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How to Prevent Hypothermia

Contributor: Luke N. Apisa, MD
7 minute read
Woman hiking in cold weather

Winter hikes and ski trips are a great way to stay active and embrace the outdoors, but extended periods in cold or wet weather also can put you at risk of a serious health condition called hypothermia.

Luke N. Apisa, MD, a Mass General Brigham emergency medicine doctor, shares his tips for staying safe in cold weather so you can prevent hypothermia and enjoy your favorite activities.

Dr. Apisa serves on the faculty of the Emergency Department’s Division of Wilderness Medicine at Massachusetts General Hospital. The division focuses on the practice of resource-limited medicine under austere conditions, including emergency care in wilderness areas, medical care in response to rural and urban disasters, and care in other challenging settings.

What causes hypothermia?

Our body temperatures typically clock in at around 98.6°F (37°C) and hypothermia develops when body temperature falls below 95°F (35°C). Most people experience hypothermia when they’re exposed to extremely cold temperatures. But you can also develop hypothermia when you’re chilled from rain, sweat, or submersion in cold water. It’s possible to experience onset of hypothermia in these cases at cool temperatures above 40°F. Prolonged exposures to cold air or water use up your body’s stored energy over time, and this leads to lower body temperature.

Ironically, one of the hardest things about temperature regulation when you’re outdoors, particularly when you’re exerting yourself in the cold, is not overheating. Sweating while exerting yourself in the cold generates water that can cool you quickly.

Luke Apisa, MD
Emergency Medicine Doctor
Mass General Brigham

Reduce your risk of hypothermia 

Here’s how to reduce your risk of developing hypothermia and serious complications:

Pack adequate water and food and carry them strategically.

If you’re hiking or taking part in other outdoor activities for extended periods in cold or unpredictable weather, it’s critical to pack the right amount of water and food. Staying hydrated and well-fed gives your body the energy it needs to stay warm.

“If it's a day hike, I would always recommend having 2 or 3 liters of water on board, particularly if you're hiking in a cold area,” advises Dr. Apisa. “Carry water in a backpacking water bladder where it's right up against your body so the water doesn't freeze. That's really important. From a nutrition standpoint, I'm a chronic food over-packer when it comes to hikes. And part of that I think is my background in wilderness and emergency medicine and seeing it go wrong.”

“I always recommend having a couple thousand calories, almost a day's worth of food with you, which is pretty easy to stack up to. If you're going for speed, 1,000 calories is OK for most day hikes, but you need to be aware that your margin for error decreases the more you run at a caloric deficit. You're rarely going to be wrong bringing more food, especially if it's just a recreational hike and you're not too worried about aggressively minimizing pack weight.”

Dr. Apisa has a tip for winter athletes who need to keep their energy levels high during long workouts: Tuck away food, gels, and other pieces of nutrition in places where you can easily access them. This makes you far more likely to actually consume them than if they’re out of reach.

Choose clothing with new wool blends or wool synthetic blends.

If you’re an avid hiker, camper, or winter athlete, you may be familiar with the saying “cotton kills.” Dr. Apisa explains why it’s important to choose the best clothing materials for extended time outdoors in winter.

“Ironically, one of the hardest things about temperature regulation when you’re outdoors, particularly when you’re exerting yourself in the cold, is not overheating. Sweating while exerting yourself in the cold generates water that can cool you quickly,” he says. “Cotton is dangerous because it holds water really well. If you end up sweating, the cotton holds all of that water. This can cool you off and make you a lot colder in the long run. New wool blends or wool synthetic blends are much, much safer to wear outdoors.”

Layer clothes loosely to optimize warmth.

Layering clothing properly to optimize warmth is critical and Dr. Apisa explains there are some tricks to doing it the right way. Bundling too many layers of clothing too tightly can actually cause harm, but with a little finesse you stay warm and safe.

“Many of us remember being a kid getting that giant bundle of clothing, like the Stay-Puft Marshmallow Man in all of this insulating gear, and layering when you go outside,” explains Dr. Apisa. “But one of the more non-intuitive things about layering is that you don't want the clothing to be layered so tightly that all of the layers compress each other.”

“A better solution is to layer clothes a little more loosely to leave room for a small air gap between. That gap of air between the layers gets warm and creates an insulating barrier between you and the outside. If you put more layers on and stack them too thickly, suddenly you don't have that insulating air barrier. All of those compressed layers start to conduct heat to the outside. And so unintuitively, more layers compressed down tightly on you can actually make you lose heat faster than a few good layers with the appropriate looseness on the body.”

Winter athletes who may exert themselves more with long periods of intense activity, like skiing or outdoor hockey, should be especially careful about layering.

“Because of their high levels of exertion, winter athletes have far more precise insulation needs. The high levels of heat generation from strenuous activity outdoors means that they need to be ready to rapidly switch from perhaps one or two light layers of insulation at peak activity to full fleece or down insulation after returning to rest.”

Limit or avoid drinking alcohol in cold weather.

Alcohol is the most common substance Dr. Apisa sees in people with hypothermia who arrive in the Massachusetts General Hospital Emergency Department each winter.

“Alcohol feels like it makes us warm because it brings all that blood flow to the skin,” he says. “But in reality, it doesn't make us any warmer; it just inhibits our ability to feel the cold.”

He describes a common scenario: “Some patients go out on a snowy night in Boston and drink too much. They get to their front door and are too intoxicated to get in. Someone later finds them on their porch and they wind up with hypothermia after being out in the cold for too long.”

Stay safe while boating in winter.

If you’re boating in cold weather, wear a lifejacket at all times to prevent drowning. Dr. Apisa also shares some guidance for hypothermia prevention that’s less intuitive.

“If you were to get into a boating accident in the winter when the water is cold, the water itself would feel, paradoxically, warmer than sitting on top of the boat while wet," he explains.

He warns this experience can be misleading and put you at harm if you stay in the water too long.

“Even though it feels colder to you, the wind is actually far less efficient at pulling heat away from your body than being immersed in the cold water is. The Navy actually did studies on boaters to research this. They put all of these temperature probes on them, had some boaters stay immersed in the water, and had others stay on a boat wet. The people in the water progressed to hypothermia two, three, or more times faster than the people on the boat. So safe boating practices are quite the opposite of what the body feels intuitively.”

If you’re in a boating accident and find yourself in cold water, Dr. Apisa recommends getting out of the water as quickly as possible if you’re close to shore. If you’re on a boat that capsizes a long distance from shore and the water’s quite cold, it would be dangerous to swim. Instead, he says the safest option is to stay on top of the boat until a rescue team can reach you.

Complete first aid and CPR training.

Taking a first aid or CPR course to learn how to do CPR and other skills is a great way to be prepared for any emergency, especially if you spend extended time outdoors in cold or wet weather. It can help you save a life while you wait for medical assistance or rescue, especially if you’re in a remote setting.

“Late-stage complications of hypothermia, like loss of pulses are rare, but having adequate first aid and CPR training commensurate with the intensity and remoteness of your outdoor pursuits is always a good idea," says Dr. Apisa.


Emergency Medicine Doctor