn.”
She further stated, “I suspect there’s a significant likelihood that after our four years of testing, we’ll conclude that limb length, body mass, surface area […] do not predominantly influence how well one endures cold and heat.”
Adaptable to a degree
In July 1995, a week-long heat wave raised Chicago temperatures above 100 °F, resulting in nearly 500 deaths. Three decades later, Ollie Jay, a physiologist at the University of Sydney, can replicate the conditions of that notably humid heat wave in a climate chamber at his lab.
“We can recreate the Chicago heat wave of ’95, the Paris heat wave of 2003, and the recent heat wave [in early July this year] in Europe,” Jay explains. “As long as we possess the temperature and humidity data, we can mimic those circumstances.”
“Everyone has a very personal experience of feeling hot, so we have 8 billion specialists on keeping cool,” he remarks. Yet our internal gauge for when heat becomes lethal is often inaccurate. Even elite athletes monitored by skilled medics have perished after overlooking dangerous warning signs. Moreover, few studies have explored how at-risk groups like the elderly, those with heart conditions, and low-income communities with limited access to cooling react to extreme heat.
Jay’s team studies the most effective methods for surviving it. He criticizes air-conditioning, asserting it consumes so much energy that it can exacerbate climate change in “a vicious cycle.” Instead, he has tracked individuals’ vital signs while using fans and skin mists to withstand three hours in humid and dry heat. His published research last year showed that fans lessened cardiovascular stress by 86% for individuals with heart disease in the humid heat familiar in Chicago.
Dry heat presented a contrasting scenario. In that simulation, fans not only didn’t assist but actually increased the rate at which core temperatures escalated in healthy older adults.
Heat is a killer. But not without resistance. Your body must maintain its internal temperature within a narrow range flanking 98 °F, deviating less than two degrees. The mere fact of existence means you are generating heat. Your body must dissipate that heat without accumulating much more. The nervous system relaxes narrow blood vessels near your skin. Your heart rate escalates, driving more warm blood to your extremities and away from vital organs. You perspire. And as that sweat evaporates, it whisks away significant body heat.
This thermoregulatory reaction can be enhanced. Studies by van Marken Lichtenbelt have indicated that exposure to moderate heat boosts sweat production, reduces blood pressure, and lowers resting heart rate. Longitudinal studies based on Finnish saunas suggest similar relationships.
The body may adapt protectively to cold conditions as well. Here, body heat is crucial for survival. Shivering and physical activity help in retaining warmth. Clothing also plays a role. Cardiovascular fatalities are believed to spike in colder weather. However, those accustomed to cold seem better equipped to redirect blood flow effectively to maintain organ warmth without allowing their temperature to drop too significantly in their extremities.
Earlier this year, biological anthropologist Stephanie B. Levy (unrelated) reported that New Yorkers living in lower average temperatures possessed more active brown fat, providing further evidence that our bodies’ innate mechanisms adjust to environmental conditions over the seasons and perhaps throughout our lives. “Do our bodies retain a biological memory of past seasons?” Levy questions. “That remains an unresolved issue. Some studies in rodent models suggest this might be accurate.”
Despite clearly acclimatizing after prolonged exposure to either cold or heat, Jay remarks, “there is a limit.” Consider perspiration: Heat exposure can augment sweating only until your skin is saturated. It’s an absolute physical constraint. Any additional sweat merely results in leaking water without removing extra heat. “I’ve heard some claim we’ll adapt biologically and evolve out of this—we’ll find a way,” Jay states. “But unless we dramatically alter our body shape, that’s implausible.”
And body shape may not even influence thermoregulation as much as previously thought. The subject I observed, a personal trainer, appeared outwardly fit for cold: his broad shoulders couldn’t even be contained in one CT scan image. Cowgill theorized that this muscle mass provided insulation. Yet when he exited his session in the 40 °F environment, he had finally begun to shiver—intensely. The researchers wrapped him in a heated blanket. He continued to shake. Driving to lunch over an hour later in a warm car, he still reported feeling chilled. An hour later, a finger prick yielded no blood, indicating that blood vessels in his extremities remained constricted. His body temperature dropped approximately half a degree C during the cold session—a notable decrease—and his broader frame did not seem to protect him from the cold as my involuntary shivering safeguarded me.
I asked Cowgill if perhaps no one is uniquely suited for heat or cold. “Absolutely,” she affirmed.
A hot mess
So if body shape doesn’t reveal much about how a person regulates body temperature, and acclimatization has its limits, then how do we ascertain when heat becomes excessively high?
In 2010, climate change researchers, Steven Sherwood and Matthew Huber, contended that parts of the world become uninhabitable at wet-bulb temperatures reaching 35 °C, or 95 °F. (Wet-bulb readings combine air temperature and relative humidity.) Beyond 35 °C, an individual will simply be incapable of unloading heat swiftly enough. However, it turns out their estimate was too optimistic.
Researchers “adopted” that number for a decade, asserts Daniel Vecellio, a bioclimatologist from the University of Nebraska, Omaha. “Yet the figure had never been empirically validated.” In 2021 a Pennsylvania State University physiologist, W. Larry Kenney, collaborated with Vecellio and others to assess wet-bulb thresholds in a climate chamber. Kenney’s lab investigates the combinations of temperature, humidity, and duration that push a person’s body past its limits.
Shortly thereafter, the researchers developed their own wet-bulb tolerance benchmark: below 31 °C in warm humid environments for the youngest demographic, individuals in their thermoregulatory prime. Their findings suggest that a day reaching 98 °F and 65% humidity, for instance, could pose a risk in mere hours, even for healthy individuals.
Cowgill and her colleagues Elizabeth Cho (top) and Scott Maddux prepare graduate student Joanna Bui for a “room-temperature test.”
In 2023, Vecellio and Huber collaborated, merging the expanding reservoir of lab data with cutting-edge climate simulations to forecast where heat and humidity pose the greatest risks to global populations: initially the Middle East and South Asia, followed by sub-Saharan Africa and eastern China. Anticipating that warming could escalate by 3 to 4 °C over preindustrial levels within this century, as projected, areas of North America, South America, and northern and central Australia will be vulnerable next.
Last June, Vecellio, Huber, and Kenney jointly published an article re-evaluating the limits that Huber had suggested in 2010. “Why not 35 °C?” elucidated the reasons why human tolerance limits have proven to be lower than anticipated. Those initial assessments overlooked that our skin temperature can rapidly exceed 101 °F in hot conditions, complicating the process of dissipating internal heat.
The Penn State team has conducted in-depth analyses on how heat tolerance varies with gender and age. Older participants’ wet-bulb limits ended up being even lower—between 27 and 28 °C in warm, humid conditions—and exhibited more variability from individual to individual than those of younger participants. “The scenarios we confront today—especially in North America and Europe, and similar regions—are significantly below the thresholds observed in our studies,” Vecellio expresses. “We recognize that heat can be deadly now.”
This rapidly expanding body of research indicates, Vecellio emphasizes, that defining heat risk cannot rely on merely one or two figures. Last year, he and colleagues at Arizona State University analyzed the warmest 10% of hours between 2005 and 2020 for each of 96 US cities. They aimed to compare recent heat-health findings with historical weather data for a fresh perspective: How often are conditions so extreme that the human body cannot manage? Over 88% of those “hot hours” reached that threshold for individuals in direct sunlight. In shaded areas, many heat waves became significantly less hazardous.
“There’s virtually no one who ‘needs’ to perish in a heatwave,” asserts Ebi, the epidemiologist. “We possess the tools. We have the knowledge. Essentially all [those] fatalities are avoidable.”
More than a number
A year after my Texas visit, I spoke with Cowgill to gather her thoughts following four summers of chamber trials. She informed me that the only principle regarding heat and cold she currently endorses is … well, none.
She recounted a recent participant—the smallest man in the study, weighing 114 pounds. “He shook like a leaf on a tree,” Cowgill notes. Typically, someone who shivers strongly heats up quickly. Core temperature might even rise slightly. “This [individual] just continued to shiver and shiver without warming,” she recounted. She is unsure why this occurred. “Every time I believe I grasp the situation, another person comes in and defies the established pattern,” she observes, adding that the tremendous variability of human bodies, both internally and externally, cannot be overlooked.
The same complexity complicates physiological research.
Jay strives to embrace bodily intricacies by enhancing physiological simulations of heat and the human strain it induces. He has conducted studies that incorporate an individual’s activity level and clothing type to project core temperatures, dehydration levels, and cardiovascular stress depending on specific heating scenarios. This facilitates assessing a person’s risk based on factors such as age and health. He’s also developing physiological models to pinpoint vulnerable populations, inform early-warning systems ahead of heat waves, and possibly advise municipalities on whether measures like fans or mists could aid in protecting residents. “Heat is a comprehensive issue for society,” Ebi contends. Authorities might enhance public preparedness for cold snaps in this context as well.
“Mortality is not the sole focus,” Jay adds. Extreme temperatures also lead to illness and strain healthcare systems: “There are numerous community-level repercussions we are completely overlooking.”
Climate change compels us to engage with the complex science of how our bodies interact with the surrounding environment. Predicting health impacts is a considerable and intricate challenge.
The initial wave of revelations from Fort Worth will emerge next year. Researchers will analyze thermal imagery to process data regarding brown fat. They’ll determine whether, as Cowgill suspects, body shape may not significantly influence temperature tolerance as previously believed. “Human variability is the norm,” she concludes, “not the exception.”
Max G. Levy is an independent journalist who focuses on chemistry, public health, and environmental issues.
