Hypothermia kills in every season, not just winter. A person who falls into a 60°F lake in summer can be hypothermic within an hour. Wet clothing in 50°F weather with wind strips heat faster than many people believe possible. The mechanism is simple: the body is losing heat faster than it is generating it, and core temperature is falling. The consequences are not simple — they are progressive cognitive impairment, cardiac dysrhythmia, and death if the process is not reversed.
The most dangerous thing about hypothermia is that it impairs the judgment needed to recognize and respond to it. People in moderate hypothermia often do not feel cold. They feel fine. They are not fine.
HOW HYPOTHERMIA DEVELOPS
Core body temperature is normally 98.6°F (37°C). Hypothermia begins when core temperature drops below 95°F (35°C). The speed of that drop depends on the heat loss mechanism:
Conduction — Direct contact with cold surfaces. Lying on cold ground, immersion in cold water. Water conducts heat away from the body 25 times faster than air at the same temperature. Wet clothing is nearly as efficient.
Convection — Moving air or water carrying heat away from the skin. Wind chill dramatically accelerates heat loss. A still day at 20°F is survivable with adequate clothing; the same temperature with a 30mph wind creates conditions that can produce hypothermia rapidly in inadequate gear.
Radiation — The body radiates heat to its environment. The head, neck, and groin radiate heat preferentially — insulating these areas is disproportionately effective.
Evaporation — Wet clothing evaporates and carries heat away. Cotton absorbs water and loses virtually all insulating value when wet. This is not a hypothetical — “cotton kills” is not a slogan, it is a documented mechanism of death.
STAGES AND SIGNS
Mild Hypothermia (Core temp 90-95°F / 32-35°C)
The body is still attempting to compensate. Signs: shivering — vigorous and uncontrollable, the body’s attempt to generate heat through muscle activity; cold, pale skin as surface vessels constrict to preserve core heat; increased heart rate and respiratory rate; impaired fine motor coordination — fumbling with zippers, buttons, or tools; mild confusion or slowed thinking; intense desire to urinate (cold diuresis — the kidneys respond to peripheral vasoconstriction by producing more urine).
Critical note: shivering is the body working. It is a sign the body has not yet lost the ability to compensate. A patient who has stopped shivering despite being cold is not warming up — they may be progressing to moderate hypothermia.
Moderate Hypothermia (Core temp 82-90°F / 28-32°C)
Compensation is failing. Signs: shivering stops or becomes weak and uncoordinated; profound confusion, slurred speech, irrational behavior — patients may paradoxically remove clothing (paradoxical undressing, caused by vasodilation as peripheral vasoconstriction fails); gross motor impairment, stumbling, inability to walk; drowsiness, difficulty staying awake; heart rate and respiratory rate slow; skin may have a bluish discoloration.
Severe Hypothermia (Core temp below 82°F / 28°C)
Cardiovascular instability and high mortality risk. Signs: unconsciousness or semiconsciousness; pulse may be difficult to detect — check carefully and for at least 60 seconds before assuming absence; breathing very slow and shallow; cardiac dysrhythmias develop, particularly ventricular fibrillation; pupils may be fixed and dilated.
The axiom in emergency medicine: a patient is not dead until they are warm and dead. Patients with severe hypothermia who appear dead have been resuscitated after prolonged cardiac arrest when core temperature was restored. Do not abandon resuscitation efforts in a severely hypothermic patient until they have been rewarmed.
FIELD MANAGEMENT
The priority is preventing further heat loss before attempting rewarming. Moving a hypothermic patient, handling them roughly, or attempting aggressive rewarming before stabilizing can trigger cardiac arrest (afterdrop — see below). Handle gently.
Step 1 — Remove from the cold environment. Get out of wind, water, rain, and off cold ground. This is the first intervention. Any shelter is better than none — a vehicle, a tent, a building, a windbreak.
Step 2 — Remove wet clothing. Wet clothing continues to extract heat. Remove it, but do it gently with the patient supine — do not have the patient stand or exert themselves. Cut clothing if necessary. Every moment in wet clothing is ongoing heat loss.
Step 3 — Insulate. Wrap the patient completely in dry insulating material — sleeping bags, blankets, clothing, emergency blankets. Focus on the head, neck, and torso — these are the priority areas. Insulate underneath the patient as well. Ground conducts heat away aggressively.
Step 4 — Gentle rewarming. For mild hypothermia in a conscious patient, passive rewarming — removing from the cold, adding insulation, allowing the body to rewarm itself — is appropriate and often sufficient if the patient is still shivering. Active rewarming (applying external heat) should be applied to the torso, armpits, and groin — not the extremities — using chemical heat packs, warm water bottles, or body heat from rescuers sharing the insulated space.
Do not apply direct heat sources (hot water, heating pads at high settings) directly to skin — a hypothermic patient’s skin is vasoconstricted and has impaired sensation, making burns likely without the patient detecting the danger.
Step 5 — Warm fluids (conscious patients only). Warm, sweet liquids — hot water with sugar or honey, warm broth — provide both heat and calories for the shivering response. Do not give alcohol — it causes peripheral vasodilation that increases heat loss. Do not give caffeine. Warm, sweet, non-alcoholic fluids only.
Step 6 — Monitor continuously. A patient recovering from mild hypothermia may deteriorate if rewarming is interrupted. A patient in moderate to severe hypothermia requires continuous monitoring of heart rate, respiratory rate, and mental status.
AFTERDROP — THE REWARMING HAZARD
Afterdrop is the phenomenon of continued core temperature decline after rewarming begins, caused by cold blood from the periphery returning to the core as peripheral circulation resumes. It can cause core temperature to drop 2-4°F after apparent stabilization, triggering cardiac dysrhythmia in patients who seemed to be recovering.
Afterdrop is the reason to rewarm the torso and core preferentially rather than the extremities — warming the limbs first drives cold peripheral blood to the core rapidly. Rewarm from the core outward. Gentle handling, minimal exertion for the patient, and core-focused warming minimize afterdrop risk.
WHAT NOT TO DO
- Do not rub or massage the extremities. This drives cold peripheral blood to the core and can trigger cardiac arrest.
- Do not give alcohol. Causes vasodilation and accelerates heat loss despite the sensation of warmth it produces.
- Do not have the patient exercise to “warm up.” Moderate to severe hypothermia patients cannot safely exert themselves. Exertion drives cold blood to the core.
- Do not apply direct heat to extremities first. Rewarm the core — torso, armpits, groin — before the extremities.
- Do not assume a pulseless hypothermic patient is dead. Check for pulse for a full 60 seconds. Begin CPR if absent. Continue until the patient is rewarmed or definitive care is reached.
PREVENTION
Hypothermia is almost entirely preventable with appropriate gear and knowledge.
Layering system: Base layer (moisture-wicking, not cotton), insulating layer (fleece or wool), shell layer (wind and waterproof). Wool and synthetic materials maintain insulating value when wet. Cotton does not.
Vapor barrier for wet conditions: In sustained wet and cold, a vapor barrier (a waterproof layer against the skin) prevents the base layer from absorbing sweat and losing insulating value. Counterintuitive but effective in sustained cold and wet.
Eat and hydrate. Shivering requires energy. A cold, underfed person becomes hypothermic faster than a well-fed one. Caloric intake and hydration are direct factors in cold tolerance.
Recognize the early signs in yourself and others. Impaired fine motor coordination is often the first observable sign. If someone cannot zip their jacket or manipulate small objects, address their cold exposure immediately.
WHAT TO STOCK
- Emergency mylar blankets (multipurpose — stock 6+ per person)
- Chemical heat packs (HeatMax, HotHands) — multiple sizes, stock generously
- Wool or fleece blanket per person
- Warm, sweet drink mix (hot chocolate, honey, electrolyte powder)
- Wool base layer and socks per person (wool maintains warmth when wet)
- A dedicated shelter option — space blanket bivy, emergency tent, or tarp setup knowledge
WHERE TO SOURCE
Emergency mylar blankets — Amazon and any outdoor retailer. Buy in bulk — $10-15 for a 10-pack. They are worth far more than they cost.
Chemical heat packs — Walmart, Amazon, and sporting goods stores. Buy the large body warmer size for serious cold. Stock 20+ per person for a winter preparedness kit.
Wool blankets — Military surplus stores (wool military blankets are inexpensive and excellent), thrift stores, and outdoor retailers. A 5-lb wool blanket at a surplus store for $15-20 is one of the best preparedness purchases available.
Cross-reference: Shock — Recognition & Response | Staying Warm Without Electricity | Grid Down — First 72 Hours