Less than three weeks ago, the Northeast US was slammed by the largest blizzard in years: winter storm Nemo. Swarms of people ran to the grocery stores and gas stations to stock up on food, batteries, flashlights, clean water supplies, and gas in preparation for the storm. Fortunately, being able to stay within the comfort and warmth of their homes, most people didn’t have to be concerned about one of the greatest threats that comes with winter weather: cold weather injury. This month, the Science in the News Flash got in touch with Dr. Murray Hamlet, an expert on cold weather physiology and retired director of the Cold Research Division for the US Army Research Institute of Environmental Medicine (Natick MA), to learn more about this fascinating, yet frightening, topic. A North Dakota native, Dr. Hamlet was originally trained as a veterinarian before entering the Army where he spent 33 years researching cold weather injury. He is the author of numerous scholarly publications on the mechanism and prevention of cold weather injury, including hypothermia, frostbite, and trench/immersion foot.

What is the human body’s normal response to cold weather?

Maintaining human core body temperature at 37.0 °C (98.6 °F) is a highly regulated and complex process. It is estimated that approximately 80% of the energy we obtain from eating food goes into sustaining this core temperature- and for good reason! While fluctuations around 0.5°C are normal, changes of only 2-3 °C can greatly impair physical and mental performance. When body temperature drops significantly, chemical reactions in the body slow down, leading to serious complications [].

“When your body is exposed to cold, it tries to conserve your core heat. You shut off blood flow to your arms and legs slowly. All that blood goes to your core,” explained Dr. Hamlet. This process is termed vasoconstriction, and occurs when cold sensory receptors in our skin respond to the cold and stimulate the blood vessels in the skin to constrict. According to Dr. Murray,  “this physiological response is to a change in temperature, rather than an absolute [temperature]…as is true for many control mechanisms in our body.”

If body temperature continues to fall, “you start to shiver,” says Dr. Hamlet. When shivering, body muscles go through cycles of relaxation and contraction, producing heat to warm the body. The extent of shivering “varies [from] person to person”, says Hamlet, and depends on numerous personal characteristics such as “glycogen [energy] stores, body weight, and how drastic the change in temperature is”. While shivering can increase the body’s heat production approximately six times the resting level, it is costly; eventually the muscles do not have enough energy to sustain shivering. According to Dr. Hamlet, a single person can shiver anywhere from 10 minutes up to a number of hours depending on when their muscles give in to fatigue. “If you stop shivering, your core body temperature has probably [already] dropped around 3° F”, explained Dr. Hamlet. At this point, Dr. Hamlet continued, one is at serious risk for cold weather injury if exposure to the cold persists.

Cold Weather Injuries

Cold weather injuries range from frostbite and hypothermia to immersion/trench foot. Frostbite, a freezing injury, results when tissue is cooled to the point where it freezes. This most commonly occurs in the extremities, such as the hands, feet, nose, and ears, where blood circulation is restricted in cold temperatures. Originally beginning as frost nip, the skin turns an unnatural shade of white and loses sensation. Left unattended, frost nip will lead to frost bite, which can be a much more serious injury. While frost nip usually results in symptoms similar to sunburn, in frostbite, the skin cells freeze to the point where they form ice crystals and die. Upon re-thawing, the skin swells and can blister, turning blue, purple, or black in the process. While superficial frostbite on the outer skin layer, or epidermis, usually does not result in permanent tissue loss, freezing into the deeper layers of the skin, muscle, and bone can result in the need for amputation of the damaged area [2,3]. Of course, the decision to amputate should be taken carefully. According to Dr. Hamlet, one should whirlpool bathe a frostbitten area everyday for two months to gently remove the dead tissue before a doctor determines if anything needs to be amputated. “At the end of two months its usually obvious what needs to be removed and what doesn’t.”

Non-freezing injuries, such as trench (or immersion) foot, result from prolonged exposure of the extremities to wet and cold conditions at temperatures above freezing but below 60° F. The name “trench foot” was termed after World War I when thousands of soldiers suffered from this cold weather injury after experiencing the cold and damp conditions in the trenches. Because water cools more effectively than air at the same temperature, being wet increases the loss of heat from the body. Dr. Hamlet explained, “As you get more wet and cold, things get serious as tissue starts to die.” Tight clothing or footwear that reduces circulation to the extremities can also contribute to injury. While this non-freezing cold injury is initially reversible, prolonged exposure can result in irreversible damage, gangrene (when a large amount of tissue dies), and the need for amputation [].

Figure 1. Trench foot poster published by the Office for Emergency Management, Office of War Information during World War II (1943-1945) to warn soldiers of the dangers of this condition caused by prolonged exposure to cold, wet, and unsanitary conditions. Image courtesy of the National Archives and Records Administration.

Hypothermia begins when the body’s core temperature falls below 95 °F. While this drop from 98.6 °F may not seem like much, it has extremely detrimental effects on the body. At this temperature, metabolism, shivering, respiratory rate, and heart rate decrease. Additionally, cognitive function is impaired and a hypothermic person can lose the ability to make rational decisions, making it even more dangerous and possibly fatal to be out in the cold. At a body temperature of 85 °F, cardiac output and oxygen consumption decrease by approximately half; shivering stops and the body no longer tries to maintain warmth. Eventually a person in this condition becomes unconscious [2,3]. Dr. Hamlet warns that even if a person with hypothermia appears dead, they may not be; “hypothermics can be resuscitated from very cold temperatures.” A popular quote when it comes to hypothermia is “nobody is dead until they are warm and dead.” According to Dr. Hamlet, hypothermics should be carefully treated in the field, and quickly moved to medical attention; “Ventilate them, warm their body, especially around the chest, and move them quickly and carefully to a hospital. Raising their oxygen concentration helps prevent their heart from fibrillating or going into cardiac arrest.”

On the bright side….

With all of this discussion of the extreme dangers of cold weather injury, you are probably ready to shut yourself into your home and avoid all exposure to the cold. But wait! Just how common are these injuries? As Dr. Hamlet explains, “In the military setting, with soldiers out in combat or in a pre-combat setting, the likelihood of injury can be high.” Depending on the environment and situation, possibly up to “1 in 5 [soldiers] can suffer a cold weather injury.” However, Dr. Hamlet elaborates, “In the civilian world, we don’t put ourselves in the threat long enough to get very serious injuries. It usually involves an accident- getting trapped, hurt, or lost.”

Though rare, we do hear stories of civilians sustaining cold weather injuries every winter. Dr. Hamlet points out that we can prepare for potentially dangerous cold situations by dressing properly and wearing layers that are insulating and will keep dry. This means not wearing cotton, which, although suitable for casual or indoor activities, absorbs and retains water. Dr. Hamlet also believes that people should “carry extra clothing in their cars” in the winter, a practice he closely follows himself, in case of any accidents. “At minimum, a pair of insulated boots [and] mittens, rather than gloves, [because mittens] give you more insulation,” recommends Dr. Hamlet. Even those who are familiar with winter environments should be attentive to cold weather safety measures. While humans can acclimatize, or physiologically adapt, to hot weather by changes such as dissipating heat more readily through the skin and sweating more, we do not acclimatize to the cold nearly as well. “Most of adapting is just learning to recognize when you are getting too cold, and taking action to do something about it. Someone from the South may not know when their hands are getting too cold, or that they can get hurt,” explained Dr. Hamlet. Those new to cold weather environments, therefore, require more teaching on how to dress properly and stay safe.

So does Science in the News or Dr. Murray Hamlet recommend you hibernate for the winter? No! Be prepared and dress appropriately, but otherwise get out there and enjoy this last bit of winter, play in the snow, and enjoy your favorite winter sporting activities!

Kelsey Taylor is a PhD student in Biological Biomedical Sciences program at Harvard University.


1. Hypothermia and Cold Weather Injuries, by Rick Curtis. <http://www.princeton.edu/~oa/safety/hypocold.shtml>

2. Medical Aspects of Cold Weather Operations, by US Army Research Institute of Environmental Medicine. <http://www.usariem.army.mil/Pages/download/weather.pdf>

3. How Humans deal with and survive extreme cold: science of the cold. <http://www.coolantarctica.com/Antarctica%20fact%20file/science/cold_humans.htm>

More Interesting Resources

Simple Method Found to Warm Cold Hands (Dr. Murray Hamlet’s research on Raynauds Disease) <http://www.nytimes.com/1988/01/19/science/simple-method-found-to-warm-cold-hands.html>

Wilderness Medicine Newsletter- a blog with contributions from Dr. Murray Hamlet in October 2006 <>

4 thoughts on “Keeping Warm: A discussion with Dr. Murray Hamlet on cold weather physiology

  1. Dr. Hamlet came to Hawaii to teach a couple thousand soldiers going on Team Spirit in South Korea in Feb. His expertise no doubt saved alot of us from incurring cold weather injuries

  2. Dr. Hamlet is indeed a great source of cold weather injury research as well as the folks at Natick, I had the good fortune to meet and learn from Dr. Hamlet when I was an instructor with the U.S. Navy Cold Weather Medicine training unit at the Marine Corps Mountain Warfare Training Center back in the mid-1980’s.

  3. I’m needing assistance from a non-freezing cold injury expert. My father is a Korean War combat veteran, having served from March 1952-1953 in the Marine Corps. He has always complained of pain to his feet/toes in both feet. He was exposed to cold/wet conditions in combat. The VA has denied his claim for peripheral neuropathy to feet. We’re appealing. Think expert opinion would help.

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