It’s the 25th of June and I’m shivering in my lab-issued underwear in Fort Worth, Texas. Libby Cowgill, an anthropologist in a furry parka, has wheeled me and my cot into a metal-walled room set to 40 °F. A loud fan pummels me from above and siphons the dregs of my body heat through the cot’s mesh from below. A large respirator fits snug over my nose and mouth. The device tracks carbon dioxide in my exhales—a proxy for how my metabolism speeds up or slows down throughout the experiment. Eventually Cowgill will remove my respirator to slip a wire-thin metal temperature probe several pointy inches into my nose.
Cowgill and a graduate student quietly observe me from the corner of their so-called “climate chamber.” Just a few hours earlier I’d sat beside them to observe as another volunteer, a 24-year-old personal trainer, endured the cold. Every few minutes, they measured his skin temperature with a thermal camera, his core temperature with a wireless pill, and his blood pressure and other metrics that hinted at how his body handles extreme cold. He lasted almost an hour without shivering; when my turn comes, I shiver aggressively on the cot for nearly an hour straight.
I’m visiting Texas to learn about this experiment on how different bodies respond to extreme climates. “What’s the record for fastest to shiver so far?” I jokingly ask Cowgill as she tapes biosensing devices to my chest and legs. After I exit the cold, she surprises me: “You, believe it or not, were not the worst person we’ve ever seen.” Climate change forces us to reckon with the knotty science of how our bodies interact with the environment.
This personal encounter with the raw edge of cold tolerance underscores a critical, global quest: to understand precisely how our bodies react to extreme temperatures. As climate change accelerates, pushing global temperatures to unprecedented highs and lows, the science of human thermoregulation has never been more vital. Researchers are racing to uncover the physiological adaptations and limits that dictate survival in an increasingly volatile world.
Unraveling Thermoregulation: Old Rules, New Science
Libby Cowgill, a 40-something anthropologist at the University of Missouri, leads the charge, spending her summers at the University of North Texas Health Science Center. Her team aims to revamp our understanding of thermoregulation, moving beyond broad strokes to fill crucial blind spots.
While we generally comprehend how people maintain their body temperature, the specifics, especially for vulnerable populations, remain elusive. Kristie Ebi, an epidemiologist with the University of Washington, highlights this challenge: “How does thermoregulation work if you’ve got heart disease?” This gap in knowledge is particularly concerning as climate change subjects millions to dangerous conditions.
Historically, our understanding of human temperature tolerance stemmed from theories like Carl Bergmann’s observation that animals grow larger in cold climates, or Joel Asaph Allen’s note about shorter appendages in cold-dwellers. Arthur Thomson even theorized about nose shapes. These ideas, often based on animal observations or comparative studies of Indigenous populations and white male control groups, were rarely tested directly in humans.
“Our bodies are constantly in communication with the environment,” notes Cara Ocobock, an anthropologist at the University of Notre Dame. Yet, the precise relationship between bodies and temperature has remained surprisingly mysterious. Modern biology, however, has evolved, revealing the body as far more malleable than once imagined.
Cowgill, Ocobock, and their colleagues Scott Maddux and Elizabeth Cho, all biological anthropologists, are now challenging these century-old assumptions. For the past four years, their team has meticulously studied how factors like metabolism, fat, sweat, blood flow, and even personal history contribute to thermoregulation, moving beyond simple body shape.
Their multi-year experiment employs cutting-edge tools: CT scans for body shape, DEXA scans for fat and muscle percentages, high-resolution 3D scans, and DNA analysis from saliva to examine ancestry. Volunteers, like myself, endure 45-minute sessions in various climate conditions – dry cold (40 °F), dry heat (112 °F with 15% humidity), and humid heat (98 °F with 85% humidity) – all designed to measure the body’s real-time response.
The Limits of Human Adaptation
One fascinating aspect of thermoregulation is adaptation. Your native climate, for instance, can influence how you handle temperature extremes. A study from 1980s Milan found that Italians raised in warmer southern Italy were more likely to survive heat waves in the country’s north.
Similarly, cold adaptation can involve “brown adipose” fat, specialized for generating heat without shivering. Wouter van Marken Lichtenbelt, who discovered brown fat in adults, has shown it can activate with cold exposure, aiding in heat generation and even metabolic regulation. This adaptability offers clues, but it also has its limits.
Physiologist Ollie Jay at the University of Sydney can replicate historical heat waves in his climate chamber. His team explores effective survival strategies, noting that while our internal sense of heat danger is unreliable, interventions like fans and skin mists can reduce cardiovascular strain in humid heat. Interestingly, fans can worsen conditions in dry heat for some.
The body’s thermoregulatory response, involving relaxed blood vessels, increased heart rate, and sweating to expel heat, can be trained. Studies show mild heat exposure can increase sweat capacity and improve cardiovascular health. Conversely, cold adaptation may improve blood flow rerouting to protect organs, as Stephanie B. Levy’s research on New Yorkers suggests.
Yet, there’s a “ceiling” to acclimatization. Jay cautions against assuming indefinite biological adaptation. Sweat capacity, for example, is finite. My own experience in Cowgill’s chamber, shivering aggressively, offered more protection than the personal trainer’s muscular build, which insulated him but didn’t prevent a significant core temperature drop. Cowgill simply stated, “Absolutely,” when asked if there’s no unique predisposition to hot or cold.
The Deadly Threshold: What’s “Too Hot” (or Cold)?
If individual body shape and general acclimatization aren’t the sole predictors, how do we define the danger zone? In 2010, climate researchers Steven Sherwood and Matthew Huber proposed a wet-bulb temperature of 35 °C (95 °F) as the point at which regions become uninhabitable. This measurement combines air temperature and relative humidity, indicating when the body can no longer dissipate heat.
However, this estimate proved too optimistic. Daniel Vecellio, a bioclimatologist, and Penn State physiologist W. Larry Kenney, along with others, empirically tested these wet-bulb limits in a climate chamber. Their findings in 2021 revealed that the human tolerance limit is actually lower—below 31 °C in warm, humid conditions for young, healthy individuals.
Their research suggests that a day reaching 98 °F and 65% humidity can pose danger in a matter of hours, even for healthy people. Older participants showed even lower wet-bulb limits, between 27 and 28 °C, highlighting increased vulnerability. As Vecellio notes, “We know that heat kills now.” These revised limits, published by Vecellio, Huber, and Kenney, accounted for factors like skin temperature quickly exceeding 101 °F in hot weather, making internal heat dissipation harder.
Beyond the Numbers: Individual Variation
Vecellio stresses that defining heat risk isn’t a simple matter of one or two numbers. Context is everything. Research showed that in the hottest 10% of hours for 96 US cities, over 88% met the criterion for bodies failing to compensate in full sun. In the shade, these heat waves became significantly less dangerous.
This complexity is echoed in Cowgill’s ongoing work. She recounts a recent participant, the smallest man in the study at 114 pounds, who “shivered like a leaf on a tree” but didn’t get any warmer. “Every time I think I get a picture of what’s going on in there, we’ll have one person come in and just kind of be a complete exception to the rule,” she admits. Human bodies vary immensely, both inside and out, complicating universal thresholds.
This messy reality informs new approaches to physiological simulations. Jay is developing models that consider activity levels, clothing, and individual health factors to predict core temperature, dehydration, and cardiovascular strain. These tools can identify vulnerable groups, inform early-warning systems for heat waves, and even guide cities on effective interventions.
As Ebi wisely states, “There’s really almost no one who ‘needs’ to die in a heat wave. We have the tools. We have the understanding. Essentially all [those] deaths are preventable.” The same proactive approach can be applied to cold snaps, addressing not just fatalities but also morbidity and the strain on healthcare systems.
Preparing for a Warmer World: Practical Insights
The quest to understand how our bodies react to extreme temperatures is no longer purely academic; it’s a matter of public health and survival. The findings from researchers like Cowgill, Jay, Vecellio, and Kenney are fundamentally changing how we perceive the limits of hot and cold, and how we can survive in a new world.
Practical insights derived from this research are crucial. They inform guidelines for safe exposure, highlight the importance of shade and hydration, and underscore the limitations of individual physiological adaptation. For instance, knowing that fans are less effective in dry heat or that older populations have lower wet-bulb limits can guide public health messaging and infrastructure planning.
Understanding the interplay between individual physiology, environmental conditions, and behavioral responses allows for targeted interventions. This includes early warning systems, cooling centers, and community support networks. Climate change demands a holistic strategy that leverages scientific data to protect the most vulnerable and enhance overall resilience.
Conclusion: The Future of Thermoregulation Research
Climate change forces us to reckon with the knotty science of how our bodies interact with the environment, predicting health effects is a big and messy matter. The research being conducted globally, from Fort Worth’s climate chambers to Sydney’s heat simulations, is crucial for navigating our warming world.
The first wave of answers from Cowgill’s team in Texas will materialize next year, offering new insights into brown fat and body shape’s role in temperature tolerance. Their work, alongside others, reinforces a profound truth: “Human variation is the rule,” Cowgill affirms, “not the exception.” As we continue to face unprecedented weather events, understanding this incredible variability within the human body will be key to saving lives and building more resilient communities.
