Hypothermia

Key takeaways

• Definition: body temperature below 95°F (35°C)
• Signs in adults: shivering, exhaustion, confusion, fumbling hands, memory loss, slurred speech, drowsiness
• Most useful single change: Dress in layers and cover your head, hands and feet in cold weather.
• Commonest mistake: Drinking alcohol to "warm up".
• See a doctor if any of the signs under "When to get help" below apply to you.

Why it matters

It can happen indoors as well as outdoors, and it affects thinking, so people may not realize it is happening.

The numbers, in one place

Published figures for Hypothermia

Measure	Figure
Definition	body temperature below 95°F (35°C)
Signs in adults	shivering, exhaustion, confusion, fumbling hands, memory loss, slurred speech, drowsiness
Signs in babies	bright red, cold skin and very low energy
Higher risk	older adults, babies, people who drink heavily and people outdoors for long periods
Can occur	even above 40°F (4°C) if a person is chilled by rain, sweat or cold water

What helps

• Dress in layers and cover your head, hands and feet in cold weather
• Change out of wet clothes as soon as possible
• Warm the center of the body first: chest, neck, head and groin
• Give warm, non-alcoholic drinks only if the person is fully alert

What to avoid

• Drinking alcohol to "warm up"
• Warming the arms and legs before the center of the body
• Giving drinks to someone who is unconscious

In depth

Your body produces heat constantly through metabolism and muscle activity, and loses it to the surroundings through the skin and breath. When heat loss outpaces heat production for long enough, core temperature falls. Below 95°F (35°C), the brain, heart and other organs begin to slow down. That is hypothermia. Water and wind speed up heat loss dramatically, which is why wet clothing or a fall into cold water can cause hypothermia even when the air does not feel especially cold.

In mild hypothermia the body fights back hard. Blood vessels in the skin narrow to keep warm blood in the core, and muscles shiver to generate heat. As temperature falls further, shivering weakens and may stop, thinking becomes slow and confused, and judgment fails. People may stumble, slur their words or even remove clothing, so they often cannot recognize what is happening or ask for help. This is why companions, family members and bystanders play such an important role.

Severe hypothermia affects the heart's rhythm and can lead to cardiac arrest. Yet the cold also protects the brain to some degree by reducing its need for oxygen, and people have recovered after long periods of very low body temperature. For this reason, emergency teams continue care and rewarming even when a person appears lifeless. Careful handling matters, because a very cold heart is sensitive, and rough movement can trigger dangerous rhythms.

Signs and symptoms

• Constant, uncontrollable shivering in the early stage
• Pale, cold skin, especially on the belly and chest rather than just hands and feet
• Slowed breathing and heart rate as temperature drops
• Poor coordination, stumbling or trouble with simple tasks like zipping a jacket
• Apathy, poor judgment or behavior that seems out of character
• Shivering that stops while the person is still cold, a sign of worsening hypothermia

Causes and risk factors

• Wet clothing from rain, snow, sweat or falling into water
• Wind, which strips heat from exposed skin
• Cold homes, especially when heating is turned down to save money
• Alcohol and some drugs, which widen blood vessels and dull awareness of cold
• Older age, very young age and low body fat
• Conditions such as an underactive thyroid, diabetes, stroke, dementia and malnutrition
• Exhaustion, injury or being unable to move, for example after a fall

How it is checked

Hypothermia is suspected from the setting and the person's signs: a cold environment, cold skin on the trunk, shivering or confusion. Standard home thermometers often cannot read very low temperatures accurately, so a reading may not show how cold the person really is. If someone is confused, very drowsy or not shivering despite being cold, treat it as serious and call 911.

In the emergency room, staff measure core temperature with a special low-reading thermometer, often rectally or through a probe in the esophagus or bladder. They monitor the heart rhythm and check blood tests including blood sugar, salts, kidney function and signs of infection or drug and alcohol use. They also look for injuries, frostbite and medical problems that may have led to the hypothermia.

Related topics

• CPR and Defibrillation: What the Evidence Actually Says (Emergency Care)
• Sun and Heat Abroad: What the Evidence Actually Says (Travel Health)
• Dry Skin: Sorting the Evidence From the Noise (Skin Health)
• The Evidence on Fever, Without the Hype (Immunity)

Where to go next

• Hypothermia: What Published Guidance Supports
• What Hypothermia Is, and Why It Matters
• Weather Safety: The Complete Guide
• Frostbite: What Published Guidance Supports
• Wildfire Smoke: What Published Guidance Supports

When to get help

Call 911 if a person's temperature is below 95°F or they are confused, very drowsy or unresponsive. If the person has no pulse, start CPR while waiting for help.

Where this comes from

• CDC — Preventing Hypothermia
• National Institute on Aging — Cold Weather Safety for Older Adults

How this page was put together

Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.

Medical disclaimer

This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.