Hypothermia: when your body temperature drops

Hypothermia is the dangerous lowering of a person’s core body temperature, also known as exposure. Worsening hypothermia can lead to unconsciousness and death if the person is not warmed.

Cold, wet clothes and wind chill

Cold temperatures, moisture and wind all combine to lower your body temperature. If you have wet clothes against your body, the evaporation of moisture takes heat from your body and makes you colder. Add in the effects of the wind, which increases the evaporation rate, and you will be chilled even more quickly.

Wind chill is the effect of air currents taking the warm layer of air away from your body, further reducing the "effective temperature". The stronger the wind, the greater its chilling effect, because it quickly whips away the warmth you are generating. Figures from the New Zealand National Institute of Weather and Atmospheric Research (NIWA) demonstrate this effect: an outside temperature of 8°C (46.4°F), combined with a wind speed of 72km/h (45 mph) brings the effective temperature down to -6°C (21.2°F) a reduction of 14°C (or 25.2°F).

Core body temperature, body function and consciousness

As the body becomes cold blood vessels constrict, first in the fingers and toes, then the hands, arms and legs - to keep warm blood around the vital organs like the heart and liver. If a person remains in the cold, the core body temperature eventually decreases and the body’s functions slow down, starting with the extremities and then inwards towards the vital organs. 

• The average normal body temperature is around 37°C (98.6°F), or between 36°C and 37.5°C (96.8-99.5°F) - the body functions best around this temperature.
• Hypothermia is when the body temperature drops below 35°C (95°F) - shivering that may have been present, slows and stops.
• At 32°C (89.6°F), the condition is a medical emergency and the person may fall unconscious. 
• At 31°C (87.8°F) the person is rarely conscious - the heartbeat will be slow and the breathing shallow.
• At 28°C (82.4°F) the heartbeat becomes very irregular and could stop at any time - the person may appear to be dead.
• Between 26°C and 24°C (75.2-78.8°F) the person could die at any time. 
• People have been known to survive body temperatures as low as 14.2°C (57.5°F).

Symptoms of hypothermia

Hypothermia can be hard to notice at first and often the person does not acknowledge anything is wrong. Signs of hypothermia gradually worsen and it is important to be alert to them and respond as quickly as possible. Signs of hypothermia may include:

• clumsiness and lack of dexterity (such as difficulty zipping a jacket)
• shivering 
• stumbling or lagging behind the rest of the group
• carelessness in protecting against the cold
• irrational or unusual behaviour
• vagueness or apathy towards activities or suggestions
• exhaustion
• slurred speech
• a pale appearance 
• feels cold to the touch.

What to do for hypothermia

As soon as you suspect hypothermia a speedy response is important:

• Stop and find or improvise shelter from the wind and moisture - it is important not to press on if anyone in your party has hypothermia. 
• Seek immediate assistance.
• Help the person to gradually warm up as much as possible.
• If possible, give warm, easily digested food and drink (not alcohol, strong coffee or tea).

If you are inside:

• The best way to warm a person is skin-to-skin, with warm dry layers around both people. This allows the best transfer of warmth.
• DO NOT expose them to direct heat, like a fire, as this could cause them to go into shock.
• Insulate them from the cold and warm them gradually.
• It is important NOT to rub their hands, arms, legs or feet, as this can draw essential blood away from the vital organs and actually lower the core body temperature, increasing hypothermia.

If you are outside:

• Add extra layers to the whole body, including the head, keeping the face clear.
• Ensure the person is insulated from the snow and moisture - improvise if necessary with packs, branches, sleeping bags.
• Huddle together to share warmth. 

If the person is unconscious:

• Handle them with great care and avoid any unnecessary movement.
• Even though there may be little sign of life, keep up your care of the person until medical assistance arrives.
• Lay the person down in the recovery position and insulate them as much as possible from the cold.
• Make sure the person’s airway stays open and follow, as much as possible, the tips above to gently warm them.
• Do not attempt to give them any food or drink.

If someone in your party begins to show symptoms of hypothermia a swift response is essential for their survival and, potentially, your own.

To prevent others in your party from developing hypothermia, ensure everyone is warm. Add extra layers once you have stopped.

Prevention is the best approach for hypothermia

Hypothermia is potentially fatal. Prevention is the key and many of the tips for prevention are simple and logical. Appropriate clothing for the cold is essential: 

• Wear several layers of clothing, including a warm hat, with a waterproof outer layer, to ensure that you remain warm and dry. 
• Layers create pockets of warm air around the body and can be removed or added as temperatures rise and fall. 
• Thermal undergarments made of wool or synthetic fibres are recommended as they draw any moisture away from the skin and can easily be wrung out.
• Cotton is a poor insulator and holds moisture, so should be avoided in cold, wet weather.
• Down is not advised, as it can become waterlogged and hold water against your body.

Hypothermia can result from:

• Falling into icy water, or even spending a long time in temperate waters.
• Insufficient protection against the cold, particularly combined with insufficient food intake, exhaustion, high altitude, windy or wet conditions.
• Remaining motionless in the cold, for example as a result of injury.

Good preparation prevents many instances of hypothermia:

• Eat a good meal before venturing out and take high energy snacks with you.
• Dress up warm in layers, with thermal (wool or synthetic) layers against the skin and a waterproof layer on the top. Take extra layers in case of a drop in temperature.
• Take a large plastic rubbish sack or, better still a "space blanket" (lightweight thermal plastic sheet) in your bag as an emergency layer of wind and rain protection.
• Know your limits and work within them - don’t stay out longer or press on further than your fitness levels allow.
• Let people know where you are going and for how long, and take your mobile phone with you in case of an emergency.

For more information about being prepared and keeping safe in the snow, visit the ACC’s website www.snowsmart.co.nz.

Original material provided by everybody and reviewed by Dr Milne Simpson, August 2009.