Thermal balance of the underwater diver

Thermal balance of a diver occurs when the total heat exchanged between the diver and their surroundings results in a stable temperature of the diver. Ideally this is within the range of normal human body temperature. Thermal status of the diver is the temperature distribution and heat balance of the diver. The terms are frequently used as synonyms. Thermoregulation is the process by which an organism keeps its body temperature within specific bounds, even when the surrounding temperature is significantly different. The internal thermoregulation process is one aspect of homeostasis: a state of dynamic stability in an organism's internal conditions, maintained far from thermal equilibrium with its environment. If the body is unable to maintain a normal human body temperature and it increases significantly above normal, a condition known as hyperthermia occurs. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia. It occurs when the body loses heat faster than producing it. The core temperature of the body normally remains steady at around 36.5–37.5 °C (97.7–99.5 °F). Only a small amount of hypothermia or hyperthermia can be tolerated before the condition becomes debilitating, further deviation can be fatal. Hypothermia does not easily occur in a diver with reasonable passive thermal insulation over a moderate exposure period, even in very cold water.^[1]

Body heat is lost by respiratory heat loss, by heating and humidifying (latent heat) inspired gas, and by body surface heat loss, by radiation, conduction, and convection, to the atmosphere, water, and other substances in the immediate surroundings. Surface heat loss may be reduced by insulation of the body surface. Heat is produced internally by metabolic processes and may be supplied from external sources by active heating of the body surface or the breathing gas.^[2] Radiation heat loss is usually trivial due to small temperature differences, conduction and convection are the major components. Evaporative heat lodd is also significant to open circuit divers, not so much for rebreathers.^[1]

Heat transfer to and via gases at higher pressure than atmospheric is increased due to the higher density of the gas at higher pressure which increases its heat capacity. This effect is also modified by changes in breathing gas composition necessary for reducing narcosis and work of breathing, to limit oxygen toxicity and to accelerate decompression. Heat loss through conduction is faster for higher fractions of helium. Divers in a helium based saturation habitat will lose or gain heat fast if the gas temperature is too low or too high, both via the skin and breathing, and therefore the tolerable temperature range is smaller than for the same gas at normal atmospheric pressure.^[2] The heat loss situation is very different in the saturation living areas, which are temperature and humidity controlled, in the dry bell, and in the water.^[3]

The alveoli of the lungs are very effective at heat and humidity transfer. Inspired gas that reaches them is heated to core body temperature and humidified to saturation in the time needed for gas exchange, regardless of the initial temperature and humidity. This heat and humidity are lost to the environment in open circuit breathing systems. Breathing gas that only gets as far as the physiological dead space is not heated so effectively. When heat loss exceeds heat generation, body temperature will fall.^[2] Exertion increases heat production by metabolic processes, but when breathing gas is cold and dense, heat loss due to the increased volume of gas breathed to support these metabolic processes can result in a net loss of heat, even if the heat loss through the skin is minimised.

The thermal status of the diver has a significant influence on decompression stress and risk, and from a safety point of view this is more important than thermal comfort. Ingassing while warm is faster than when cold, as is outgassing, due to differences in perfusion in response to temperature perception, which is mostly sensed in superficial tissues. Maintaining warmth for comfort during the ingassing phase of a dive can cause relatively high tissue gas loading, and getting cold during decompression can slow the elimination of gas due to reduced perfusion of the chilled tissues, and possibly also due to the higher solubility of the gas in chilled tissues. Thermal stress also affects attention and decision making, and local chilling of the hands reduces strength and dexterity.^[4]