During firefighting, firefighters experience a great deal of heat stress at the fire site [118-121]. The physical activities firefighters perform require large amounts of energy, and this energy is mainly produced through oxidation (cell respiration process) of the macronutrients (eg, carbohydrate, protein, fat, water) stored in their bodies. In the process of energy production through oxidation, about 80% of the stored potential energy in the macronutrients converts into metabolic heat energy (Eq. 3.4). This metabolic heat increases firefighters’ normal core body temperature (37°C) if the heat dissipation to the environment is too low . If the core temperature variation of 0.1-0.5°C is sensed by firefighters’ central and/or peripheral nervous receptors, these receptors send the information to the hypothalamus, which will induce thermoregulatory measures to fight the heat . This condition results in a vasodilatation process, which increases the blood flow and transfers the body’s metabolic heat towards the skin. There are various models which demonstrate the metabolic heat transfer mechanism through a human body. These models are (1) the bio-heat transfer model, (2) the Chen and Homes model, and (3) the Weinbaum, Jiji, and Lemos model.