Hypothermia happens when extreme exposure to the cold causes your core body temperature to drop below 95 ºF. In other words, a person coming down with hypothermia is losing heat faster than their body can produce or retain heat.

Hypothermia can overcome people stranded in the outdoors or unprepared in severe cold and wet weather conditions, such as the 21 runners who unexpectedly died in an ultramarathon in China. But it can also impact people indoors and in cities as well, like the tragic winter storm that knocked out Texas’ power grid and was directly responsible for 246 deaths.

There are approximately 1,500 annual deaths due to hypothermia in the United States. The death rate from hypothermia is at 7% when body temperatures fall between 95-90 ºF. This mortality rate increases to 23% when body temperatures reach below 90 ºF.

That is why the most important thing to do to prevent hypothermia is to keep warm and dry.

How your body reacts

Hypothermia slows down the speed of your metabolism. This inhibits how fast nerves transmit their messages that help your body naturally keep you warm and ultimately alive.

When your body temperature begins to drop, it first responds by shivering. Shivering is a rapid squeeze of muscles and your body’s first defense. This can be often observed as teeth chattering, when the jaw muscles rapidly spasm. These quick muscle movements can produce 5-6x more heat than normal.

As your body’s temperature continues to drop, the cold disrupts the hypothalamus (the thermostat of the brain) and you lose the ability to shiver. When shivering stops, which happens at around 86 ºF, the heart begins beating at about 50% efficiency, and with even further drop in temperatures, the unstable heart rhythms can lead to death.

Continued cold exposure can cause a person’s cognitive and mental state to drastically diminish. This is because for every 1.8 ºF drop in temperature, your brain’s ability to function decreases by 7%.

The signs are there when someone has hypothermia: irritability, lethargy, appearing intoxicated, confusion, sleepiness, and eventual unconsciousness.

You don’t need a thermometer to diagnose someone’s hypothermia. Just watch for the symptoms!

a chart listing the levels of danger for hypothermia according to their symptoms

Fact or fiction: Ways to keep warm

In rapidly declining temperatures, the goal is to prevent hypothermia by keeping the core body temperature as warm as possible. While there are common things we all do to keep warm, they may not be the best method to keep warm.

Let’s break down what’s fact and fiction when it comes to staying warm in the cold.

Do you lose most of your heat through your head?

No. The head represents approximately 10% of the total body surface area, but the head cools at the same rate similar to any other exposed part of the body. This means you do not lose heat faster through the scalp than any other portion of the body with the same surface area.

So, similar to covering any other exposed skin on your body, it’s a good idea to also cover your head with a warm hat – not because it loses heat faster, but because it loses heat just the same as anywhere else, such as your hands and neck, which you are likely keeping covered with gloves and a scarf.

The best advice is just to keep everything covered and insulated, always.

Does drinking hot beverages keep you warmer?

Not really. While a warm drink will feel comforting in the cold, it is the amount of calories that you ingest that actually warms you up. Carbohydrates in drinks (like sugar, not synthetic sweeteners) are burned by the cells that provide energy and warmth. These simple sugars are calories or fuel that allow the body to keep you shivering.

It is the ingested carbohydrates through drinks and high calorie foods, not the temperature of your drink, that will keep you warmer in the cold. So a sugary juice or soda would actually be the wiser choice over a hot cup of tea (which has no calories).

Take extra note to avoid alcohol in the cold because it causes blood vessels to expand, which makes you lose heat.

How effective is shared body heat?

There is the idea that shared body heat, being body-to-body, can help a cold person warm up from hypothermia. As cozy as this sounds (with consent), the research does not support it.

Someone who is hypothermic with a low core temperature has constricted blood vessels (vasoconstriction). The body does this to minimize heat loss by keeping the warm blood closer to the core and vital organs, away from radiating into the cold surrounding environment. The vessels closest to the skin are impacted and constrict first. This state of vasoconstriction limits the ability for someone to absorb the heat from a warm person.

Exposure to a warm person will trick the skin into thinking it is warmer than the cold body core, and blocks the shivering response, leading to efforts that are no greater at rewarming than if they were shivering alone.

Body-to-body rewarming may make the hypothermic person more comfortable and should only be considered if it does not delay evacuation.

Will a hot tub help?

A warm shower, bath, or soak in a hot tub should be avoided as an initial tactic to rewarm a hypothermic person. The hot water increases blood flow to the skin (as the blood vessels expand to warm the blood), and all the cold blood in the extremities floods back to the central circulation and the heart.

Any action that increases blood flow in hypothermia (like hot water immersion, standing and walking, exercise) will cause the core temperatures to drop further. This “afterdrop” phenomenon is important to attend to in more severe hypothermia cases, where even a small drop in temperature can lead to unstable and potentially fatal heart rhythms.

Hot water immersion should be considered after an initial period of rewarming in mild hypothermia.

Can chemical heat packs be used?

While chemical heat packs should be applied to the hands and feet to prevent frostbite in the extremities (fingers and toes), they should not be used for rewarming for hypothermia.

The heat packs could instead cause thermal burns to the skin if applied to other parts of your body (rather than where intended such as your hands and feet).

No matter how tempting it might be, it does not provide sufficient heat to warm a person.

Keep dry

If you fall into a body of freezing water or get soaked in a winter storm, removing wet clothes is critical to warming up and preventing hypothermia.

Water conducts heat 24 times faster than air. What this means is that wet clothes will suck warmth away from your body that much faster.

Remove wet clothes, put on warm and insulative layers, and protect yourself from further exposure from the wind and elements. If available, a waterproof “vapor barrier” (like garbage bags with a hole for the head, space blanket, etc.) can be wrapped around a wet person to keep the insulation dry and is an effective method to reduce total heat loss.

The bottom line on hypothermia

Preventing hypothermia starts before your trip, in how you prepare. Layering properly, packing right, and paying attention to changes in weather conditions can all help ensure an enjoyable time. However, accidents can happen. Knowing what symptoms to look out for can help you spot hypothermia before it gets worse. And when faced with the onset of hypothermia, keep the person dry and their core temperature from dropping too low. The GOES app is a single resource that can help you manage all of this, before and during your trip.