Hypothermia begins when your core body temperature drops below 95°F (35°C). Normal body temperature sits around 98.6°F (37°C), so hypothermia represents a drop of roughly 3.5 degrees or more from your body’s ideal operating point. That gap might sound small, but your cells, heart, and brain depend on staying within a narrow temperature range to function properly.
The Three Stages of Hypothermia
Hypothermia isn’t a single condition. It’s classified into three stages based on how far your core temperature has fallen, and each stage looks and feels noticeably different.
Mild hypothermia (95°F to 89.6°F / 35°C to 32°C) is the earliest stage. Your body is still actively fighting the cold. You’ll shiver, sometimes intensely, and your coordination will suffer. Fine motor tasks like zipping a jacket or texting become surprisingly difficult. Heart rate may increase slightly, but blood pressure stays normal. Thinking gets slower, and you may struggle to make good decisions, which is part of what makes even mild hypothermia dangerous: you’re less capable of helping yourself right when you need to act.
Moderate hypothermia (89.6°F to 82.4°F / 32°C to 28°C) brings more obvious changes. Coordination worsens significantly, mental clarity drops, and alertness fades. Shivering may still be present but often becomes weaker or irregular. Speech slurs. Drowsiness sets in, and confusion deepens. People at this stage sometimes stop recognizing that they’re in trouble.
Severe hypothermia (below 82.4°F / below 28°C) is life-threatening. Shivering stops entirely because the body no longer has the energy or neural signaling to sustain it. Consciousness fades toward coma. The heart becomes electrically unstable, meaning dangerous rhythm problems can develop. Breathing slows. Muscles stiffen. At this point, a person may appear dead even though survival is still possible with proper medical treatment.
Why Your Body Loses the Fight
Your body runs a constant balancing act between producing heat and losing it. A set of automatic responses, including constricting blood vessels near your skin, generating heat through shivering, and raising your metabolic rate, normally keeps your core within a degree or two of 37°C. Hypothermia happens when heat escapes faster than your body can replace it.
Cold water is the fastest route. Water pulls heat from your body roughly 25 times more efficiently than air at the same temperature, which is why falling into cold water can trigger hypothermia within minutes. But prolonged exposure to cool air, especially with wind or wet clothing, can do the same thing over hours. Your body’s fuel reserves matter too. Once glycogen (stored energy in your muscles and liver) runs low from sustained shivering, heat production drops and core temperature can fall quickly.
Who Is Most Vulnerable
Certain people enter hypothermia faster or at temperatures that wouldn’t normally be dangerous. Older adults lose the ability to regulate body temperature efficiently, and they may not sense cold as sharply, meaning they can become hypothermic indoors in a poorly heated home. Infants have a high surface-area-to-body-mass ratio and limited ability to shiver, making them vulnerable even in mildly cool environments.
Several medical conditions lower the temperature at which your body can defend itself. An underactive thyroid gland reduces your baseline heat production. Low blood sugar deprives muscles of fuel for shivering. Conditions affecting the pituitary or adrenal glands can blunt the hormonal signals that drive your cold-defense responses. Brain injuries or neurological conditions can impair the thermoregulatory system directly. Alcohol is a common contributor because it widens blood vessels near the skin, accelerating heat loss, while simultaneously impairing judgment about seeking warmth.
What Happens During Rewarming
How hypothermia is treated depends entirely on the stage. For mild hypothermia, where the body is still shivering and generating its own heat, the approach is straightforward: get out of the cold, remove wet clothing, wrap up in warm blankets, and drink warm fluids. The body can largely rewarm itself at this stage.
Moderate hypothermia typically requires active external warming, such as forced warm-air blankets, because the body’s own heat production is faltering. For severe hypothermia, external warming alone isn’t enough. Core rewarming through medical interventions is necessary, especially if blood pressure is dangerously low or the heart has stopped.
Rewarming happens slowly on purpose. A safe target is raising core temperature by about 1°C per hour. Warming someone too quickly, particularly by immersing them in a hot bath or shower, can cause blood vessels near the skin to open suddenly, dropping blood pressure and potentially triggering shock. There’s also a phenomenon called “afterdrop,” where cold blood from the extremities circulates back to the core during rescue, temporarily pushing core temperature even lower. This is why people with moderate or severe hypothermia need to be moved gently and kept as horizontal as possible.
Recognizing Hypothermia Without a Thermometer
In real-world situations, you rarely have a way to measure someone’s core temperature. The Wilderness Medical Society recommends staging hypothermia based on visible signs instead. A person who is shivering and alert but clumsy is likely in the mild range. Someone who is confused, drowsy, or has slurred speech with weakening shivering has likely crossed into moderate territory. If shivering has stopped, the person is barely conscious or unresponsive, and muscles are rigid, assume severe hypothermia.
One critical point: a person with severe hypothermia can have a pulse so slow and faint that it’s nearly undetectable. If you’re checking for a pulse, feel for at least a full minute before concluding there isn’t one. Starting chest compressions on someone whose heart is still beating, even barely, can push an unstable heart into a fatal rhythm. The guiding principle in wilderness and emergency medicine is that a hypothermic person is not considered dead until they are “warm and dead,” meaning full resuscitation efforts continue until core temperature has been restored.
Indoor Hypothermia Is More Common Than You’d Think
Hypothermia doesn’t require a blizzard or a frozen lake. Indoor hypothermia accounts for a significant number of cases, particularly among elderly adults living alone in homes kept below 64°F (18°C). Air conditioning set very low, inadequate heating during winter, or sleeping in cool rooms with insufficient blankets can all push vulnerable people below the 95°F threshold over the course of hours. The onset is gradual enough that the person may not realize what’s happening, especially if cognitive changes from the cold itself impair their awareness. Checking on elderly neighbors and family members during cold snaps is one of the simplest ways to prevent hypothermia deaths that never needed to happen.