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Heat exchange between a person and the environment is governed by five key factors: the temperature difference between the body and surroundings, the amount of surface area in contact with the environment, the duration of exposure, the insulation or resistance provided by clothing and gear, and the rate of air movement around the person.

The greater the temperature difference, the more heat moves either into or out of the body. More exposed surface area (skin plus PPE surfaces) increases the pathways for heat transfer. Longer exposure accumulates more heat gained or lost. Strong insulation from clothing or gear slows heat transfer, while reduced insulation speeds it up. Faster air movement around the individual enhances convective heat transfer, boosting the rate of heat exchange.

These five factors collectively determine how quickly heat is gained or lost, which is why five is the correct count.