Three years ago, I was pursuing a herd of elk down a steep gully into a remote mountain valley in southwest Montana when it began to snow. The storm was unexpected and hyper-local; it often is in the mountains. It was only the middle of September, so I wore thin base layers under soft-shell pants, a thin fleece jacket, and low-top hiking boots. I opted to leave my rain gear and insulation in my truck, six miles away, to travel as fast and light as possible.

　　As the snow turned heavier and wetter, it soaked through my layers and into my boots, leaving me totally drenched. I really began to worry when I stopped shivering.

　　Hypothermia is the cause of around 1,500 deaths a year in the United States, according to a study published in the New England Journal of Medicine.

　　Hypothermia begins to occur when your body’s core temperature falls below 95 degrees, according to Chris Adams, a flight nurse for the Life Flight Network, a nonprofit transport network that takes high-risk patients to hospitals by helicopter, working out of Coeur dAlene, Idaho. Adams says he treats hypothermia virtually every time he rescues a trauma victim.

　　The majority of hypothermia cases happen in rural environments where emergency services may be unavailable or slow to respond. And in many colder places, hypothermia is the second most common cause of unintentional death, after vehicle accidents. Hypothermia is particularly dangerous because its occurrence often involves the unexpected. Just like during my elk hunt.

　　My ATV was waiting on a trail 1,000 feet above me, at least a 30-minute hike away. My efforts to reach the vehicle partially dried my torso and legs, but my hands and feet were still totally numb by the time I climbed the gully. Riding the ATV back to my truck was a challenge. The numbness in my fingers made it hard to operate the controls, and the urgency to reach safety had to be balanced with the additional windchill created by speed.

　　By the time I got to my truck and clumsily climbed into the driver’s seat, a glance in the rearview mirror revealed blue lips surrounded by my ghastly pale face. I cranked the heat, turned on my seat warmer, and sat in silence for half an hour while the shivers returned and stabbing pain crept into my extremities.

　　According to Adams, I was probably in a stage of mild severity while hiking up the mountain, then into moderate hypothermia by the time I’d reached the truck. Preparing to survive hypothermia is an essential skill for those of us who recreate outdoors in cold weather.

　　How Can You Tell If You Have Hypothermia? Luckily, hypothermia has clear indicators. Watch for the umbles—stumbles, mumbles, fumbles, and grumbles which show changes in motor coordination and levels of consciousness,” reads a white paper on hypothermia published by Princeton University.

　　Medical professionals parse the stages of hypothermia by internal body temperature. But since you cant get an accurate read of your bodys internal temperature with oral thermometers, according to a University of Minnesota paper, you and I are better off looking at symptoms.

　　According to Adams, symptoms of hypothermia include:

　　Feeling cold Loss of motor control, including both fine (operating zippers) and gross (the ability to walk) Impaired mental abilities impacting speech and consciousness The slowing of respiratory and heart rates A mildly hypothermic person will still be shivering, but begin to lose fine motor control. A telltale sign of moderate hypothermia is when the victim stops shivering, and when walking and standing become difficult. In a severe stage of hypothermia a person may be unable to stand or walk, and will likely lose consciousness altogether. Beyond that, the body approaches death as its heart rate slows to just a few beats per minute, breathing stops, and eventually the heart fails.

　　Uncontrolled shivering, slurred speech, confusion, and reduced coordination can quickly spiral into unconsciousness, John Barklow tells Outside. After serving as a Navy diver, Barklow trained Navy SEALs in cold weather survival techniques (including self treatment of mild to moderate hypothermia), designed clothing systems intended to reduce the odds of Special Operations Forces experiencing hypothermia, and now works as the lead designer for Bozeman, Montana-based technical clothing brand Sitka Gear, while still teaching survival classes and seminars.

　　How to Prevent Hypothermia Beyond wearing enough insulation to remain warm in a given temperature, its important to consider the materials youre wearing.

　　The worst of those is cotton. Because cotton fibers are hollow and carry a negative electrical charge, they absorb and retain positively charged water. Cotton fabrics can hold up to 27 time their own weight in water, then refuse to dry out.

　　Wool is a lot better. It absorbs only 30 percent of its own weight in water, and the microscopic structure of its fibers can work to break the bonds between hydrogen and oxygen atoms, producing a tiny amount of heat.

　　Down, even varieties treated with hydrophobic coatings designed to repel water, loses its ability to loft (and keep you warm) when wet.

　　Best are synthetic fabrics and insulations like polyester and nylon, which only absorb around 0.4 to four percent of their weight in water, respectively. Because synthetic materials dry so much faster as a result, they’re a much safer option in cold, wet conditions, or when you run a risk of submersion. I’ve recently transitioned to an all-synthetic clothing system for backcountry adventures, for that reason.

　　In what Barklow calls the rewarming drill, he submerges participants in freezing-cold water, then instructs those students to add layers of synthetic insulation, drink water, and consume easily-digested calories. Patients huddle inside breathable rain shells designed to keep out the weather and prevent convective heat loss. The warmth generated by their bodies, held in by warm-when-wet synthetic insulation, is able to force water first away from their skin. Moisture gets drawn out through the layers of clothing, enabling their bodies to return to safe temperatures.

　　“With a great clothing system there’s no need to carry extra [equipment],” Barklow says.

　　Adams backs this up. This is a really good idea, if you have the right clothes, he states. Preparation is everything.

　　The nurse also says that if you begin to experience hypothermia symptoms, and youre alone in the backcountry, you should focus on creating body heat. You can hike up a hill really fast, he says.

　　How to Treat Hypothermia Barklow’s method also tracks with survival advice given by the Wilderness Medical Society, which prescribes protecting yourself or a patient from the environment, drying them out, and then warming them up using a heat source. With Barklow’s method, which requires synthetic clothing layers, you don’t need to remove any clothing, or — and you don’t need a fire.

　　Should you lack such a clothing system, the approach becomes a little more complicated. You’ll need shelter, dry insulation, and a heat source. In mild stages of hypothermia, you can use something as simple as a tent, a dry sleeping bag, and another person’s body heat, warm water bottle, or chemical heat packs. (Place the latter two items on the neck, arm pits, and groin where large arteries pass close to the skin). As you progress into moderate and severe cases, hypothermia will require more significant sources of heat, like a heated structure or vehicle or a wood stove, and ultimately treatment by medical professionals.

　　Adams recommends paying close attention to preventing convective heat loss through contact with the cold ground, and suggests chemical heat blankets (like those sold by Ready Heat) as a heat source. I lay down a wool blanket, put a heated blanket on top of that, lay the patient down, then layer heated blankets and another wool blanket on top of that, describes Adams. Then we just crank the heat in the helicopter until the patient warms back up.

　　Adams is careful to caution against shocking a hypothermia victim with too much heat, though, saying you shouldnt submerge them in a hot bath or shower. Its gotta be slow, he says. Just focus on getting the ambient temperature nice and high, and rewarming slowly.

　　What about CPR? In severe stages of hypothermia, a person’s pulse may not be detectable at the wrist due to severely constricted blood vessels, and when checked at the carotid artery may be as slow as just a few beats-per-minute. Adams warns against chest compressions as a result, but says blowing warm air into an unconscious victims lungs may help increase their core temperature.

　　Barklow recommends that you dont just leave survival skills up to chance. You need to train in realistic conditions to ensure you and your gear perform as expected, he says.

　　Ensuring your layers are a match for the conditions is something you should first try in a safe environment. You don’t want to realize that you and your kit aren’t up to the task of saving your life after you swim a glacier-fed rapid while on a remote packraft trip in Alaska, he says.

　　Adams offers one final piece of advice: don’t give up. Due to the protective effects of cold temperatures, complete recovery can be possible even in severe, prolonged hypothermia cases.

　　Wes Siler recently returned from a trip to Yellowknife, in northern Canada, where temperatures were as low as -38 degrees Fahrenheit. He was warm, comfortable, and safe throughout. You can ask him more detailed questions about outdoors gear and other topics on Substack.