Hypothermia is a dangerously low body temperature (below 35°C or 95°F), while hyperthermia is a dangerously high body temperature. Both are failures of the body’s ability to maintain its normal core temperature of about 37°C (98.6°F), but they move in opposite directions, produce different symptoms, and require very different emergency responses.
How the Body Regulates Temperature
Your brain’s thermoregulatory center acts like a thermostat. It monitors core body temperature and triggers responses to keep you near 37°C: dilating blood vessels and producing sweat to shed heat, or constricting blood vessels and triggering shivering to generate warmth. Hypothermia and hyperthermia both represent situations where these mechanisms are overwhelmed, but the breakdown happens differently in each case.
Hyperthermia occurs when the body’s thermostat is working correctly but simply can’t keep up. Heat builds faster than it can be lost. The set point stays normal, but the body overheats anyway, typically from environmental exposure or intense physical exertion. This is distinct from a fever, where the brain deliberately raises its temperature set point in response to infection. In a fever, the body heats itself on purpose. In hyperthermia, the heat is unwanted and uncontrolled.
Hypothermia is the mirror image: the body loses heat faster than it can produce it. Cold environments, wet clothing, wind exposure, and immersion in cold water can all strip heat away more quickly than shivering and blood vessel constriction can compensate.
Stages and Symptoms of Hypothermia
Hypothermia progresses through three stages, each more dangerous than the last. The progression can be gradual, which makes it deceptive. People in the early stages often don’t realize how much trouble they’re in.
Mild (32–35°C / 90–95°F): The body fights hard to rewarm itself. Shivering is the most obvious sign, along with pale skin, fatigue, nausea, and hunger. Heart rate, blood pressure, and breathing all increase as the body ramps up heat production. Thinking becomes fuzzy. Judgment, memory, and coordination start to decline, which is why hikers and swimmers in the early stages of hypothermia often make poor decisions that worsen their situation.
Moderate (28–32°C / 82–90°F): The body starts losing the fight. Shivering typically stops once core temperature drops to around 30–32°C because the muscles no longer have the energy to sustain it. This is a dangerous turning point: the person may feel warmer even as their condition worsens. Lethargy sets in, reflexes slow, and heart rhythm becomes irregular. A strange behavior called paradoxical undressing sometimes appears at this stage, where victims strip off their clothing. Researchers believe this happens because the blood vessels near the skin, which had been tightly constricted to conserve heat, suddenly relax, flooding the skin with warm blood and creating a false sensation of overheating. It’s one of the body’s final, misguided efforts before collapse.
Severe (below 28°C / 82°F): Blood flow to the brain drops sharply. The person becomes unresponsive. Blood pressure, heart rate, and breathing all fall to dangerously low levels. The heart may develop life-threatening rhythm problems. Without intervention, cardiorespiratory failure follows. Irreparable organ damage can occur once core temperature drops below about 32°C (90°F).
The Spectrum of Hyperthermia
Hyperthermia exists on a spectrum from uncomfortable to fatal, with heat exhaustion and heat stroke as the two most important points along it.
Heat exhaustion is the body’s response to losing too much water and salt through heavy sweating. Symptoms include headache, nausea, dizziness, weakness, irritability, heavy sweating, and decreased urine output. Core temperature rises, but the brain is still functioning normally. This is the stage where intervention is most effective: moving to a cool environment, drinking fluids, and resting can reverse it.
Heat stroke is a medical emergency. It occurs when the body’s cooling system fails entirely and temperature rises rapidly. The hallmark that separates heat stroke from heat exhaustion is neurological involvement: confusion, slurred speech, seizures, or loss of consciousness. Skin may be hot and dry (because sweating has stopped) or still profusely sweaty. Core temperatures above 41.1°C (106°F) can cause irreversible damage to organs. The transition from heat exhaustion to heat stroke can happen quickly, which is why early warning signs should never be dismissed.
Who Is Most at Risk
Very young children and older adults are vulnerable to both conditions because their bodies regulate temperature less efficiently. But beyond age, a long list of common medications can quietly impair thermoregulation and increase risk in either direction.
Beta-blockers reduce the body’s ability to widen blood vessels near the skin, making it harder to shed heat. Diuretics lower fluid volume and blunt thirst, setting the stage for dehydration in hot weather. Anticholinergic drugs, including some antihistamines like diphenhydramine, decrease sweating. Antipsychotic medications interfere with both central temperature regulation and sweating. Stimulant medications can directly raise body temperature. Even blood pressure medications like ACE inhibitors can increase fainting risk during heat exposure by lowering blood pressure and reducing thirst.
For hypothermia, alcohol is one of the most significant risk factors. It dilates blood vessels near the skin, which feels warming but actually accelerates heat loss. It also impairs judgment, making people less likely to seek shelter. Homelessness, malnutrition, and thyroid disorders all increase vulnerability as well.
How Emergency Cooling and Rewarming Work
The treatment principles are straightforward: cool the overheated person down, warm the cold person up. But the methods and speed matter enormously.
For hypothermia, rewarming falls into two categories. Passive rewarming, used for mild cases, means removing the person from the cold environment and insulating them with blankets to let the body reheat itself. Active rewarming adds external heat sources like forced warm air, heated blankets, or warmed fluids. The key danger with rewarming is doing it too aggressively. Rapid surface warming can cause cold blood from the extremities to rush back to the heart, potentially triggering dangerous heart rhythms. This is why moderate and severe hypothermia cases need careful, controlled rewarming.
For heat stroke, speed is everything. Cold water immersion is the most effective method, cooling the body at roughly 0.13°C per minute. Evaporative cooling, where water is misted on the skin while fans blow air across it, works at about 0.05°C per minute, less than half the rate. Every minute of delay increases the risk of organ damage, which is why getting the person into cold water as quickly as possible is the priority.
Mortality and Prevention
Both conditions kill. In the United States, 1,024 deaths were attributed to excessive cold or hypothermia in 2023 alone. Heat-related deaths are tracked separately and have been rising in recent years due to more frequent extreme heat events. Both numbers almost certainly undercount the true toll, since heat and cold can worsen existing heart and lung conditions without being listed as the official cause of death.
Prevention for both conditions comes down to the same core principle: respect the environment your body is in. For cold exposure, that means layering clothing, staying dry, and recognizing early signs like shivering and confusion before they progress. For heat, it means staying hydrated, taking breaks from exertion, and paying attention to headaches, dizziness, or nausea as early warning signals. If you take any medications that affect sweating, blood pressure, or fluid balance, you have a narrower margin of safety in both extremes.