I had the unfortunate opportunity to witness the effects of hypothermia firsthand during a midsummer mountain misadventure in the Canadian Rockies.
The event began to unfold near our mountain summit destination. An unexpected and violent thunderstorm caught our party of five high up on the ridge, exposed to ear-splitting claps of thunder and lightning strikes so close that we could feel the electrical current running through the rocks under our feet, hands and any other body part in contact with the ground. With the valley floor thousands of feet below us and nature’s fury bellowing all around us, we started an immediate and desperate descent. But the going was slow, with technical climbing difficulties hampering our progress and wind, rain and icy snow pummeling us most of the way down.
By the time we finally reached the talus and scree slopes that marked the last stretch of mountain terrain before the meadow and our car beyond, the storm had moved on, leaving us in the dark after nightfall, wet and exhausted. Still, I felt a bit of relief, now that the worst was behind us. The relief was short-lived. I noticed that Jenny (not her real name) was stumbling through the broken and unstable rock rubble, walking erratically. She mumbled something that didn’t make any sense, her speech slurred. A scant few moments later, her condition seemed to be getting worse, her words incoherent and responses sluggish. I fought off a wave of dread, recognizing the symptoms of hypothermia. I wasn’t sure of the onset, since all of us were too focused on getting off the peak to notice the signs that Jenny’s core body temperature was sliding away from normal and toward a level that could, if untreated, lead to her death.
I alerted my husband and our two other friends, who were carefully picking their own way through the boulders. Between us, we focused on managing Jenny, prodding and pulling her, all the while working to keep the rest of us safe. When we reached our vehicle, we set the car heater on high and bundled her into the back seat between two of us, sharing our body heat. We were able to warm her up before the situation became critical and required intervention by emergency medical professionals.
We were lucky to have averted a dire outcome. The episode stayed with me and raised all kinds of questions. What caused or contributed to Jenny’s hypothermia? And how could I keep it from happening again, to me or someone else? Getting more familiar with what happens in hypothermia and owning the lessons learned from that day would help answer those questions.
The human body performs an intricate physical and biochemical dance to keep its core temperature around 98.6 degrees (F). But there are limitations to the body’s ability to maintain that normal temperature. Hypothermia occurs when protective heat regulation processes become overwhelmed and core temperature falls to a level where the brain and muscles can’t function. Untreated hypothermia progresses through distinct stages of body system shutdown: mild, moderate and severe. With declining brain function, hypothermia victims are unaware that they’re in danger. They make poor decisions, or no decisions at all, don’t take care of themselves to mitigate the effects of hypothermia, become apathetic to their situation and make dangerous mistakes in judgment. Like a falling line of dominoes, the core temperature continues to drop toward severe hypothermia, which can lead to death.
Understanding the body’s thermoregulatory equation is important in preventing and responding to hypothermia in the field. It’s a balancing act between heat production, heat conservation and heat loss.
Our basal metabolism, the continuous hum of our body’s basic processes, produces heat. In addition, the body burns glucose to power other chemical reactions that produce heat. Muscle activity, such as exercise, also generates heat.
To retain heat, blood vessels in the skin constrict to minimize radiant heat loss. In another subtle maneuver, hairs on the skin’s surface stand on end (goosebumps) to trap air and increase the insulating property of air around the body. Sweating ceases to halt evaporation. Postures that conserve heat, such as pulling the arms in close, mimic insulating layers or form barriers to cold.
Heat loss occurs through four main mechanisms: radiation, evaporation, conduction and convection.
Radiation. Electromagnetic energy releases heat. A wood stove emitting warmth is an example of radiant heat.
Evaporation. When moisture evaporates from the body into the air, it cools. Skin or clothing wet from rain, sweat, snow, ice, fog or from stream, river and lake water promotes evaporative heat loss.
Conduction. Conduction is a transfer of heat from one surface to a cooler adjacent surface. A warm human sitting on cold rock or ground loses heat through conduction.
Convection. A thin, warm layer of air surrounds us close to the skin’s surface. Air movement carries this layer away. If the air is cooler than the warm layer next to the skin, the warm air is replaced by the cooler air, resulting in heat loss. Wind blowing across the body cools. Higher wind velocity increases this effect.
Applying these principles to our Canadian Rockies experience revealed some significant lessons. All four of the heat loss mechanisms had been in play that day.
Our bodies radiated heat as a normal function throughout the day. But we had underestimated the length of our adventure and misjudged the potential for foul weather and difficulties descending. The day stretched into night, extending radiant heat loss. Our “wood stoves” had been radiating heat for many hours.
Our skin, hair and clothes were soaked from rain, snow, low-lying, moisture-laden storm clouds and a sweaty start on the uphill climb. Jenny had worn a pair of cotton sweatpants, which became sodden and ineffective as insulation. Cotton fibers have no insulating value when wet. Wet clothing presents a double-whammy effect through a combination of evaporative and conductive heat loss. Jenny also wasn’t carrying any extra, dry clothing. The rest of us were somewhat better equipped. Even though it was mid-summer and hypothermia had not been top-of-mind as a potential problem, three of us were wearing lightweight wool pants that provided some insulation when wet.
Sunshine in the morning devolved into cloudy skies, followed by the storm’s colder air temperatures, rain, snow and icy graupel. Any direct contact with the cold rock, cold ground and cold equipment provided ample additional opportunities for conductive heat loss.
Wind? Yes, we suffered plenty of heat loss from convection. A fierce wind dragged away any warm layers of air next to our skin, allowing the colder air to flow in and occupy that space.
Had we been producing enough body heat to balance the losses? Calories from our breakfast had fueled the uphill climb. Too intent on reaching our objective throughout the day, we hadn’t eaten much, if at all. Glucose heat production was in short supply by the time the storm rolled over us. Our haste to get down from the mountainside trumped any stops to refuel with snacks or uneaten lunch. The balance scale had been tipping toward heat loss.
Were there other individual risk factors involved in maintaining a normal core temperature? Jenny had a slight build, a smaller body mass to generate heat than the rest of us. Shivering can produce some heat, but it also fatigues the muscles over time, and becomes less effective for heat production. Jenny’s body hadn’t been able to balance the heat gain–heat loss equation.
As core temperature approaches 96 degrees, involuntary shivering kicks in, the body’s attempt to generate heat with involuntary muscle activity. A core temperature drop to below 95 degrees results in an inability to care for oneself, and the beginning of trouble. The hypothermic victim is unaware of the potential danger ahead. Judgment begins to fail, accompanied by decreases in self-protective behavior and survival instincts. For a hypothermic individual, an established goal, such as reaching a summit, may take priority over self-preservation. The brain is busy sending signals to the body to produce and conserve heat, leaving higher levels of thought and reasoning to dissipate.
Jenny’s slurred speech, stumbling, tripping, inability to walk on her own and apparent apathy to her situation fell fully in the realm of mild hypothermia, a core body temperature between 90 degrees and 95 degrees.
Any additional drop in her core temperature would have precipitated a dire trajectory. At 92 degrees, victims are unable to walk. Moderate hypothermia arrives at core temperatures between 90 degrees and 82 degrees and results in harm to basic vital body functions, depressing the central nervous system with accompanying changes in level of consciousness and disturbances to the heart, lungs and blood volume. A core temperature below 80 degrees, or severe hypothermia, causes unconsciousness and cardiovascular collapse.
When we reached flatter terrain at the base of the mountain, we worked to keep Jenny moving, not letting her stop or sit down. Without an emergency shelter, where we might have been able to get protection from the elements, get Jenny out of her wet clothes and gather close together to make the most of our collective heat, the best option seemed to be to hold her up and make her walk as best we could. The movement of the large muscles in her legs generated some heat. It was nearly midnight when we finally got home safely to hot food and drink, hot showers, dry clothes and sleep.
That was over thirty years ago. I am an older, more knowledgeable and more experienced hiker now. Hypothermia isn’t a hazard encountered only in winter or colder climates. Rain, wind, high-altitude temperatures, sudden nonseasonal weather, an accidental slip into a stream swollen with spring run-off, insufficient food for fuel, excessive sweating, being “benighted” without shelter—any of these on their own, or more often in cascading combination, can lead to a case of hypothermia in any season.
I’ve incorporated my Canadian Rockies lessons into self-care strategies for avoiding hypothermia, and for addressing it in its early stages if I run into it again. And I know that prevention is key. Consider using “4Cs” as a mnemonic checklist for trip preparation, hypothermia prevention and treatment.
Environmental. Have you determined, as much as possible, the forecasted air and wind chill temperatures, the current and incoming weather, wind speeds and directions, your proposed route with mileage and elevations, as well as alternative routes, if needed? What’s the precipitation outlook, the potential for getting wet, even if from sweating in warmer weather?
Personal and group. Are you traveling solo, or with a group? Are you aware of your own and others’ fitness level, capabilities and temperament for the terrain and the upcoming adventure? Are there physical limitations or considerations? Older hikers may be at greater risk for hypothermia due to decreased physiological reserves, or from the effects of certain medical conditions or medications that impair the body’s ability to compensate for insufficient heat production or increased heat loss. Diabetes, neuropathies, Parkinsonism, hypothyroidism and anorexia are conditions that can impair thermoregulation. Medications such as beta blockers, antihyperglycemics, antidepressants, antipsychotics, antianxiety medications and opioids can impact the body’s responses. Alcohol induces dilation of the blood vessels, which increases heat loss.
Clothing. Are you wearing clothing in layers, able to add or remove items as needed? Do you have something extra for an emergency? Does the weather warrant a hat and gloves? Have you avoided cotton clothing, opting instead for synthetic or wool fabrics, which have some insulating value even when wet? Are you attentive to your body’s heat gain–heat loss efforts? With the knowledge that wet clothes, from sweat or external moisture, can contribute to hypothermia risk, are you staying dry? Stopping to take clothing off if you are too warm? Stopping to put on clothing if you’re beginning to feel cold? Don’t wait for others to call for a break. Take care of yourself.
Calories (and hydration). Do you have enough food to fuel yourself for the duration of the trip, with some extra, just in case? Do you have water to stay hydrated (your body also needs fluids to work properly)? Are the food and fluid easily accessible? Are you willing to delay trip objectives to eat and drink, or finding a way to refuel while moving? Is your water protected from freezing in cold temperatures?
Communication. Are you speaking up for yourself when you need to make an adjustment for your own self-care? Have you been observing the others on a group outing and supporting them to initiate self-care if it seems they may need it? Are members in a group watching out for each other’s welfare, including yours? Don’t hesitate to say something before trouble with hypothermia begins. If you are alone, are you checking in with yourself often, as if you had a buddy with you who was checking on your status?
We don’t head out the door for a hike expecting drama and difficulties. Mostly, we’re focused on our trip’s objectives, anticipating an experience that satisfies us with solitude or camaraderie, or challenge and accomplishment, or a new view or perspective. We don’t go looking for trouble. Hypothermia is trouble. Be prepared to handle it, if trouble finds you.
Pack effective extra layers in your pack—waterproof, windproof, insulating.
Keep extra clothes and gear dry with a pack cover, or pack items inside a large plastic bag.
Keep hat and gloves, if warranted by the weather, easily accessible, to put on and take off as you self-regulate.
Have trail mix or other finger food easily accessible in pockets.
Consider adding some “quick hit” glucose items to your snack stash—chocolate or other candy, “gummy” snacks, squeeze packets of pureed fruit, such as applesauce.
In warmer weather without risk of water freezing, make your water easily accessible with a water bladder and drinking tube system.
In cold weather when water may freeze, carry a small water bottle on a lanyard around your neck and under your clothing, where it is easy to get to and won’t freeze. To keep the lids of water bottles in your pack from freezing, turn the water bottles upside down and put them in insulated covers or thick socks.
Carry some type of emergency shelter. Two large plastic garbage bags can be used.
Carry a foam pad to put on the ground as a conductive heat loss barrier.
Organize your pack so that you know what is where. It will minimize stop times and keep you from accidentally dumping items onto the ground while looking for something.
Zafren, Ken, MD, and C. Crawford Mechem, MD. “Accidental hypothermia in adults.” UpToDate. uptodate.com. Accessed 27 January 2023.
Hubbell, Frank, DO. SOLO’s Field Guide to Wilderness First Aid. Fifth Edition. TMC Books, LLC, 2020.
Gagne, Ty. Where You’ll Find Me: Risk, Decisions, and the Last Climb of Kate Matrosova. TMC Books, LLC, 2017.