Decompression sickness in the context of Emily Warren Roebling

Pearl hunting, also known as pearl fishing or pearling, is the activity of recovering or attempting to recover pearls from wild molluscs, usually oysters or mussels, in the sea or freshwater. Pearl hunting was prevalent in India and Japan for thousands of years. On the northern and north-western coast of Western Australia pearl diving began in the 1850s, and started in the Torres Strait Islands in the 1860s, where the term also covers diving for nacre or mother of pearl found in what were known as pearl shells.

In most cases the pearl-bearing molluscs live at depths where they are not manually accessible from the surface, and diving or the use of some form of tool is needed to reach them. Historically the molluscs were retrieved by freediving, a technique where the diver descends to the bottom, collects what they can, and surfaces on a single breath. The diving mask improved the ability of the diver to see while underwater. When the surface-supplied diving helmet became available for underwater work, it was also applied to the task of pearl hunting, and the associated activity of collecting pearl shell as a raw material for the manufacture of buttons, inlays and other decorative work. The surface supplied diving helmet greatly extended the time the diver could stay at depth, and introduced the previously unfamiliar hazards of barotrauma of ascent and decompression sickness.

In physiology, dehydration is a lack of total body water that disrupts metabolic processes. It occurs when free water loss exceeds intake, often resulting from excessive sweating, health conditions, or inadequate consumption of water. Mild dehydration can also be caused by immersion diuresis, which may increase risk of decompression sickness in divers.

Most people can tolerate a 3–4% decrease in total body water without difficulty or adverse health effects. A 5–8% decrease can cause fatigue and dizziness. Loss of over 10% of total body water can cause physical and mental deterioration, accompanied by severe thirst. Death occurs with a 15 and 25% loss of body water. Mild dehydration usually resolves with oral rehydration, but severe cases may need intravenous fluids.

An environmental suit is a suit designed specifically for a particular environment, usually one otherwise hostile to humans. An environment suit is typically a one-piece garment, and many types also feature a helmet or other covering for the head. Where the surrounding environment is especially dangerous the suit is completely sealed.

The first environmental suits were diving suits designed to protect a diver from the surrounding water (see timeline of underwater technology). Later developments were designed to protect the wearer from the cold (for example wetsuits and other ambient pressure suits) or from undersea high pressure and the resulting decompression sickness (for example atmospheric diving suits). Protecting the wearer from cold is also a feature of ski suits.

Barotrauma 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 liquid. The initial damage is usually due to over-stretching the tissues in tension or shear, either directly by an expansion of the gas in the closed space or by pressure difference hydrostatically transmitted through the tissue. Tissue rupture may be complicated by the introduction of gas into the local tissue or circulation through the initial trauma site, which can cause blockage of circulation at distant sites or interfere with the normal function of an organ by its presence. The term is usually applied when the gas volume involved already exists prior to decompression. Barotrauma can occur during both compression and decompression events.

Barotrauma generally manifests as sinus or middle ear effects, lung overpressure injuries and injuries resulting from external squeezes. Decompression sickness is indirectly caused by ambient pressure reduction, and tissue damage is caused directly and indirectly by gas bubbles. However, these bubbles form out of supersaturated solution from dissolved gases, and are not generally considered barotrauma. Decompression illness is a term that includes decompression sickness and arterial gas embolism caused by lung overexpansion barotrauma. It is also classified under the broader term of dysbarism, which covers all medical conditions resulting from changes in ambient pressure.

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 to full ocean depth, and remotely controlled or robotic machines can reduce risk to humans.

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or reducing work of breathing and allowing safer deep diving.

There are several categories of decompression equipment used to help divers decompress, which is the process required to allow ambient pressure divers to return to the surface safely after spending time underwater at higher ambient pressures.

Decompression obligation for a given dive profile must be calculated and monitored to ensure that the risk of decompression sickness is controlled. Some equipment is specifically for these functions, both during planning before the dive and during the dive. Other equipment is used to mark the underwater position of the diver, as a position reference in low visibility or currents, or to assist the diver's ascent and control the depth.

Hyperbaric medicine is medical treatment in which an increase in barometric pressure of typically air or oxygen is used. The immediate effects include reducing the size of gas emboli and raising the partial pressures of the gases present. Initial uses were in decompression sickness, and it also effective in certain cases of gas gangrene and carbon monoxide poisoning. There are potential hazards. Injury can occur at pressures as low as 2 psig (13.8 kPa) if a person is rapidly decompressed. If oxygen is used in the hyperbaric therapy, this can increase the fire hazard.

Hyperbaric oxygen therapy (HBOT), is the medical use of greater than 99% oxygen at an ambient pressure higher than atmospheric pressure, and therapeutic recompression. The equipment required consists of a pressure vessel for human occupancy (hyperbaric chamber), which may be of rigid or flexible construction, and a means of a controlled atmosphere supply. Treatment gas may be the ambient chamber gas, or delivered via a built-in breathing system. Operation is performed to a predetermined schedule by personnel who may adjust the schedule as required.

An atmospheric diving suit (ADS), atmospheric pressure diving suit or single atmosphere diving suit is a small one-person articulated submersible which resembles a suit of armour, with pressure-tight joints to allow articulation while maintaining a constant internal volume and an internal pressure of one atmosphere. An ADS can enable diving at depths of up to 2,300 feet (700 m) for many hours by eliminating the majority of significant physiological dangers associated with deep diving. The occupant of an ADS does not need to decompress, and there is no need for special breathing gas mixtures, so there is no danger of decompression sickness or nitrogen narcosis when the ADS is functioning properly. An ADS can permit less-skilled swimmers to complete deep dives, albeit at the expense of dexterity.

Atmospheric diving suits in current use include the Newtsuit, Exosuit, Hardsuit and the WASP, all of which are self-contained hard suits that incorporate propulsion units. The Hardsuit is constructed from cast aluminum (forged aluminum in a version constructed for the US Navy for submarine rescue); the upper torso hull is made from cast aluminum, while the bottom dome is machined aluminum. The WASP is of glass-reinforced plastic (GRP) body tube construction.

The decompression of a diver is the reduction in ambient pressure experienced during ascent from depth or depressurisation of a diving chamber. It is also the process of elimination of dissolved metabolically inert gases from the diver's body tissues which accumulated during the dive. Gas elimination also occurs during pauses in the ascent known as decompression stops, where the pressure is held constant, and after surfacing, until the gas concentrations reach equilibrium. Divers breathing gas at ambient pressure need to ascend at a rate determined by their recent exposure to pressure and the breathing gas in use. A diver who only breathes gas at atmospheric pressure when free-diving or snorkelling will not usually need to decompress. Divers using an atmospheric diving suit do not need to decompress as they are never exposed to high ambient pressure.

When an ambient pressure diver descends in the water, the hydrostatic pressure, and therefore the ambient pressure, rises. Because breathing gas is supplied at ambient pressure, some of this gas dissolves into the diver's blood and is transferred by the blood to other tissues where it may accumulate by diffusion. Inert gas such as nitrogen or helium continues to be taken up until the gas dissolved in the diver is in a state of equilibrium with the breathing gas in the diver's lungs, at which point the diver is saturated for that depth and breathing mixture, or the depth, and therefore the pressure, is changed, or the partial pressures of the gases are changed by modifying the breathing gas mixture. During ascent, the ambient pressure is reduced, and at some stage the inert gases dissolved in any given tissue will be at a higher concentration than the equilibrium state and start to diffuse out again to the blood and be eliminated in the lungs. If the pressure reduction is sufficient, excess gas may form bubbles, which may lead to decompression sickness, a possibly debilitating or life-threatening condition. It is essential that divers manage their decompression to avoid excessive bubble formation and decompression sickness. A mismanaged decompression usually results from reducing the ambient pressure too quickly for the amount of gas in solution to be eliminated safely. These bubbles may block arterial blood supply to tissues or directly cause tissue damage. If the decompression is effective, the asymptomatic venous microbubbles present after most dives are eliminated from the diver's body in the alveolar capillary beds of the lungs. If they are not given enough time, or more bubbles are created than can be eliminated safely, the bubbles grow in size and number causing the symptoms and injuries of decompression sickness. The immediate goal of controlled decompression is to avoid development of symptoms of bubble formation in the tissues of the diver, and the long-term goal is to avoid complications due to sub-clinical decompression injury.

Nitrox refers to any gas mixture composed (excepting trace gases) of nitrogen and oxygen. It is usually used for mixtures that contain less than 78% nitrogen by volume. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness (also known as the bends). The two most common recreational diving nitrox mixes are 32% and 36% oxygen, which have maximum operating depths of about 110 feet (34 meters) and 95 feet (29 meters) respectively.

Nitrox is used to a lesser extent in surface-supplied diving, as these advantages are reduced by the more complex logistical requirements for nitrox compared to the use of simple low-pressure compressors for breathing gas supply. Nitrox can also be used in hyperbaric treatment of decompression illness, usually at pressures where pure oxygen would be hazardous. Nitrox is not a safer gas than compressed air in all respects; although its use can reduce the risk of decompression sickness, it increases the risks of oxygen toxicity and fire.

Diving medicine, also called undersea and hyperbaric medicine (UHB), is the diagnosis, treatment and prevention of conditions caused by humans entering the undersea environment. It includes the effects on the body of pressure on gases, the diagnosis and treatment of conditions caused by marine hazards and how aspects of a diver's fitness to dive affect the diver's safety. Diving medical practitioners are also expected to be competent in the examination of divers and potential divers to determine fitness to dive.

Hyperbaric medicine is a corollary field associated with diving, since recompression in a hyperbaric chamber is used as a treatment for two of the most significant diving-related illnesses, decompression sickness and arterial gas embolism.

To prevent or minimize decompression sickness, divers must properly plan, conduct, and monitor decompression. Divers follow a decompression model to allow the release of excess inert gases dissolved in their body tissues at acceptable risk, which accumulated as a result of breathing at ambient pressures greater than surface atmospheric pressure. Decompression models take into account variables such as depth and time of dive, breathing gasses, altitude, and equipment to develop appropriate procedures for safe ascent.

Decompression may be continuous or staged, where the ascent is interrupted by stops at regular depth intervals, but the entire ascent is part of the decompression, and ascent rate can be critical to harmless elimination of inert gas. What is commonly known as no-decompression diving, or more accurately no-stop decompression, relies on limiting ascent rate for avoidance of excessive bubble formation. Staged decompression may include deep stops depending on the theoretical model used for calculating the ascent schedule. Omission of decompression theoretically required for a dive profile exposes the diver to significantly higher risk of symptomatic decompression sickness, and in severe cases, serious injury or death. The risk is related to the severity of exposure and the level of supersaturation of tissues in the diver. Procedures for emergency management of omitted decompression and symptomatic decompression sickness have been published. These procedures are generally effective, but vary in effectiveness from case to case.