Thermal Physiology 101: Human Heat Stress
Humans are amazingly complex and creative creatures, capable of surviving in the most extreme environments of our planet. Through physical processes, cultural adaptations, and human ingenuity, our species thrives everywhere from the heat of equatorial jungles to subtropical deserts. This article introduces the complex and fascinating fundamentals of human thermal physiology, providing a basic understanding of how humans adapt and endure in high heat conditions.
Metabolic Heat Fundamentals
At a basic level, all mammals are heat engines, burning fuel to perform work. To illustrate this point, consider the analogy of an internal combustion engine. Engines burn gasoline (or diesel) because of the high density of chemical energy stored in hydrocarbons.[1] Similar to a car engine burning gasoline to perform work, the human body converts the chemical energy of the food we eat (fats, proteins, and carbohydrates) to perform biological functions required for daily living. In both the car engine and the human body, this process releases heat; in humans, this is referred to as metabolic heat. Lacking a car’s mechanical radiator, humans rely on a far more adaptive and fascinating ways of dissipating or avoiding this heat burden. However, like an overworked engine, when a human body generates more heat than it can dissipate, biological processes cease to function properly. For the average car, this results in calling a tow truck. For humans, exceeding the ability to dissipate metabolic heat is potentially fatal.
A biological requirement for any human is maintaining a stable core temperature, classifying us (and other mammals) as endotherms (from the Greek endo for “within” and therm for “heat). Generally around 98.6oF, there is some slight variation in body core temperature depending on individual factors and time of day, but only by about 1oF. The further body core temperature strays from this range, the greater the likelihood of negative health outcomes. If core temperature drops too far, we experience a condition known as hypothermia (Greek for “below heat”), with symptoms ranging from shivering to unconsciousness to death. Core temperature can dip surprisingly low before becoming fatal. In 1999, a Swedish woman was resuscitated after being trapped under ice despite a body temperature of 56.7oF – an astounding 42oF below normal!
At the other end of the spectrum, hyperthermia (Greek for “above heat”) is defined as an abnormally high body temperature caused by the body’s inability to dissipate heat. There is a distinction between core temperature rise from hyperthermia and the same condition resulting from a fever. However uncomfortable, the fever is intentional; our body is deliberately resetting its internal thermostat as a response to infection. In contrast, hyperthermia results from the body’s inability maintain the desired stable temperature despite its best effort; our internal thermostat remains around 98.6oF but core temperature rises anyway.
Since both too hot and too cold can be deadly, it’s no surprise that humans have a climate niche, a goldilocks zone where temperatures aren’t generally hot or cold enough to be deadly. This niche is found at an annual average temperature is about 52-59oF. People certainly live in areas outside this range - think indigenous populations in the Arctic and the Kalahari desert, both which developed very different cultural practices based on their environments - but at a global scale, populations even today remain concentrated in parts of the world corresponding around this annual average temperature range.
Hyperthermia and Heat Illness
When the body’s ability to maintain a stable core temperature is overwhelmed, heat illness becomes a significant health concern. Heat illness occurs along a spectrum ranging from relatively mild conditions such as heat cramps and heat syncope (i.e., fainting from heat) to heat stroke, a medical emergency characterized by, among other things, central nervous system dysfunction (i.e., delirium, convulsions, and coma) and potentially death. Heat illness is a real and growing challenge even in the United States; the Center for Disease Control reports that in 2023 there were 119,605 emergency room visits for heat related illnesses, with record-hot months between May-September resulting in record high visits.
Heat stroke survivors commonly exhibit long-term neurological and organ complications. Heat stroke itself is divided into two types, exertional and non-exertional heat illness. Although both result from the body’s inability to prevent core temperature from rising, causal pathways are different.
Non-exertional heat stroke is experienced primarily by the elderly and the very young. In this population, the body’s response mechanisms for cooling down, such as increasing heartrate and sweating, are compromised by advanced age or a child’s immature biological responses. External factors, such as temperature, are the primary drivers increasing core temperature. Extending our car engine metaphor, in non-exertional heat stroke victims the body’s radiator is worn out or not fully functioning. A high proportion of fatalities during the most deadly heatwaves in recent history- including the more than 30,000 fatalities during a 2003 European heatwave and more than 500 fatalities in a Chicago area heatwave in 1995 - were among the elderly. It’s no coincidence that ambulance dispatches increase by 18% during extreme heatwaves.
In contrast, exertional heat stroke is more often experienced in younger and often physically fit, healthy populations, including athletes, military personnel, and outdoor workers. A British Medical Journal review found exertional heat stroke to be the third leading cause of mortality in athletes during physical activity with mortality rates exceeding 1-in-4 instances. Heat illness in the U.S. military is severe enough that Congress has mandated an annual report on the problem.
While external factors like temperature and humidity play a role, exertional heat stroke results primarily from increases in core temperature from physical activity. Human muscles are surprisingly inefficient; depending on activity type, anywhere from 75% to 80% of the calories we burn results in waste heat that must be dissipated. The more intense the physical activity, the more our metabolic rate (i.e., calories burned) goes up. Compared with resting, physically laborious activity can generate up to ten times more metabolic heat. During some activity, metabolic heat production reaches a tipping point where physiological cooling mechanisms of even young, healthy individuals can’t keep up with the rate of heat generated; in medical terms, the body moves from “compensable” to “uncompensable” heat stress. Drawing on our car analogy again, exertional heat stroke is comparable to a heavy truck travelling up a steep mountain road. Anyone who’s driven the interstate across the Rocky Mountains will have noticed that even well-maintained trucks slow down and use truck lanes up steep mountain grades, often to prevent the truck from overheating.
Keeping Cool - A human superpower
Maintaining a stable core temperature requires a constant balancing act. Even resting, basic physiological processes to sustain life (e.g., keeping your heart beating and brain working) generate metabolic heat. If we couldn’t dissipate this heat, our core temperature would rise by about 2oF an hour. Fortunately, humans usually excel at thermal regulation.
Thermoregulation and Physiology
At a physiological level, we shed heat in three ways – convection, radiation, and evaporation (a fourth, conduction, is generally negligible – but more on this below). Heat loss from convection and radiation are similar; both results from heat moving from a hot surface (such as your skin) to a cooler surface (such as the air around you). Convection differs from radiation in that it requires a fluid (i.e., air) to transfer heat, while radiation is energy transfer using electromagnetic waves. We’ll ignore the physics, but people give off radiation in the infrared spectrum, a fact used by mosquitoes and militaries to find targets.
For humans, transferring heat by convection and radiation is a two-way street. The Second Law of Thermodynamicsstates that heat flows from hot to cold. All else equal, heat also flows faster if the temperature difference between objects is greater (described by Fourier’s Law of Heat Conduction, for the curious). On cooler days, convection can be sufficient to dissipate excess body heat. This is not true in warm weather. Since the surface of your skin is about 95oF, the surrounding environment must be cooler than this to dissipate body heat through convection or radiation; any warmer and you actually gain heat from the environment! At the same time, we are constantly generating metabolic heat. Balancing the rate of metabolic heat production with heat loss through convection and radiation alone requires an environment no warmer than about 87oF.
Obviously, you can survive temperatures above 87oF! This is because we have a superpower almost unique in the animal world – the ability to sweat copiously. Sweating removes body heat when sweat evaporates from the skin surface. Since the energy requirement to turn liquid water into vapor (i.e., evaporate) is high, large amounts of heat can be dissipated. In hot conditions, heartrate also typically increases and blood vessels near the skin surface dilate, allowing the body to quickly move heat from our core to the skin and, through sweat evaporation, to the environment. This heat transfer occurs in only one direction, away from the person.[2] Sweating is so effective that achieving the same (theoretical maximum) cooling potential by convection requires “standing outside, naked, in a 2 mph wind during the coldest day ever recorded in the United States.”
Unfortunately, achieving the maximum cooling potential of sweating is unrealistic. For sweating to work, sweat must evaporate from the skin surface; dripping sweat has no cooling ability. Like any Hollywood superpower, sweat has a weakness – humidity. When relative humidity is high, air already holding large amounts of water vapor reduces sweat evaporation; at relative humidity of 100%, no evaporation occurs. All else equal, heat loss from sweating decreases as humidity increases. This is why, given enough water, a healthy individual can work outdoors at higher temperatures in dry conditions compared to humid conditions.
Thermoregulation and Adaptation
Being humans, we have another way to excel at thermal regulation. Cultural and behavioral practices, along with innovation, help us deal with hot days. Cultural practices range from the selection of traditional clothing, such as Bedouin dress historically worn in the Arabian desert, to the siesta, rest periods often taken during the hottest part of the day in many cultures.
Human ingenuity also plays an increasingly important role. The development and widespread adoption of air conditioning led to a drastic decrease in heat related fatalities in the U.S., to the point that now only socially vulnerable groups without reliable access to air conditioning are typically considered at risk of non-exertional heat illness.
As high temperature records continue to be broken, new innovations allow working and recreating in the heat. Products such as the Qore Performance ICEPLATE are examples of ways to assist the human body with heat management. Keeping a Qore vest against your torso introduces direct conductive cooling, allowing heat transfer between two objects of different temperatures in direct contact (i.e., your body and the vest). Without this innovative equipment, conductive heat exchange is generally negligible or even an impediment to thermoregulation due to the insulating properties of gear and safety equipment.
Conclusion
Humans possess an extraordinary ability to adapt to diverse climates thanks to our unique physiology and innovative spirit. Our physiology operates like a heat engine, burning food to obtain energy. This process produces metabolic heat that must be dissipated to maintain a stable core temperature. Failure to do so can lead to severe health issues, including potentially fatal heat stroke.
Fortunately, humans have developed multiple strategies to manage heat. Physiologically, we rely on sweating as a primary cooling mechanism. Culturally and behaviorally, we adopt practices such as wearing appropriate clothing and taking rest periods during peak heat. Technological innovations, like air conditioning and advanced cooling gear, further enhance our ability to thrive in hot conditions.
As temperatures continue to rise and heat records are broken, these adaptive strategies and innovations become increasingly vital. By leveraging human physiological strengths and creative solutions, we can continue to work and play safely in the heat.
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About the author: Erik is a doctoral candidate at Duke University where he conducts research on the challenges rising temperatures pose for military training. An Army veteran, Erik has served in a variety of extreme climates ranging from deserts in the U.S. Southwest and Middle East (120oF) to Arctic conditions in central Alaska (-42oF).
[1] Electric cars aren’t heat engines, although if charged from a thermal power plant the concept is similar, with the intermediate step of converting chemical energy from coal or natural gas to electricity performed at the power plant. Our analogy breaks down if the electric car is charged from nuclear energy, solar energy, or wind (kinetic energy).
[2] As an interesting side note, in wet saunas, your skin surface is usually the coolest object around. Water vapor in the saturated sauna air condenses onto your skin, adding heat. This is the reverse processes of sweat evaporation but operates according to the same principle.