How hypothermia develops

Heat is produced by metabolism, mainly in the muscles and internal organs. A control centre in the brain constantly compares the temperature with its set point, much like a thermostat. When it gets too cold, it starts two defence mechanisms. First, the blood vessels of the skin narrow, especially in the hands, feet and ears. Less warm blood then reaches the cold surface and the core loses less heat – at the expense of fingers and toes.

Second, shivering starts. Muscles contract quickly and involuntarily. This produces a lot of heat but also costs a lot of energy. Someone who is exhausted, has not eaten for a long time or is very thin cannot keep this heating going for long.

If the countermeasures are not enough, the core temperature falls. Below 35 °C, medicine speaks of hypothermia. The brain is particularly sensitive: thinking, judgement and coordination decline, and later consciousness too. In severe hypothermia shivering can stop because the body can no longer find the energy for it – an alarm sign, not a sign of improvement.

Early signs are feeling cold, shivering, cold hands and declining fine motor skills: zips and knots suddenly become difficult. A well-known memory aid is the “umbles”: stumbles, mumbles, fumbles, grumbles. Anyone in a group who notices this in someone should act, even if the person says everything is fine.

Because the brain is affected, hypothermic people often misjudge their situation. They want to keep going, refuse an extra jacket or react irritably. That is why it is important to look out for each other rather than relying on self-assessment. The 35 °C threshold can hardly be measured in the field without a special thermometer – in practice, the signs are what count.

Hypothermia is not just a problem in ice and snow. Many cases happen at temperatures above freezing, when wet, wind and exhaustion come together: a rainy autumn day in the hills, a hike that takes longer than planned, or a car breakdown. Wet clothing loses much of its insulation, and wind carries heat away as well.

Cold water is particularly treacherous, because water conducts heat much better than air. Anyone who falls in cools down considerably faster than in air at the same temperature. At first, the greatest danger is often not hypothermia but cold shock with violent gasping, and later the declining strength in arms and legs.

It can also happen at home, for example when the heating fails during a winter power cut. Older people, small children, sick and exhausted people are at particular risk. Children have a lot of body surface relative to their weight and therefore cool down faster. Alcohol increases the risk: it widens the skin's blood vessels so that more heat is lost, and it clouds judgement.

This entry explains how hypothermia develops and is not a treatment guide. With confusion, marked drowsiness, reduced consciousness, or if shivering stops while the person is still cold, call the local emergency number (112 in the EU). Until help arrives: get out of wind and wet, insulate from the ground, cover the head, handle the person gently and do not move them unnecessarily.

Do not give anything to drink or eat to people with reduced consciousness, because they may choke. If, on the other hand, someone is awake and can swallow safely, warm sweet drinks and food help the body to produce heat itself. Further first-aid steps are in the first-aid entry on hypothermia.

Most cases can be prevented by interrupting the heat-loss paths early. Staying dry is the most important part: put on rain protection in good time and take off a layer when climbing before you sweat through. A dry spare layer packed waterproof in the backpack is a small insurance policy.

Eating and drinking regularly keeps the internal heating running. Breaks in the lee of the wind and a pad against the cold ground reduce heat loss. And anyone who notices that a trip is taking longer than planned or the weather is turning is better off turning back early than getting caught in darkness and wet.