Thermal Physiology 201: Human Cold Stress and Response
“No One is Dead Until They are Warm and Dead.”
Human physiology in extreme cold environments is fascinating. One of the most surprising aspects of cold temperature physiology is the extremely low core temperature humans can, in some rare cases, experience before death. Anna Bagenholm holds the distinction of having the lowest core temperature ever recorded in a person who survived. After falling through ice while skiing in a remote location, Bagenholm was able to find a pocket of air but wasn’t rescued until 80 minutes after being submerged in icy water. At that point, she was clinically dead - no heartbeat, no breathing – but friends immediately performed CPR. In the hospital two and a half hours after the incident, her core temperature was recorded at an astounding 56.7oF – an amazingly low measurement! Her nearly full recovery was possible due to specialized medical treatment and because, in deep hypothermia, human metabolism slows to a crawl. Although her core temperature is the lowest recorded in anyone who subsequently survived, Bagenholm’s experience is not unique. Over one 5-year period a Norwegian hospital revived nine hypothermic cases with no heartbeat (a success rate of 37.5%).
Although humans can survive low core temperatures, resuscitation in such extreme conditions requires luck and specialized medical response. More typically, cold weather injuries are dangerous, disfiguring, and potentially deadly. In this article we explore the basics of cold weather injury, hypothermia, and our physiological responses to cold environments.
Cold Weather Injuries
The U.S. Army classifies cold weather injuries into three categories: non-freezing, freezing, and hypothermia. Non-freezing injuries result from prolonged blood vessel constriction (more on this below); over a long enough period, such blood flow restriction destroys tissue. Trench foot is the classic example of a non-freezing cold weather injury and can occur in temperatures well above freezing.
Freezing injuries, commonly called frostbite, result from fluid in the skin or tissue literally freezing solid. Temperatures need to be below 30oF for frostbite to occur. Frostbite is most common in exposed parts of the body like fingers, toes, ears, and the nose. These body parts are most likely to be exposed to the environment and to experience reduced blood flow as the body seeks to retain warmth for internal organs; this can reduce the temperature of these body parts enough to allow freezing.
Frostbite can also occur quickly by touching a piece of very cold metal; this is called contact frostbite. To prevent contact frostbite, a common practice in Arctic military units is to “warm soak” a vehicle inside a maintenance bay or heated structure for several hours prior to using or maintaining it. Once stationed in the Arctic, I know from personal experience the reaction of a careless soldier touching a vehicle just brought into a maintenance bay during the winter – my ungloved hand recoiled from the surface as if I had touched a boiling pot of water!
Superficial frostbite (i.e., skin surface only) can be treated by careful, slow rewarming. Deeper, severe frostbite is likely to result in amputation of the affected area. Body fluids, which expand during freezing, rupture cell membranes, destroying tissue beyond recovery. This dead tissue needs to be cut away to prevent gangrene. Such extreme medical intervention isn’t limited to polar explorers or arctic workers; in January 2024, dozens of people required treatment (and twelve amputations) after extreme cold weather descended on the Kansas City, MO area. Some of these unfortunate patients developed frostbite while attending a Chiefs football game!
Frostbite may be disfiguring, by hypothermia can be deadly. Hypothermia is defined as a body core temperature reduction to 95oF or lower and is a medical emergency. Hypothermic conditions occur when the body cannot generate or retain enough heat to prevent internal cooling; as discussed previously in this blog, humans can only survive if core temperature is maintained in a narrow range centered around 98.6oF. Given the potentially deadly consequences of hypothermia, the remainder of this article focuses on our physiological responses to prevent this condition.
Basic Physiological Reactions
Thermoregulatory responses and an individual’s ability to maintain stable core temperature in extreme cold depends on body shape and size. All else equal, individuals with greater body surface area compared to weight (i.e., tall, skinny people) lose heat fastest, and individuals with more subcutaneous (i.e., below skin surface) fat retain heat better. There is mixed consensus about the role of physical fitness in cold adaptation; lean, fit endurance athletes have better vasoconstrictive responses (more on that later), providing a thermoregulatory advantage, but also tend to have less insulating fat than non-athletic individuals.
Regardless of body type or fitness, at a physiological level we all respond to cold temperatures in two ways: by increasing heat production and seeking to minimize heat loss.
Reducing Heat Loss
One of the first reactions to the cold is for the body to try and minimize heat loss. This is done through vasoconstriction across the surface of the body, generally starting at the hands. (Vasoconstriction is the narrowing of blood vessels.) Narrowing blood vessels restricts blood flow, reducing the amount of warm blood circulated between our relatively well-insulated body core (where organs need to stay warm) and our skin surface (where warm blood loses heat to the cold environment). Put simply, vasoconstriction is an attempt to reduce heat loss occurring when blood flows near the cold skin surface.
In seeking to minimize heat loss through vasoconstriction, your body is willing to accept colder skin surface and extremities (like hands and feet) to maintain a warmer center. After all, to survive, you don’t necessarily need your fingers, but you certainly need a beating heart and functioning lungs.
Humans have an interesting adaptation to the challenge of keeping extremities like fingers warm despite vasoconstriction. Termed the hunting reaction (or Lewis reaction, after the researcher who described the effect in 1930), we undergo periodic rises and falls of skin temperature during prolonged cold exposure. These pulses of warmth occur by re-expansion of the blood vessels in arms and hands, briefly accepting increased heat loss in an attempt to rewarm hands and fingers to prevent frostbite (the reaction has also been documented in other body parts like the ears and nose).
Increasing Heat Production: Resting Metabolic Rate
In addition to decreasing heat loss, the body responses to the cold by increasing metabolic heat production. In extremely cold environments, metabolic heat production is critical survival. Depending on environment, degree of acclimatization, and availability of calories, humans have two primary methods to increase heat production. We can do so either voluntarily (e.g., moving around or exercising) or involuntarily by shivering.
The most cold-adapted people respond to extreme cold by increasing internal (metabolic) heat production even when at rest. Studies have found native polar populations and well acclimatized people in the Arctic have higher resting metabolic rates than non-native populations. In some of these populations, the higher resting metabolic rate can be nearly 20% greater than what would be expected in other populations. Put another way, given sufficient calories, these extreme-cold acclimatized populations “run hotter” than most, burning extra calories to generate warmth even when resting. People with this adaptation have significantly less cold-induced constriction of surface blood vessels in the cold, an advantage in preventing the loss of fingers and toes due to frostbite.
Increasing Heat Production: Shivering
While increasing baseline metabolic rate is a great adaptation to extreme cold, most of us don’t live in arctic conditions or experience the lengthy, sustained cold extremes required to develop an increased resting metabolic rate. We are more likely to increase our metabolic rate by shivering.
Shivering, the involuntary and rhythmic contraction of skeletal muscles, can increase metabolic heat production by about a factor of four. Shivering starts when skin temperature decreases and becomes more intense as core temperature drops. Violent, maximum shivering occurs when a person’s core temperature is about 93oF, or 5.5oF below normal (a core temperature here is considered mildly hypothermic).
Research cited by the U.S. Army Research Institute of Environmental Medicine found metabolic rates typically generate about 200-250 watts when resting (and shivering) in cold air; this is about the same heat production generated by walking. The study notes metabolic rates as high as 763 watts recorded by other researchers conducting cold water immersion experiments, but even this greater rate of heat production can’t necessarily protect against hypothermia. For example, wearing light clothing in a temperature of 26oF and a brisk wind result in the loss of 1300 watts of heat [1]- much greater than the 250 watts of heat generated by shivering. (A watts measures the rate of energy transfer; with conversion, it can be expressed as the equivalent to calories per second). Clearly, shivering alone cannot generate the required metabolic heat to maintain a stable core temperature in cold environments.
In addition to providing useful-but-limited amounts of heat, shivering can result in muscular fatigue. Over prolonged periods, muscles tire, producing less metabolic heat, especially if a person, having burned through most of their blood sugar reserve by exercise or physical labor, is hypoglycemic. In studies of people subjected to multiple bouts of cold or following longer periods of sustained physical effort, shivering begins later in fatigued muscles, delaying the onset of critical extra metabolic heat production.
Aside from fatigue, shivering is problematic when working outdoors. Trembling arms and legs make it difficult to operate tools at a construction site or maintain steady aim while hunting or at the range. Tasks that require finesse, for example, utility work, setting up a shelter, or assembling equipment on a winter morning, are further complicated by the loss of sensitive in fingertips cause by the reduced blood flow. Reduced hand dexterity and slower reaction time in cold environments have been well documented in many experimental studies.
Behavior Strategies
While humans have some ability to acclimatize to the cold, the most important factor for cold weather thermoregulation is our behavioral strategies. We dress for the cold, find a heated shelter, or move indoors. When the mission requires being out in the cold, more advanced forms of adaptation also exist. The Qore Performance ICEPLATE® system provides 52 watts of heating (or 104 watts with two plates). This heat reduces the need for metabolically intensive and fatiguing shivering, allowing those of us operating in the cold to focus our energies on the mission or job. By helping to prevent core temperature from falling, ICEPLATE® also reduces vasoconstriction, allowing us to retain dexterity and reducing the negative cognitive impacts associated with heat loss.
---
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] For those who read Thermophysiology 101 and Thermophysiology 102: the 1,300 Watts of heat loss comes from 200 Watts of radiative heat loss and 1,100 Watts of convective heat loss.