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 stabilityDynamic stability is the ability of a system to maintain operational stability after disturbance by automatic regulation 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 human body normally remains steady at around . 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.
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. Radiation heat loss is usually trivial due to small temperature differences, conduction and convection are the major components. Evaporative heat load is also significant to open circuit divers, not so much for rebreathers.
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. 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.
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. 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.
Chilling
Core temperature reduction will eventually result in hypothermia, but even lesser temperature drops will cause reduced physical and mental capacity. Peripheral chilling generally causes vasoconstriction, which slows the rate of ingassing and outgassing (washout) in those tissues. This affects the risk of decompression sickness in surface oriented diving and saturation diving excursions from storage depth with significant depth change.
Hand temperature reduction below results in loss of hand and finger strength and dexterity which can make the diver unable to perform necessary tasks, or delay getting them done, which can reduce their ability to recover from dangerous incidents, and increase risk of injury or fatality. Maintaining core temperature of the body does not preserve hand temperature or perfusion when the hands are exposed to cold water, even when passive insulation of the hands is provided. In sufficiently cold water, non-freezing cold injury is possible.
Hypothermia is reduced core body temperature that happens when a body dissipates more heat than it absorbs and produces for long enough. Clinical hypothermia occurs when the core temperature drops below , though the American Heart Association uses . Heat loss is a major limitation to swimming or diving in cold water. The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which consequently increases the risk of other injuries. Reduced capacity for rational decision making increases risk due to other hazards, and loss of strength in chilled muscles also affects the capacity to manage both routine and emergency situations. Low tissue temperatures and reduced peripheral perfusion affect inert gas solubility and the rate of ingassing and outgassing, thereby affecting decompression stress and risk. Body heat is lost much more quickly in water than in air, so water temperatures that would be quite reasonable as outdoor air temperatures can lead to severe chilling, and eventually hypothermia in inadequately protected divers, although it is not often the direct clinical cause of death.
Most divers are sufficiently protected by their diving suits that clinical hypothermia is rare for scuba divers, but it is considered a significant hazard for surface supplied saturation divers, largely due to core heat loss to the breathing gas at depths where the gas density causes high thermal capacity of the gas. Head loss to the breathing gas is slightly reduced by helium content of the breathing gas, as although it has greater thermal conductivity, its heat capacity is lower than nitrogen.
According to Pollock (2023) the unprotected adult human takes somewhere in the order of 30 minutes to become hypothermic in near freezing water. This is because core heat loss is significantly retarded by peripheral vasoconstriction. This period will be greatly extended by even a damaged diving suit, which will reduce convective heat loss, without reducing vasoconstriction. The diver will feel cold, and the peripheral tissues will chill, possibly to the extent of marked loss of strength and dexterity, but the core temperature will drop relatively slowly, and may take much longer than 30 minutes to drop below . The diver is cold and uncomfortable, but not hypothermic, and there is no immediate risk to life as long as the diver can still function competently and make good decisions.
In freediving, the physiological responses to cold are more marked, and may be seen as part of the diving reflex. breathhold after inspiration increases intrathoracic pressure. This triggers baroreceptors and cardiac stretch receptors which in turn trigger parasympathetic nervous system (PNS) responses. Breathing in preparation for apnoea causes a drop in blood carbon dioxide and lactate increase causes carotid chemoreceptors to activate the PNS and lower heart rate. Splenic contraction frees more red blood cells carrying oxygen into the circulation, and myoglobin, neuroglobin and cytoglobin oxygen stores become available.
Overheating
In humans, hyperthermia is defined as a temperature greater than , depending on the reference used, that occurs without a change in the body's temperature set point. The normal human body temperature can be as high as . Hyperthermia requires an elevation from the temperature that would otherwise be expected. Such elevations range from mild to extreme; body temperatures above can be life-threatening.
Overheating of divers is much less common than excessive heat loss, but there are situations where commercial divers must dive in relatively hot water, where the diver will be heated by the diving medium. In these cases it is necessary for the diver to be actively chilled, as insulation alone cannot reduce the temperature of the diver. Sweating is the mechanism normally used to cool the body when the temperature rises above , but it is ineffective in water, as the sweat cannot evaporate, and in a dry suit the internal gas as saturated in a very short time, after which no further evaporation can occur. A system using vests chilled with ice has been tested successfully, and tubesuits chilled with water at have been tested in ambient water temperatures of up to . Cold breathing gas can help cool the diver, and preventing the gas supply from being heated by the ambient water may be useful. The peripheral blood vessels will dilate when body temperature is high, as this promotes sweating, the normal reaction to overheating. In the absence of evaporative cooling the increased perfusion will increase heat transfer to the core. As in the case of excessive heat loss, effective insulation of the diver inside a chilled suit will minimise the cooling load. The effects of hyperthermia on diver performance and physiology are incompletely understood, particularly during exercise. Stand-by divers using dry suits should be monitored for heat stress when the temperature is above .
Thermal management The thermally neutral air temperature for an unprotected resting human is about , and the thermally neutral temperature in water is about , much closer to normal body temperature. This difference is due to the very different physical properties of the media. If the metabolic rate is raised, these ambient temperatures will become uncomfortably high.
Thermally neutral temperatures