Hypothermia is a dangerous drop in your core body temperature below 95°F (35°C). Your body normally holds steady around 98.6°F, and even a few degrees below that threshold can impair your brain, heart, and muscles. It can happen to anyone exposed to cold long enough, but it doesn’t require extreme arctic conditions. Cool, wet, windy weather can cause hypothermia even at temperatures well above freezing.
How Your Body Loses Heat
Your body loses heat through four pathways: radiation, convection, conduction, and evaporation. Radiation is the biggest contributor, accounting for roughly 60% of heat loss under normal conditions. This is simply your body releasing warmth into the surrounding air, the same way a hot mug cools on a countertop.
Convection strips heat away when air or water moves across your skin, which is why wind makes cold temperatures feel so much worse. Conduction transfers heat directly through contact with a cold surface, and water conducts heat away from your body about 100 times faster than air. That’s why falling into cold water is so much more dangerous than standing in cold air at the same temperature. Evaporation pulls heat from your body as sweat or moisture dries on your skin, which is why wet clothing accelerates cooling dramatically.
How Symptoms Progress
Hypothermia sets in gradually, and early signs are easy to dismiss. The first reliable signal is shivering. Shivering actually means your body’s heat-regulation system is still working, generating warmth through rapid muscle contractions. As your temperature continues to fall, things get worse in predictable ways.
Outdoor safety instructors teach a memory aid called “the umbles”: grumbling, mumbling, stumbling, and fumbling. These capture the four hallmarks of worsening hypothermia. A person becomes irritable and withdrawn (grumbling), their speech slurs (mumbling), their coordination deteriorates (stumbling), and they lose fine motor control in their hands (fumbling). Exhaustion, confusion, and memory loss follow.
In mild hypothermia (roughly 90–95°F), you’ll shiver intensely and your thinking slows. In moderate hypothermia (roughly 82–90°F), shivering may actually stop, which is a bad sign. Your muscles stiffen, confusion deepens, and drowsiness takes over. In severe hypothermia (below about 82°F), a person may become unresponsive, their pulse faint or undetectable, and their breathing extremely slow.
Paradoxical Undressing
One of the strangest features of severe hypothermia is paradoxical undressing. People near death from cold sometimes strip off their clothing. This happens because the blood vessels near the skin, which had been clamped down to conserve heat, suddenly dilate. The rush of warm blood to the skin creates an intense, false sensation of overheating, and the person tears off layers in confusion. In forensic studies, nearly all victims found partially undressed were also discovered wedged into small, enclosed spaces, under beds or behind furniture. Researchers believe this “terminal burrowing” is a primitive, unconscious protective behavior triggered by the brain stem, similar to how hibernating animals burrow before shutting down.
Who Is Most Vulnerable
Older adults are at higher risk because the body’s ability to regulate temperature and sense cold weakens with age. An elderly person in a poorly heated home can develop hypothermia without ever going outside. Infants are also vulnerable because they have a large surface area relative to their body weight and can’t generate heat efficiently through shivering.
Alcohol is a major contributor. It dilates blood vessels near the skin, which feels warming but actually accelerates heat loss from your core. It also impairs judgment, making people less likely to seek shelter or recognize danger. Certain medications that affect the nervous system, blood pressure, or sedation can blunt your body’s shivering response or interfere with blood vessel constriction, both key defenses against cold.
What to Do If Someone Has Hypothermia
The priority is stopping further heat loss. Gently move the person out of the cold, or at minimum shield them from wind, especially around the head and neck. Insulate them from the ground with a blanket or sleeping pad underneath. Gently remove any wet clothing and replace it with dry layers or blankets. The word “gently” matters here. In moderate to severe hypothermia, rough handling or sudden movements can trigger a fatal heart rhythm disturbance.
If further warming is needed, focus on the center of the body. Apply warm, dry compresses to the neck, chest, and groin. An electric blanket works if available. If using a hot water bottle or chemical heat pack, wrap it in a towel first. Offering warm, sweet, nonalcoholic drinks can help if the person is alert enough to swallow safely.
Several common instincts are actually dangerous:
- Don’t rewarm too quickly with a hot bath or heating lamp. Rapid warming causes blood vessels in the skin and limbs to open suddenly, sending a surge of cold blood back toward the heart and potentially causing a dangerous drop in blood pressure.
- Don’t heat or massage the arms and legs. This can stress the heart and lungs for the same reason.
- Don’t give alcohol or cigarettes. Alcohol speeds up heat loss, and tobacco restricts the circulation needed for rewarming.
Why Afterdrop Happens During Rewarming
Even after rewarming begins, a person’s core temperature often continues to fall for a period before it starts rising. This phenomenon is called afterdrop, and it’s one of the reasons hypothermia can be deadly even after a person is rescued. The traditional explanation was that cold blood from the limbs rushes back to the heart as circulation improves. But research using physical models with no circulation at all (including, memorably, a bag of gelatin and a leg of beef) showed that afterdrop occurs purely from how heat moves through tissue. The cold outer layers of the body continue pulling heat away from the warmer core, like a cold sponge absorbing warmth from the center outward, until the temperature gradient evens out.
“Not Dead Until Warm and Dead”
There’s a well-known principle in emergency medicine: no one is dead until they are warm and dead. Hypothermic patients who appear lifeless, with no detectable pulse, fixed dilated pupils, and no visible breathing, have survived with full neurological recovery. Cold slows the body’s metabolism so dramatically that the brain and organs can tolerate far longer periods without oxygen than they could at normal temperature.
The chances of survival and good neurological outcomes are actually higher for hypothermic cardiac arrest than for cardiac arrest at normal body temperature. This holds true even for unwitnessed arrests and prolonged periods without a pulse, but only if the hypothermia developed before the heart stopped. None of the usual signs that would normally indicate death, including asystole (a flatline heart rhythm), fixed pupils, old age, or even major trauma, are reasons to stop resuscitation in a hypothermic patient. CPR should continue, sometimes for hours, until the person’s core temperature has been restored to a normal range.