Mechanisms of Heat Loss

It is important to understand the Mechanisms of Heat Loss to better treat someone with hypothermia or to avoid it in the first place.

There are 5 main Mechanisms of Heat Loss

• Radiation (from skin)

• Convection (air/wind/water)

• Conduction (contact with ground/water/fuel)

• Evaporation (moisture from skin)

• Respirations (each breath is heated)

Radiation

Radiation is the transmission of electromagnetic energy through space from one object to another.  Your body radiates a significant amount of heat.  Some of that can be reflected back with a space blanket or snow.

Setting up fire in front of a large rock or a wall will reflect some of the radiant heat back towards the fire.  Setting up a lean to shelter behind you will also reflect heat back at you. 

Convection 

This is the movement of heat in gasses and liquids.  We also consider heat loss through wind as convectional heat loss.  Your body heats the air directly next to it.  This warmed air is quickly replaced with cold air by wind.  This hastens cooling of your skin and increases the risk of frostbite. 

Wind Chill does not lower the ambient temperature, but it does make it feel colder and again, hastens cooling. 

Conduction

Conduction is the transfer of heat from direct contact with a solid or liquid.  We lose heat from our body when we sit or lie on a cold surface or stand or float in cold water.

Heat loss can be reduced by insulating your body from contact with a cold surface, such as using insulated boots and insoles or standing/sitting/lying on a closed cell foam pad.

Being exposed to water can result in significant heat loss.  Water conducts heat about 25x faster than air, so getting wet in cold environments can be deadly. 

Contact with metal or liquid fuel in subfreezing temperatures can result in flash frostbite. 

Evaporation

When wet, your body heats the water on your skin and clothing.  Higher energy (warmer) molecules escape, leaving behind lower temperature molecule as water changes its state from a liquid to a gas.  This results in significant cooling.  This is why we sweat in hot temperatures, to help cool our bodies.  In the cold, water from sweat, rain, melted snow and mud puddles can result in constant cooling until you are dried off.

Respirations

A significant amount of heat and water is lost in cold temperatures.  

A 1990 study in the Journal of Respiration Physiology suggest that "heat loss due to respiration was between 25 and 30% of the resting metabolic and between 15 and 20% of the working metabolic rate."

Mechanisms of Heat Gain

Body's Heat Generation

• A resting human body normally produces about 100 watts of heat

• Shivering increases this number by approximately 3-5 times to 300-500W

• A severely hypothermic subject who is not shivering, only generates about 40 watts of heat or less

• Diet-induced thermogenesis

  • Eating and processing food produces a small amount of heat

  • Calories are also needed to power metabolism and shivering

• Heat output can be increased with exercises

  • May not be possible or safe with a hypothermic subject

  • Avoid exercising to the point that you begin to sweat

Other Sources of Heat Gain

• Sun

• Camp fire

• Heater

• Water bottle filled with heated water

  • Do not place bottle filled with hot water directly against the skin

  • Cover bottle with adequate layer of fabric

  • Hypothermic skin is easily burned. 

Resources:

• Wilderness Medical Society Clinical Practice Guidelines for the Out-of-Hospital Evaluation and Treatment of Accidental Hypothermia: 2019 Update

• Wilderness Medical Society Clinical Practice Guidelines for the Prevention and Treatment of Frostbite: 2019 Update

• Baby It's Cold Outside - Hypothermia Awareness and Educational Program