Underwater diving
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For the Olympic sport, see Diving (sport). For other uses, see Diving.
Surface-supplied divers riding a stage to the underwater workplace
Underwater diving, as a human activity, is the practice of descending below the water’s surface to interact with the environment. It is also often referred to as diving, an ambiguous term with several possible meanings, depending on context. Immersion in water and exposure to high ambient pressure have physiological effects that limit the depths and duration possible in ambient pressure diving. Humans are not physiologically and anatomically well-adapted to the environmental conditions of diving, and various equipment has been developed to extend the depth and duration of human dives, and allow different types of work to be done.
In ambient pressure diving, the diver is directly exposed to the pressure of the surrounding water. The ambient pressure diver may dive on breath-hold (freediving) or use breathing apparatus for scuba diving or surface-supplied diving, and the saturation diving technique reduces the risk of decompression sickness (DCS) after long-duration deep dives. Atmospheric diving suits (ADS) may be used to isolate the diver from high ambient pressure. Crewed submersibles can extend depth range, and remotely controlled or robotic machines can reduce risk to humans.
The environment exposes the diver to a wide range of hazards, and though the risks are largely controlled by appropriate diving skills, training, types of equipment and breathing gases used depending on the mode, depth and purpose of diving, it remains a relatively dangerous activity. Professional diving is usually regulated by occupational health and safety legislation, while recreational diving may be entirely unregulated. Diving activities are restricted to maximum depths of about 40 metres (130 ft) for recreational scuba diving, 530 metres (1,740 ft) for commercial saturation diving, and 610 metres (2,000 ft) wearing atmospheric suits. Diving is also restricted to conditions which are not excessively hazardous, though the level of risk acceptable can vary, and fatal incidents may occur.
Recreational diving (sometimes called sport diving or subaquatics) is a popular leisure activity. Technical diving is a form of recreational diving under more challenging conditions. Professional diving (commercial diving, diving for research purposes, or for financial gain) involves working underwater. Public safety diving is the underwater work done by law enforcement, fire rescue, and underwater search and recovery dive teams. Military diving includes combat diving, clearance diving and ships husbandry. Deep sea diving is underwater diving, usually with surface-supplied equipment, and often refers to the use of standard diving dress with the traditional copper helmet. Hard hat diving is any form of diving with a helmet, including the standard copper helmet, and other forms of free-flow and lightweight demand helmets. The history of breath-hold diving goes back at least to classical times, and there is evidence of prehistoric hunting and gathering of seafoods that may have involved underwater swimming. Technical advances allowing the provision of breathing gas to a diver underwater at ambient pressure are recent, and self-contained breathing systems developed at an accelerated rate following the Second World War.
Physiological constraints on diving[edit]
Main article: Human physiology of underwater diving
Immersion in water and exposure to cold water and high pressure have physiological effects on the diver which limit the depths and duration possible in ambient pressure diving. Breath-hold endurance is a severe limitation, and breathing at high ambient pressure adds further complications, both directly and indirectly. Technological solutions have been developed which can greatly extend depth and duration of human ambient pressure dives, and allow useful work to be done underwater.[1]
Immersion[edit]
Main article: Diving reflex
Immersion of the human body in water affects the circulation, renal system, fluid balance, and breathing, because the external hydrostatic pressure of the water provides support against the internal hydrostatic pressure of the blood. This causes a blood shift from the extravascular tissues of the limbs into the chest cavity,[2] and fluid losses known as immersion diuresis compensate for the blood shift in hydrated subjects soon after immersion.[3][2] Hydrostatic pressure on the body from head-out immersion causes negative pressure breathing which contributes to the blood shift.[3]
The blood shift causes an increased respiratory and cardiac workload. Stroke volume is not greatly affected by immersion or variation in ambient pressure, but slowed heartbeat reduces the overall cardiac output, particularly because of the diving reflex in breath-hold diving.[2] Lung volume decreases in the upright position, owing to cranial displacement of the abdomen from hydrostatic pressure, and resistance to air flow in the airways increases because of the decrease in lung volume.[3] There appears to be a connection between pulmonary edema and increased pulmonary blood flow and pressure, which results in capillary engorgement. This may occur during higher intensity exercise while immersed or submerged.[2]
The diving reflex is a response to immersion that overrides the basic homeostatic reflexes.[4][5] It optimises respiration by preferentially distributing oxygen stores to the heart and brain, which allows extended periods underwater. It is exhibited strongly in aquatic mammals (seals,[6] otters, dolphins and muskrats),[7] and also exists in other mammals, including humans. Diving birds, such as penguins, have a similar diving reflex.[4] The diving reflex is triggered by chilling the face and holding the breath.[4][8] The cardiovascular system constricts peripheral blood vessels, slows the pulse rate, redirects blood to the vital organs to conserve oxygen, releases red blood cells stored in the spleen, and, in humans, causes heart rhythm irregularities.[4] Aquatic mammals have evolved physiological adaptations to conserve oxygen during submersion, but apnea, slowed pulse rate, and vasoconstriction are shared with terrestrial mammals.[5]
Exposure[edit]
Cold shock response is the physiological response of organisms to sudden cold, especially cold water, and is a common cause of death from immersion in very cold water,[9] such as by falling through thin ice. The immediate shock of the cold causes involuntary inhalation, which if underwater can result in drowning. The cold water can also cause heart attack due to vasoconstriction;[10] the heart has to work harder to pump the same volume of blood throughout the body, and for people with heart disease, this additional workload can cause the heart to go into arrest. A person who survives the initial minute after falling into cold water can survive for at least thirty minutes provided they do not drown. The ability to stay afloat declines substantially after about ten minutes as the chilled muscles lose strength and co-ordination.[9]
Hypothermia is reduced core body temperature that occurs when a body loses more heat than it generates.[11] It is a major limitation to swimming or diving in cold water.[12] The reduction in finger dexterity due to pain or numbness decreases general safety and work capacity, which in turn increases the risk of other injuries.[12][13] Non-freezing cold injury can affect the extremities in cold water diving, and frostbite can occur when air temperatures are low enough to cause tissue freezing. Body heat is lost much more quickly in water than in air, so water temperatures that would be tolerable as outdoor air temperatures can lead to hypothermia, which may lead to death from other causes in inadequately protected divers.[12]
Breath-hold limitations[edit]
Breath-hold diving by an air-breathing animal is limited to the physiological capacity to perform the dive on the oxygen available until it returns to a source of fresh breathing gas, usually the air at the surface. As this internal oxygen supply reduces, the animal experiences an increasing urge to breathe caused by buildup of carbon dioxide and lactate in the blood,[14] followed by loss of consciousness due to cerebral hypoxia. If this occurs underwater, it will drown.[15]
Blackouts in freediving can occur when the breath is held long enough for metabolic activity to reduce the oxygen partial pressure sufficiently to cause loss of consciousness. This is accelerated by exertion, which uses oxygen faster, and can be exacerbated by hyperventilation directly before the dive, which reduces the carbon dioxide level in the blood. Lower carbon dioxide levels increase the oxygen-haemoglobin affinity, reducing availability of oxygen to brain tissue towards the end of the dive (Bohr effect); they also suppress the urge to breathe, making it easier to hold the breath to the point of blackout. This can happen at any depth.[16][17]
Ascent-induced hypoxia is caused by a drop in oxygen partial pressure as ambient pressure is reduced. The partial pressure of oxygen at depth may be sufficient to maintain consciousness at that depth and not at the reduced pressures nearer the surface.[15][17][18]
Ambient pressure changes[edit]
Mild barotrauma to a diver caused by mask squeeze
Barotrauma, an type of dysbarism, is physical damage to body tissues caused by a difference in pressure between a gas space inside, or in contact with the body, and the surrounding gas or fluid.[19] It typically occurs when the organism is exposed to a large change in ambient pressure, such as when a diver ascends or descends. When diving, the pressure differences which cause the barotrauma are changes in hydrostatic pressure.[20]
The initial damage is usually due to over-stretching the tissues in tension or shear, either directly by expansion of the gas in the closed space, or by pressure difference hydrostatically transmitted through the tissue.[19]
Barotrauma generally manifests as sinus or middle ear effects, decompression sickness, lung over-expansion injuries, and injuries resulting from external squeezes.[19] Barotraumas of descent are caused by preventing the free change of volume of the gas in a closed space in contact with the diver, resulting in a pressure difference between the tissues and the gas space, and the unbalanced force due to this pressure difference causes deformation of the tissues resulting in cell rupture.[19] Barotraumas of ascent are also caused when the free change of volume of the gas in a closed space in contact with the diver is prevented. In this case the pressure difference causes a resultant tension in the surrounding tissues which exceeds their tensile strength. Besides tissue rupture, the overpressure may cause ingress of gases into the adjoining tissues and further afield by bubble transport through the circulatory system. This can cause blockage of circulation at distant sites, or interfere with the normal function of an organ by its presence.[19]
Breathing under pressure[edit]
See also: Physiology of decompression and Work of breathing
Provision of breathing gas at ambient pressure can greatly prolong the duration of a dive, but there are other problems that may result from this technological solution. Absorption of metabolically inert gases is increased as a function of time and pressure, and these may both produce undesirable effects immediately, as a consequence of their presence in the tissues in the dissolved state, such as nitrogen narcosis and high pressure nervous syndrome,[21][22] or cause problems when coming out of solution within the tissues during decompression.[23]
Other problems arise when the concentration of metabolically active gases is increased. These range from the toxic effects of oxygen at high partial pressure,[24] through buildup of carbon dioxide due to excessive work of breathing, increased dead space,[25] or inefficient removal, to the exacerbation of the toxic effects of contaminants in the breathing gas due to the increased concentration at high pressures.[26] Hydrostatic pressure differences between the interior of the lung and the breathing gas delivery, increased breathing gas density due to ambient pressure, and increased flow resistance due to higher breathing rates may all cause increased work of breathing, fatigue of the respiratory muscles, and a physiological limit to effective ventilation.[2][27]
Sensory impairment[edit]
Views through a flat mask, above and below water
Underwater vision is affected by the clarity and the refractive index of the medium. Visibility underwater is reduced because light passing through water attenuates rapidly with distance, leading to lower levels of natural illumination. Underwater objects are also blurred by scattering of light between the object and the viewer, resulting in lower contrast. These effects vary with the wavelength of the light, and the colour and turbidity of the water. The human eye is optimised for air vision, and when it is immersed in direct contact with water, visual acuity is adversely affected by the difference in refractive index between water and air. Provision of an airspace between the cornea and the water can compensate, but causes scale and distance distortion. Artificial illumination can improve visibility at short range.[28] Stereoscopic acuity, the ability to judge relative distances of different objects, is considerably reduced underwater, and this is affected by the field of vision. A narrow field of vision caused by a small viewport in a helmet results in greatly reduced stereoacuity,[28] and an apparent movement of a stationary object when the head is moved.[29] These effects lead to poorer hand-eye coordination.[28]
Water has different acoustic properties from those of air. Sound from an underwater source can propagate relatively freely through body tissues where there is contact with the water as the acoustic properties are similar. When the head is exposed to the water, some sound is transmitted by the eardrum and middle ear, but a significant part reaches the cochlea independently, by bone conduction.[30][31] Some sound localisation is possible, though difficult.[30] Human hearing underwater, in cases where the diver’s ear is wet, is less sensitive than in air.[30] Frequency sensitivity underwater also differs from that in air, with a consistently higher threshold of hearing underwater; sensitivity to higher frequency sounds is reduced the most.[30] The type of headgear affects noise sensitivity and noise hazard depending on whether transmission is wet or dry.[30] Human hearing underwater is less sensitive with wet ears than in air, and a neoprene hood causes substantial attenuation. When wearing a helmet, hearing sensitivity is similar to that in surface air, as it is not greatly affected by the breathing gas or chamber atmosphere composition or pressure.[30] Because sound travels faster in heliox than in air, voice formants are raised, making divers’ speech high-pitched and distorted, and hard to understand for people not used to it.[32] The increased density of breathing gases under pressure has a similar and additive effect.[33]
Tactile sensory perception in divers may be impaired by the environmental protection suit and low temperatures. The combination of instability, equipment, neutral buoyancy and resistance to movement by the inertial and viscous effects of the water encumbers the diver. Cold causes losses in sensory and motor function and distracts from and disrupts cognitive activity. The ability to exert large and precise force is reduced.[34]
Balance and equilibrium depend on vestibular function and secondary input from visual, organic, cutaneous, kinesthetic and sometimes auditory senses which are processed by the central nervous system to provide the sense of balance. Underwater, some of these inputs may be absent or diminished, making the remaining cues more important. Conflicting input may result in vertigo, disorientation and motion sickness. The vestibular sense is essential in these conditions for rapid, intricate and accurate movement.[34] Proprioceptive perception makes the diver aware of personal position and movement, in association with the vestibular and visual input, and allows the diver to function effectively in maintaining physical equilibrium and balance in the water.[34] In the water at neutral buoyancy, the proprioceptive cues of position are reduced or absent. This effect may be exacerbated by the diver’s suit and other equipment.[34]
Taste and smell are not very important to the diver in the water but more important to the saturation diver while in accommodation chambers. There is evidence of a slight decrease in threshold for taste and smell after extended periods under pressure.[34]
Diving modes[edit]
There are several modes of diving distinguished largely by the breathing gas supply system used, and whether the diver is exposed to the ambient pressure. The diving equipment and support equipment are largely determined by the mode.
Freediving[edit]
Main article: Freediving
Recreational breath-hold divers in basic equipment with floats and catch bags suitable for collecting lobster or shellfish
The ability to dive and swim underwater while holding one’s breath is considered a useful emergency skill, an important part of water sport and Navy safety training, and an enjoyable leisure activity.[35] Underwater diving without breathing apparatus can be categorised as underwater swimming, snorkelling and freediving. These categories overlap considerably. Several competitive underwater sports are practised without breathing apparatus.[36][37][38][39][40]
Freediving precludes the use of external breathing devices, and relies on the ability of divers to hold their breath until resurfacing. The technique ranges from simple breath-hold diving to competitive apnea dives. Fins and a diving mask are often used in free diving to improve vision and provide more efficient propulsion. A short breathing tube called a snorkel allows the diver to breathe at the surface while the face is immersed. Snorkelling on the surface with no intention of diving is a popular water sport and recreational activity.[35][41]
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