In this article, you'll learn:
- The most common cause of death in a heat wave
- The true, evidence-based heat limit for a healthy young adult
- How heat acclimatization can dramatically reduce your risk
As the planet warms and extreme heat events become more frequent, understanding how the human body copes with high temperatures is more important than ever. Larry Kenney has spent his career studying exactly that. A professor of physiology and kinesiology and an expert in human thermoregulation, his research focuses on how heat affects people of all ages, especially older adults and other vulnerable groups. In this Q&A, he shares what decades of research have revealed about the body’s limits, why some populations are more at risk, and what we can do to stay safe in a hotter world.
What inspired your decades-long focus on human temperature regulation, and how did your early experiences shape your research into how the body copes with high temperatures?
My path began as a collegiate volleyball and baseball player, then as a coach, which led me to seek ways to stay connected to athletics while pursuing my scientific interests. I came to Penn State with the intention of eventually teaching at the college level and coaching, but I quickly became fascinated by the human physiology research being done at Noll Laboratory, especially studies placing people in environmental chambers to observe how disparate groups of individuals respond to extreme heat and cold.
That interest deepened just after the Three Mile Island nuclear accident occurred. My advisor had a contract to study heat stress among workers entering the damaged reactor building, where temperatures could reach 160°F and protective clothing made conditions even more dangerous. This convergence of scientific curiosity and a pressing real-world problem solidified my passion for studying environmental physiology and human temperature regulation, combining foundational research with practical impact.
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As global temperatures rise and heat waves become more frequent, what are the most dangerous effects of extreme heat on the human body, and what are some groups at greater risk?
Extreme heat and humidity pose two major physiological threats to the human body. The first is a rise in core body temperature. If that temperature climbs beyond what the body can regulate, the risk of heat-related illnesses increases. This can range from heat exhaustion to heat stroke, which can be life-threatening. The second and more common effect is cardiovascular strain. As the body works to cool itself by redirecting blood flow to the skin and supporting sweat production, the heart must work harder to maintain that effort.
While young, healthy individuals can typically tolerate this increased cardiovascular load, older adults face greater risk. Individuals over 65, even if otherwise healthy, may have difficulty increasing their heart rate or maintaining adequate stroke volume under heat stress. When age-related conditions like heart failure, prior cardiac events, significant hypertension, or diabetes are present, the danger increases significantly. In fact, data from heat waves around the world show that the most common cause of death isn't heat stroke or dehydration—it’s cardiovascular failure brought on by prolonged heat exposure.
Our team at Penn State leads the HEAT Project (Human Environmental Age Thresholds), a multi-year, NIH-funded effort aimed at understanding how hot and humid it can get before basic daily activities become unsafe.
We’re not studying athletes or military personnel, but rather ordinary people of all ages performing routine tasks like cooking, showering, or light housework. Our goal is to define the upper environmental limits at which the body can still regulate temperature and cardiovascular function—what we call limits of livability. This information helps us to identify where dangerous conditions already exist or are likely to emerge due to climate change, and it provides an evidence-based foundation for safety standards that can better protect vulnerable populations.
Your lab has conducted hundreds of experiments to find the “critical environmental limit” for human heat tolerance. What have you discovered about how hot and humid it can get before even healthy people are at risk, and how does this challenge previous assumptions?
For years, a widely cited 2010 paper by Sherwood and Huber suggested that the upper limit for human survivability was a wet bulb temperature of 35°C or 95°F, which is 95°F at 100% humidity. The idea was that at that point, sweat can no longer evaporate, and the body loses its ability to cool itself. In theory, even resting in the shade under those conditions would be fatal within hours. This concept gained traction in both scientific circles and the popular press, but it had never been empirically tested in humans—until now.
Through our work at Penn State as part of the HEAT Project, we’ve conducted extensive lab experiments to determine the actual limits of human heat tolerance. Our research has shown that the true threshold for healthy, young adults is closer to a wet bulb temperature of 31°C, or about 87°F at 100% humidity. That limit is even lower for older adults. The key difference is that our results are based on real human physiology, not theoretical models. People don't simply continue sweating as heat increases, especially when that sweat stops evaporating. We respond in more complex and adaptive ways.
Importantly, while the 35°C wet bulb threshold has rarely—if ever—been sustained on Earth, the 31°C threshold has already been reached in various parts of the world. This provides a more realistic, evidence-based explanation for why heat waves are already proving deadly. It aligns with observed mortality patterns and underscores the need to revise outdated assumptions. Our findings not only offer more physiologically valid thresholds for heat tolerance, but they also provide practical value. Understanding these limits helps guide public health policies and inform early warning systems in a warming world.
With climate change making summers hotter and more humid, what practical steps can people take to protect themselves and their loved ones from heat-related illnesses, especially during heat waves?
The most effective protection is access to air conditioning, even if it means going to a library, shopping mall, or other public space. But there are also physiological strategies. One of the most effective methods is heat acclimatization: gradually exposing the body to warmer temperatures to help it slowly and safely adapt. For instance, spending 20–30 minutes a day doing light activity outdoors as temperatures rise in the spring can improve the body’s ability to manage heat stress.
This principle is especially important in athletics. Heat-related deaths in sports like football are most common in the first days of preseason practices when athletes are not yet acclimated. After just a few sessions, the risk drops significantly, highlighting how powerful gradual adaptation can be.
Why is your research on heat and human health especially relevant today, and how do your findings help communities and policymakers prepare for a hotter future?
Today’s climate challenges require interdisciplinary collaboration, and I am actively involved in several U.S. and global climate change summits that bring together experts from medicine, physiology, biometeorology, insurance, and other fields. Our goal is to develop practical, science-based solutions to address the rising threat of extreme heat worldwide.
Our research has gained traction because it provides a clear, physiologically grounded threshold indicating when heat stress reaches dangerous levels and action should be taken. One major challenge in the U.S. is the widespread reliance on either air temperature alone or the heat index, which is a measure designed by the National Weather Service to express how hot it feels by combining temperature and humidity. While the heat index is useful for describing perceived discomfort at rest, it does not accurately reflect the true physiological strain on the body during everyday activities or exertion.
This mismatch limits public understanding and policy effectiveness. For example, the heat index may underestimate risk for individuals who are physically active, as their bodies experience greater heat stress than the index suggests, and safety thresholds based on the heat index are not valid for older individuals. At these climate summits, it’s become widely accepted that relying solely on air temperature or the heat index is insufficient to define environmental impact on human health.
Our research offers more precise, physiology-based metrics that better capture the true effects of heat on the body in real-world conditions. These insights help communities and policymakers design targeted interventions, establish safety standards, and develop early warning systems that protect vulnerable populations as the planet warms.
Larry Kenney is the Marie Underhill Noll Chair in Human Performance and a Professor of Physiology and Kinesiology at Penn State, internationally recognized for his expertise in human thermoregulation and the health impacts of heat. His pioneering research explores how the body responds to heat stress, with a special focus on vulnerable populations and the growing risks posed by climate change.
