Hypothermia is a dangerous drop in your body’s core 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 brain function, muscle control, and heart rhythm. In the United States, roughly 700 people die from hypothermia each year, with adults over 65 accounting for about 60% of those deaths.
How Your Body Loses Heat
Your body constantly radiates warmth into the surrounding air, the same way a hot cup of coffee cools on a counter. That’s radiation, and it accounts for the largest share of heat loss under normal conditions. But three other pathways speed the process. Conduction transfers heat directly into anything cold that touches your skin, like wet clothing, snow, or cold ground. Convection strips warmth away when wind or water moves across your body, which is why a 40°F day feels far colder in a strong breeze. Evaporation pulls heat from your skin whenever sweat or water dries off the surface.
When heat escapes faster than your body can produce it, your core temperature starts to fall. Your brain responds by triggering shivering, constricting blood vessels near the skin, and redirecting warm blood toward your vital organs. These defenses work well against mild cold, but they have limits. Once your body can no longer keep up with the rate of heat loss, hypothermia sets in.
The Three Stages of Hypothermia
Hypothermia is classified by how far core temperature has dropped, and each stage looks noticeably different.
Mild: 90–95°F (32–35°C)
Shivering is intense and uncontrollable. You may notice clumsiness, slurred speech, poor judgment, and difficulty with fine motor tasks like zipping a jacket. Outdoor survival instructors sometimes call these “the umbles”: you stumble, mumble, fumble, and grumble. At this stage, most people still recognize something is wrong, but their ability to help themselves is already declining.
Moderate: 82–90°F (28–32°C)
Shivering slows or stops entirely, which can trick bystanders into thinking the person is improving. In reality, the body has exhausted its ability to generate heat through muscle contractions. Confusion deepens, drowsiness takes over, and coordination deteriorates further. Heart rate and breathing slow. Some people at this stage experience “paradoxical undressing,” a disoriented behavior where they strip off their clothing because damaged nerve signals create a sudden, false sensation of burning heat.
Severe: Below 82°F (28°C)
At this point, the person may be unconscious or barely responsive. Breathing becomes shallow and difficult to detect. The heart is increasingly unstable. Below about 82°F, the risk of life-threatening heart rhythm problems rises sharply. The heart may slip into atrial fibrillation first, then progress to ventricular fibrillation, a chaotic rhythm that can’t pump blood. Without intervention, cardiac arrest follows.
Who Is Most Vulnerable
Older adults face the highest risk. Their bodies are slower to detect temperature changes, produce less heat at rest, and often carry less insulating body fat. The CDC data on hypothermia deaths in the U.S. found that 64% of victims were male and 60% were over age 65.
Infants are also highly vulnerable because of their large body surface area relative to their weight, which means they radiate heat quickly. They can’t shiver effectively and obviously can’t add layers or move to a warmer spot on their own.
Several medical conditions make hypothermia more likely even in moderately cold environments. An underactive thyroid slows metabolism and reduces heat production. Diabetes can damage the nerves that sense temperature changes. Malnutrition leaves the body with fewer calories to burn for warmth. Alcohol is a major contributor: it dilates blood vessels near the skin, which feels warm but actually accelerates core heat loss. It also impairs judgment, making people less likely to seek shelter. Some medications, including certain blood pressure drugs, can interfere with the body’s ability to regulate temperature by limiting blood vessel constriction or reducing metabolic rate.
Hypothermia doesn’t require extreme cold. Wet, windy conditions in the 40s and 50s°F can cause it, especially if you’re underdressed, exhausted, or immersed in water. Cold water conducts heat away from the body roughly 25 times faster than air at the same temperature.
How Hypothermia Is Diagnosed
A standard home or hospital oral thermometer often can’t read low enough to detect moderate or severe hypothermia. Many stop registering below about 94°F. Emergency medical teams use specialized thermometers placed in the esophagus or rectum to get an accurate core temperature reading. Even among these methods, readings can vary depending on placement, so clinicians often combine temperature data with the person’s symptoms and level of consciousness to gauge severity.
What to Do if Someone Has Hypothermia
If you suspect hypothermia, getting the person out of the cold is the first priority. Move them indoors or into a sheltered area and remove any wet clothing. Replace it with dry layers or blankets. If you can’t get indoors, insulate them from the ground with a sleeping pad, extra clothing, or anything that creates a barrier.
Warming should focus on the center of the body: the chest, neck, and groin. Apply warm, dry compresses to these areas, or use an electric blanket if one is available. If you’re using a hot water bottle or chemical heat pack, wrap it in a towel first to prevent burns on cold, numb skin. Offer warm, sweet, nonalcoholic drinks if the person is alert enough to swallow safely.
Several common instincts can actually make things worse:
- Don’t rub or massage the arms and legs. This can push cold blood from the extremities back toward the heart and lungs, stressing them further.
- Don’t use a hot bath or heating lamp. Rewarming too quickly can cause a dangerous drop in blood pressure or trigger heart rhythm problems.
- Don’t offer alcohol. Despite the warming sensation, it speeds up heat loss and impairs the rewarming process.
- Don’t offer cigarettes. Tobacco constricts blood vessels in a way that interferes with circulation needed for rewarming.
For moderate or severe hypothermia, call emergency services immediately. Gentle handling is critical because a cold heart is electrically unstable and rough movement can trigger a fatal arrhythmia.
Dangerous Complications During Rewarming
The rewarming phase carries its own risks. When cold blood from the arms and legs returns to the core, it can temporarily lower core temperature even further, a phenomenon called “afterdrop.” This is one reason why warming should focus on the torso rather than the extremities.
Heart rhythm problems are the most serious complication. When core temperature sits between about 82°F and 86°F, the risk of life-threatening arrhythmias rises significantly, especially if the person also has electrolyte imbalances from prolonged cold exposure. The heart may initially develop atrial fibrillation, an irregular but survivable rhythm, then deteriorate into ventricular fibrillation, which is a medical emergency. This is why rewarming in a hospital setting, where the heart can be continuously monitored, matters so much for anyone beyond the mild stage.
Prevention in Practical Terms
Layering clothing is more effective than a single heavy garment. A moisture-wicking base layer keeps sweat off the skin, an insulating middle layer traps warmth, and a wind- and water-resistant outer layer blocks convection and wet conditions. Cotton is a poor choice for cold environments because it holds moisture against the skin and loses its insulating ability when wet.
Covering your head, neck, and hands matters more than people sometimes assume, since these areas have high blood flow near the surface. Eating regularly provides the calories your body needs to generate heat, and staying hydrated helps maintain circulation.
For older adults living alone, keeping indoor temperatures at or above 68°F is a straightforward safeguard. Hypothermia can develop gradually indoors if heating is inadequate, and the early signs of confusion and drowsiness can prevent the person from recognizing what’s happening or calling for help.