Scuba diving

Breathing apparatus

Recreational diver putting on his scuba set before diving.

The defining equipment used by a scuba diver is the eponymous scuba, the self-contained underwater breathing apparatus which allows the diver to breathe while diving, and is transported by the diver.

As one descends, in addition to the normal atmospheric pressure at the surface, the water exerts increasing hydrostatic pressure of approximately 1 bar (14.7 pounds per square inch) for every 10 m (33 feet) of depth. The pressure of the inhaled breath must balance the surrounding or ambient pressure to allow inflation of the lungs. It becomes virtually impossible to breathe air at normal atmospheric pressure through a tube below three feet under the water.^[2]

Most recreational scuba diving is done using a half mask which covers the diver's eyes and nose, and a mouthpiece to supply the breathing gas from the demand valve or rebreather. Inhaling from a regulator's mouthpiece becomes second nature very quickly. The other common arrangement is a full face mask which covers the eyes, nose and mouth, and often allows the diver to breathe through the nose. Professional scuba divers are more likely to use full face masks, which protect the diver's airway if the diver loses consciousness.^[43]

Open-circuit

Main article: Diving regulator

Aqualung Legend second stage (demand valve) regulator

Aqualung 1st stage regulator

Gekko dive computer with attached pressure gauge and compass

Suunto submersible pressure gauge display

Open circuit scuba has no provision for using the breathing gas more than once for respiration.^[1] The gas inhaled from the scuba equipment is exhaled to the environment, or occasionally into another item of equipment for a special purpose, usually to increase buoyancy of a lifting device such as a buoyancy compensator, inflatable surface marker buoy or small lifting bag. The breathing gas is generally provided from a high-pressure diving cylinder through a scuba regulator. By always providing the appropriate breathing gas at ambient pressure, demand valve regulators ensure the diver can inhale and exhale naturally and without excessive effort, regardless of depth, as and when needed.^[23]

The most commonly used scuba set uses a "single-hose" open circuit 2-stage demand regulator, connected to a single back-mounted high-pressure gas cylinder, with the first stage connected to the cylinder valve and the second stage at the mouthpiece.^[1] This arrangement differs from Émile Gagnan's and Jacques Cousteau's original 1942 "twin-hose" design, known as the Aqua-lung, in which the cylinder pressure was reduced to ambient pressure in one or two stages which were all in the housing mounted to the cylinder valve or manifold.^[23] The "single-hose" system has significant advantages over the original system for most applications.^[44]

In the "single-hose" two-stage design, the first stage regulator reduces the cylinder pressure of up to about 300 bars (4,400 psi) to an intermediate pressure (IP) of about 8 to 10 bars (120 to 150 psi) above ambient pressure. The second stage demand valve regulator, supplied by a low-pressure hose from the first stage, delivers the breathing gas at ambient pressure to the diver's mouth. The exhaled gases are exhausted directly to the environment as waste through a non-return valve on the second stage housing. The first stage typically has at least one outlet port delivering gas at full tank pressure which is connected to the diver's submersible pressure gauge or dive computer, to show how much breathing gas remains in the cylinder.^[44]

Rebreather

An Inspiration electronic fully closed circuit rebreather

Main article: Rebreather

Less common are closed circuit (CCR) and semi-closed (SCR) rebreathers which, unlike open-circuit sets that vent off all exhaled gases, process all or part of each exhaled breath for re-use by removing the carbon dioxide and replacing the oxygen used by the diver.^[45] Rebreathers release few or no gas bubbles into the water, and use much less stored gas volume, for an equivalent depth and time because exhaled oxygen is recovered; this has advantages for research, military,^[1] photography, and other applications. Rebreathers are more complex and more expensive than open-circuit scuba, and special training and correct maintenance are required for them to be safely used, due to the larger variety of potential failure modes.^[45]

In a closed-circuit rebreather the oxygen partial pressure in the rebreather is controlled, so it can be maintained at a safe continuous maximum, which reduces the inert gas (nitrogen and/or helium) partial pressure in the breathing loop. Minimising the inert gas loading of the diver's tissues for a given dive profile reduces the decompression obligation. This requires continuous monitoring of actual partial pressures with time and for maximum effectiveness requires real-time computer processing by the diver's decompression computer. Decompression can be much reduced compared to fixed ratio gas mixes used in other scuba systems and, as a result, divers can stay down longer or require less time to decompress. A semi-closed circuit rebreather injects a constant mass flow of a fixed breathing gas mixture into the breathing loop, or replaces a specific percentage of the respired volume, so the partial pressure of oxygen at any time during the dive depends on the diver's oxygen consumption and/or breathing rate. Planning decompression requirements requires a more conservative approach for a SCR than for a CCR, but decompression computers with a real time oxygen partial pressure input can optimise decompression for these systems. Because rebreathers produce very few bubbles, they do not disturb marine life or make a diver's presence known at the surface; this is useful for underwater photography, and for covert work.^[36]

Gas mixtures

A cylinder decal to indicate that the contents are a Nitrox mixture

Nitrox cylinder marked up for use showing maximum safe operating depth (MOD)

Main article: Breathing gas

For some diving, gas mixtures other than normal atmospheric air (21% oxygen, 78% nitrogen, 1% trace gases) can be used,^[1][2] so long as the diver is competent in their use. The most commonly used mixture is nitrox, also referred to as Enriched Air Nitrox (EAN), which is air with extra oxygen, often with 32% or 36% oxygen, and thus less nitrogen, reducing the risk of decompression sickness or allowing longer exposure to the same pressure for equal risk. The reduced nitrogen may also allow for no stops or shorter decompression stop times or a shorter surface interval between dives. A common misconception is that nitrox can reduce narcosis, but research has shown that oxygen is also narcotic.^[46][2]:304

The increased partial pressure of oxygen due to the higher oxygen content of nitrox increases the risk of oxygen toxicity, which becomes unacceptable below the maximum operating depth of the mixture. To displace nitrogen without the increased oxygen concentration, other diluent gases can be used, usually helium, when the resultant three gas mixture is called trimix, and when the nitrogen is fully substituted by helium, heliox.^[3]

For dives requiring long decompression stops, divers may carry cylinders containing different gas mixtures for the various phases of the dive, typically designated as Travel, Bottom, and Decompression gases. These different gas mixtures may be used to extend bottom time, reduce inert gas narcotic effects, and reduce decompression times.^[47]

Diver mobility

To take advantage of the freedom of movement afforded by scuba equipment, the diver needs to be mobile underwater. Personal mobility is enhanced by swimfins and optionally diver propulsion vehicles. Fins have a large blade area and use the more powerful leg muscles, so are much more efficient for propulsion and manoeuvering thrust than arm and hand movements, but require skill to provide fine control. Several types of fin are available, some of which may be more suited for manoeuvering, alternative kick styles, speed, endurance, reduced effort or ruggedness.^[3] Streamlining dive gear will reduce drag and improve mobility. Balanced trim which allows the diver to align in any desired direction also improves streamlining by presenting the smallest section area to the direction of movement and allowing propulsion thrust to be used more efficiently.^[48]

Occasionally a diver may be towed using a "sled", an unpowered device towed behind a surface vessel which conserves the diver's energy and allows more distance to be covered for a given air consumption and bottom time. The depth is usually controlled by the diver by using diving planes or by tilting the whole sled.^[49] Some sleds are faired to reduce drag on the diver.^[50]

Buoyancy control and trim

Diver under the Salt Pier in Bonaire.

To dive safely, divers must control their rate of descent and ascent in the water^[2] and be able to maintain a constant depth in midwater.^[51] Ignoring other forces such as water currents and swimming, the diver's overall buoyancy determines whether they ascend or descend. Equipment such as diving weighting systems, diving suits (wet, dry or semi-dry suits are used depending on the water temperature) and buoyancy compensators can be used to adjust the overall buoyancy.^[1] When divers want to remain at constant depth, they try to achieve neutral buoyancy. This minimises the effort of swimming to maintain depth and therefore reduces gas consumption.^[51]

The buoyancy force on the diver is the weight of the volume of the liquid that they and their equipment displace minus the weight of the diver and their equipment; if the result is positive, that force is upwards. The buoyancy of any object immersed in water is also affected by the density of the water. The density of fresh water is about 3% less than that of ocean water.^[52] Therefore, divers who are neutrally buoyant at one dive destination (e.g. a fresh water lake) will predictably be positively or negatively buoyant when using the same equipment at destinations with different water densities (e.g. a tropical coral reef).^[51] The removal ("ditching" or "shedding") of diver weighting systems can be used to reduce the diver's weight and cause a buoyant ascent in an emergency.^[51]

Diving suits made of compressible materials decrease in volume as the diver descends, and expand again as the diver ascends, causing buoyancy changes. Diving in different environments also necessitates adjustments in the amount of weight carried to achieve neutral buoyancy. The diver can inject air into dry suits to counteract the compression effect and squeeze. Buoyancy compensators allow easy and fine adjustments in the diver's overall volume and therefore buoyancy.^[51]

Neutral buoyancy in a diver is an unstable state. It is changed by small differences in ambient pressure caused by a change in depth, and the change has a positive feedback effect. A small descent will increase the pressure, which will compress the gas filled spaces and reduce the total volume of diver and equipment. This will further reduce the buoyancy, and unless counteracted, will result in sinking more rapidly. The equivalent effect applies to a small ascent, which will trigger an increased buoyancy and will result in accelerated ascent unless counteracted. The diver must continuously adjust buoyancy or depth in order to remain neutral. Fine control of buoyancy can be achieved by controlling the average lung volume in open circuit scuba, but this feature is not available to the closed circuit rebreather diver, as exhaled gas remains in the breathing loop. This is a skill which improves with practice until it becomes second nature.^[51]

Buoyancy changes with depth variation are proportional to the compressible part of the volume of the diver and equipment, and to the proportional change in pressure, which is greater per unit of depth near the surface. Minimising the volume of gas required in the buoyancy compensator will minimise the buoyancy fluctuations with changes in depth. This can be achieved by accurate selection of ballast weight, which should be the minimum to allow neutral buoyancy with depleted gas supplies at the end of the dive unless there is an operational requirement for greater negative buoyancy during the dive.^[35] Buoyancy and trim can significantly affect drag of a diver. The effect of swimming with a head up angle of about 15°, as is quite common in poorly trimmed divers, can be an increase in drag in the order of 50%.^[48]

The ability to ascend at a controlled rate and remain at a constant depth is important for correct decompression. Recreational divers who do not incur decompression obligations can get away with imperfect buoyancy control, but when long decompression stops at specific depths are required, the risk of decompression sickness is increased by depth variations while at a stop. Decompression stops are typically done when the breathing gas in the cylinders has been largely used up, and the reduction in weight of the cylinders increases the buoyancy of the diver. Enough weight must be carried to allow the diver to decompress at the end of the dive with nearly empty cylinders.^[35]

Underwater vision

A diver wearing an Ocean Reef full face mask

Water has a higher refractive index than air – similar to that of the cornea of the eye. Light entering the cornea from water is hardly refracted at all, leaving only the eye's crystalline lens to focus light. This leads to very severe hypermetropia. People with severe myopia, therefore, can see better underwater without a mask than normal-sighted people.^[53] Diving masks and helmets solve this problem by providing an air space in front of the diver's eyes.^[1] The refraction error created by the water is mostly corrected as the light travels from water to air through a flat lens, except that objects appear approximately 34% bigger and 25% closer in water than they actually are. The faceplate of the mask is supported by a frame and skirt, which are opaque or translucent, therefore total field-of-view is significantly reduced and eye–hand coordination must be adjusted.^[53]

Divers who need corrective lenses to see clearly outside the water would normally need the same prescription while wearing a mask. Generic corrective lenses are available off the shelf for some two-window masks, and custom lenses can be bonded onto masks that have a single front window or two windows.^[54]

As a diver descends, they must periodically exhale through their nose to equalise the internal pressure of the mask with that of the surrounding water. Swimming goggles are not suitable for diving because they only cover the eyes and thus do not allow for equalisation. Failure to equalise the pressure inside the mask may lead to a form of barotrauma known as mask squeeze.^[1][3]

Masks tend to fog when warm humid exhaled air condenses on the cold inside of the faceplate. To prevent fogging many divers spit into the dry mask before use, spread the saliva around the inside of the glass and rinse it out with a little water. The saliva residue allows condensation to wet the glass and form a continuous film, rather than tiny droplets. There are several commercial products that can be used as an alternative to saliva, some of which are more effective and last longer, but there is a risk of getting the anti-fog agent in the eyes.^[55]

Environmental protection

"Shorty" style wetsuit

Scientific divers wearing dry suits

Protection from heat loss in cold water is usually provided by wetsuits or dry suits. These also provide protection from sunburn, abrasion and stings from some marine organisms. Where thermal insulation is not important, lycra suits/diving skins may be sufficient.^[58]

A wetsuit is a garment, usually made of foamed neoprene, which provides thermal insulation, abrasion resistance and buoyancy. The insulation properties depend on bubbles of gas enclosed within the material, which reduce its ability to conduct heat. The bubbles also give the wetsuit a low density, providing buoyancy in water. Suits range from a thin (2 mm or less) "shortie", covering just the torso, to a full 8 mm semi-dry, usually complemented by neoprene boots, gloves and hood. A good close fit and few zips help the suit to remain waterproof and reduce flushing – the replacement of water trapped between suit and body by cold water from the outside. Improved seals at the neck, wrists and ankles and baffles under the entry zip produce a suit known as "semi-dry".^[59][58]

A dry suit also provides thermal insulation to the wearer while immersed in water,^{[60][61][62][63]} and normally protects the whole body except the head, hands, and sometimes the feet. In some configurations, these are also covered. Dry suits are usually used where the water temperature is below 15 °C (60 °F) or for extended immersion in water above 15 °C (60 °F), where a wetsuit user would get cold, and with an integral helmet, boots, and gloves for personal protection when diving in contaminated water.^[64] Dry suits are designed to prevent water from entering. This generally allows better insulation making them more suitable for use in cold water. They can be uncomfortably hot in warm or hot air, and are typically more expensive and more complex to don. For divers, they add some degree of complexity as the suit must be inflated and deflated with changes in depth in order to avoid "squeeze" on descent or uncontrolled rapid ascent due to over-buoyancy.^[64]

Monitoring and navigation

A scuba dive computer

Unless the maximum depth of the water is known, and is quite shallow, a diver must monitor the depth and duration of a dive to avoid decompression sickness. Traditionally this was done by using a depth gauge and a diving watch, but electronic dive computers are now in general use, as they are programmed to do real-time modelling of decompression requirements for the dive, and automatically allow for surface interval. Many can be set for the gas mixture to be used on the dive, and some can accept changes in the gas mix during the dive. Most dive computers provide a fairly conservative decompression model, and the level of conservatism may be selected by the user within limits. Most decompression computers can also be set for altitude compensation to some degree.^[35]

If the dive site and dive plan require the diver to navigate, a compass may be carried, and where retracing a route is critical, as in cave or wreck penetrations, a guide line is laid from a dive reel. In less critical conditions, many divers simply navigate by landmarks and memory, a procedure also known as pilotage or natural navigation. A scuba diver should always be aware of the remaining breathing gas supply, and the duration of diving time that this will safely support, taking into account the time required to surface safely and an allowance for foreseeable contingencies. This is usually monitored by using a submersible pressure gauge on each cylinder.^[65]

Safety equipment

Cutting tools such as knives, line cutters or shears are often carried by divers to cut loose from entanglement in nets or lines. A surface marker buoy on a line held by the diver indicates the position of the diver to the surface personnel. This may be an inflatable marker deployed by the diver at the end of the dive, or a sealed float, towed for the whole dive. A surface marker also allows easy and accurate control of ascent rate and stop depth for safer decompression. A bailout cylinder provides breathing gas sufficient for a safe emergency ascent.^[66]

Various surface detection aids may be carried to help surface personnel spot the diver after ascent. In addition to the surface marker buoy, divers may carry mirrors, lights, strobes, whistles, flares or emergency locator beacons.^[66]

Accessories

Divers may carry underwater photographic or video equipment, or tools for a specific application in addition to diving equipment.

Preparation for the dive

The purpose of dive planning is to ensure that divers do not exceed their comfort zone or skill level, or the safe capacity of their equipment, and includes scuba gas planning to ensure that the amount of breathing gas to be carried is sufficient to allow for any reasonably foreseeable contingencies. Before starting a dive both the diver and their buddy^{[note 2]} do equipment checks to ensure everything is in good working order and available. Recreational divers are responsible for planning their own dives, unless in training, when the instructor is responsible.^[67][68] Divemasters may provide useful information and suggestions to assist the divers, but are generally not responsible for the details unless specifically employed to do so. In professional diving teams all team members are usually expected to contribute to planning and to check the equipment they will use, but the overall responsibility for the safety of the team lies with the supervisor as the appointed on-site representative of the employer.^{[43][69][70][71]}

Standard diving procedures

Two divers giving the sign that they are "OK".

Some procedures are common to almost all scuba dives, or are used to manage very common contingencies. These are learned at entry level and may be highly standardised to allow efficient cooperation between divers trained at different schools.^[72][73][6]

• Water entry and descent procedures are carried out first to enter the water without injury or loss of/damage to equipment. These procedures also cover how to descend at the right place, time, and rate; with the correct breathing gas available; and without losing contact with the other divers in the group.^[6][73]
• Equalisation of pressure in gas spaces to avoid barotraumas. The expansion or compression of enclosed air spaces may cause discomfort or injury while diving. Critically, the lungs are susceptible to over-expansion and subsequent collapse if a diver holds their breath while ascending: during training divers are taught to never hold their breath while diving. Ear clearing is another critical equalisation procedure, usually requiring conscious intervention by the diver.^[6][74]
• Mask and regulator clearing may be needed to ensure the ability to see and breathe in case of flooding. This can easily happen and is not considered an emergency.^[6][73]
• Buoyancy control and diver trim require frequent adjustment (particularly during depth changes) to ensure safe and convenient underwater mobility during the dive.
• Buddy checks, breathing gas monitoring, and decompression status monitoring are carried out to ensure that the dive plan is followed and that members of the group are safe and available to help each other in an emergency.^[6][73]
• Ascent, decompression, and surfacing: to ensure that dissolved gases are safely released, that barotraumas of ascent are avoided, and that it is safe to surface.^[6][73]
• Water exit procedures: to leave the water again without injury, loss of, or damage to equipment.^[73][6]
• Underwater communication: Divers cannot talk underwater unless they are wearing a full-face mask and electronic communications equipment, but they can communicate basic and emergency information using hand signals, light signals, and rope signals, and more complex messages can be written on waterproof slates.^[74][6][73]

Decompression

Inert gas components of the diver's breathing gas accumulate in the tissues during exposure to elevated pressure during a dive, and must be eliminated during the ascent to avoid the formation of symptomatic bubbles in tissues where the concentration is too high for the gas to remain in solution. This process is called decompression.^[75] Most recreational and professional scuba divers avoid obligatory decompression stops by following a dive profile which only requires a limited rate of ascent for decompression, but will commonly also do an optional short shallow decompression stop known as a safety stop to further reduce risk before surfacing. In some cases, particularly in technical diving, more complex decompression procedures are necessary. Decompression may follow a pre-planned series of ascents interrupted by stops, or may be monitored by a personal decompression computer.^[76]

Post-dive procedures

These include debriefing where appropriate, and equipment maintenance, to ensure that the equipment is kept in good condition for later use.^[74][6]

Buddy, team or solo diving

Buddy and team diving procedures are intended to ensure that a recreational scuba diver who gets into difficulty underwater is in the presence of a similarly equipped person who understands and can render assistance. Divers are trained to assist in those emergencies specified in the training standards for their certification, and are required to demonstrate competence in a set of prescribed buddy assist skills. The fundamentals of buddy/team safety are centred on diver communication, redundancy of gear and breathing gas by sharing with the buddy, and the added situational perspective of another diver.^[77]

Solo divers take responsibility for their own safety and compensate for the absence of a buddy with skill, vigilance and appropriate equipment. Like buddy or team divers, properly equipped solo divers rely on the redundancy of critical articles of dive gear which may include at least two independent supplies of breathing gas and ensuring that there is always enough available to safely terminate the dive if any one supply fails. The difference between the two practices is that this redundancy is carried and managed by the solo diver instead of a buddy. Agencies that certify for solo diving require candidates to have a high level of dive experience – usually about 100 dives or more.^[78][79]

Since the inception of scuba, there has been ongoing debate regarding the wisdom of solo diving with strong opinions on both sides of the issue. This debate is complicated by the fact that the line which separates a solo diver from a buddy/team diver is not always clear.^[80] For example, should a scuba instructor (who supports the buddy system) be considered a solo diver if their students do not have the knowledge or experience to assist the instructor through an unforeseen scuba emergency? Should the buddy of an underwater photographer consider themselves as effectively diving alone since their buddy (the photographer) is giving most or all of their attention to the subject of the photograph? This debate has motivated some prominent scuba agencies such as Global Underwater Explorers (GUE) to stress that its members only dive in teams and "remain aware of team member location and safety at all time."^[81] Other agencies such as Scuba Diving International (SDI) and Professional Association of Diving Instructors (PADI) have taken the position that divers might find themselves alone (by choice or by accident) and have created certification courses such as the "SDI Solo Diver Course" and the "PADI Self-Reliant Diver Course" in order to train divers to handle such possibilities.^[82][83]

Emergency procedures

The most urgent underwater emergencies usually involve a compromised breathing gas supply. Divers are trained in procedures for donating and receiving breathing gas from each other in an emergency, and may carry an independent alternative air source if they do not choose to rely on a buddy.^[74][6][73] Divers may need to make an emergency ascent in the event of a loss of breathing gas which cannot be managed at depth. Controlled emergency ascents are almost always a consequence of loss of breathing gas, while uncontrolled ascents are usually the result of a buoyancy control failure.^[84]

Divers may be trained in procedures which have been approved by the training agencies for recovery of an unresponsive diver to the surface, where it might be possible to administer first aid. Not all recreational divers have this training as some agencies do not include it in entry level training. Professional divers may be required by legislation or code of practice to have a standby diver at any diving operation, who is both competent and available to attempt rescue of a distressed diver.^[74][73]

Two basic types of entrapment are significant hazards for scuba divers: Inability to navigate out of an enclosed space, and physical entrapment which prevents the diver from leaving a location. The first case can usually be avoided by staying out of enclosed spaces, and when the objective of the dive includes penetration of enclosed spaces, taking precautions such as the use of lights and guidelines, for which specialised training is provided in the standard procedures.^[85] The most common form of physical entrapment is getting snagged on ropes, lines or nets, and use of a cutting implement is the standard method of dealing with the problem. The risk of entanglement can be reduced by careful configuration of equipment to minimise those parts which can easily be snagged, and allow easier disentanglement. Other forms of entrapment such as getting wedged into tight spaces can often be avoided, but must otherwise be dealt with as they happen. The assistance of a buddy may be helpful where possible.^[5]

Scuba diving in relatively hazardous environments such as caves and wrecks, areas of strong water movement, relatively great depths, with decompression obligations, with equipment that has more complex failure modes, and with gases that are not safe to breathe at all depths of the dive require specialised safety and emergency procedures tailored to the specific hazards, and often specialised equipment. These conditions are generally associated with technical diving.^[47]

Hazards

Scuba diving in a cave

Divers touring a World War II Shipwreck.

Divers operate in an environment for which the human body is not well suited. They face special physical and health risks when they go underwater or use high pressure breathing gas. The consequences of diving incidents range from merely annoying to rapidly fatal, and the result often depends on the equipment, skill, response and fitness of the diver and diving team. The hazards include the aquatic environment, the use of breathing equipment in an underwater environment, exposure to a pressurised environment and pressure changes, particularly pressure changes during descent and ascent, and breathing gases at high ambient pressure. Diving equipment other than breathing apparatus is usually reliable, but has been known to fail, and loss of buoyancy control or thermal protection can be a major burden which may lead to more serious problems. There are also hazards of the specific diving environment, and hazards related to access to and egress from the water, which vary from place to place, and may also vary with time. Hazards inherent in the diver include pre-existing physiological and psychological conditions and the personal behaviour and competence of the individual. For those pursuing other activities while diving, there are additional hazards of task loading, of the dive task and of special equipment associated with the task.^[94][95]

The presence of a combination of several hazards simultaneously is common in diving, and the effect is generally increased risk to the diver, particularly where the occurrence of an incident due to one hazard triggers other hazards with a resulting cascade of incidents. Many diving fatalities are the result of a cascade of incidents overwhelming the diver, who should be able to manage any single reasonably foreseeable incident.^[96] Although there are many dangers involved in scuba diving, divers can decrease the risks through proper procedures and appropriate equipment. The requisite skills are acquired by training and education, and honed by practice. Open-water certification programmes highlight diving physiology, safe diving practices, and diving hazards, but do not provide the diver with sufficient practice to become truly adept.^[96]

Scuba divers by definition carry their breathing gas supply with them during the dive, and this limited quantity must get them back to the surface safely. Pre-dive planning of appropriate gas supply for the intended dive profile lets the diver allow for sufficient breathing gas for the planned dive and contingencies.^[97] They are not connected to a surface control point by an umbilical, such as surface-supplied divers use, and the freedom of movement that this allows, also allows the diver to penetrate overhead environments in ice diving, cave diving and wreck diving to the extent that the diver may lose their way and be unable to find the way out. This problem is exacerbated by the limited breathing gas supply, which gives a limited amount of time before the diver will drown if unable to surface. The standard procedure for managing this risk is to lay a continuous guide line from open water, which allows the diver to be sure of the route to the surface.^[85]

Most scuba diving, particularly recreational scuba, uses a breathing gas supply mouthpiece which is gripped by the diver's teeth, and which can be dislodged relatively easily by impact. This is generally easily rectified unless the diver is incapacitated, and the associated skills are part of entry-level training.^[6] The problem becomes severe and immediately life-threatening if the diver loses both consciousness and the mouthpiece. Rebreather mouthpieces which are open when out of the mouth may let in water which can flood the loop, making them unable to deliver breathing gas, and will lose buoyancy as the gas escapes, thus putting the diver in a situation of two simultaneous life-threatening problems.^[98] Skills to manage this situation are a necessary part of training for the specific configuration. Full-face masks reduce these risks and are generally preferred for professional scuba diving, but can make emergency gas sharing difficult, and are less popular with recreational divers who often rely on gas sharing with a buddy as their breathing gas redundancy option.^[99]

Risk

The risk of dying during recreational, scientific or commercial diving is small, and on scuba, deaths are usually associated with poor gas management, poor buoyancy control, equipment misuse, entrapment, rough water conditions and pre-existing health problems. Some fatalities are inevitable and caused by unforeseeable situations escalating out of control, but the majority of diving fatalities can be attributed to human error on the part of the victim. Equipment failure is rare in open circuit scuba.^[84]

According to death certificates, over 80% of the deaths were ultimately attributed to drowning, but other factors usually combined to incapacitate the diver in a sequence of events culminating in drowning, which is more a consequence of the medium in which the accidents occurred than the actual accident. Scuba divers should not drown unless there are other contributory factors as they carry a supply of breathing gas and equipment designed to provide the gas on demand. Drowning occurs as a consequence of preceding problems such as unmanageable stress, cardiac disease, pulmonary barotrauma, unconsciousness from any cause, water aspiration, trauma, environmental hazards, equipment difficulties, inappropriate response to an emergency or failure to manage the gas supply.^[100] and often obscures the real cause of death. Air embolism is also frequently cited as a cause of death, and it, too is the consequence of other factors leading to an uncontrolled and badly managed ascent, possibly aggravated by medical conditions. About a quarter of diving fatalities are associated with cardiac events, mostly in older divers. There is a fairly large body of data on diving fatalities, but in many cases the data is poor due to the standard of investigation and reporting. This hinders research which could improve diver safety.^[84]

Fatality rates are comparable with jogging (13 deaths per 100,000 persons per year) and are within the range where reduction is desirable by Health and Safety Executive (HSE) criteria,^[101] The most frequent root cause for diving fatalities is running out of or low on gas. Other factors cited include buoyancy control, entanglement or entrapment, rough water, equipment misuse or problems and emergency ascent. The most common injuries and causes of death were drowning or asphyxia due to inhalation of water, air embolism and cardiac events. Risk of cardiac arrest is greater for older divers, and greater for men than women, although the risks are equal by age 65.^[101]

Several plausible opinions have been put forward but have not yet been empirically validated. Suggested contributing factors included inexperience, infrequent diving, inadequate supervision, insufficient predive briefings, buddy separation and dive conditions beyond the diver's training, experience or physical capacity.^[101]

Decompression sickness and arterial gas embolism in recreational diving have been associated with specific demographic, environmental, and diving behavioural factors. A statistical study published in 2005 tested potential risk factors: age, asthma, body mass index, gender, smoking, cardiovascular disease, diabetes, previous decompression illness, years since certification, number of dives in the previous year, number of consecutive diving days, number of dives in a repetitive series, depth of the previous dive, use of nitrox as breathing gas, and use of a dry suit. No significant associations with risk of decompression sickness or arterial gas embolism were found for asthma, body mass index, cardiovascular disease, diabetes or smoking. Greater dive depth, previous decompression illness, number of consecutive days diving, and male biological gender were associated with higher risk for decompression sickness and arterial gas embolism. The use of dry suits and nitrox breathing gas, greater frequency of diving in the previous year, greater age, and more years since certification were associated with lower risk, possibly as indicators of more extensive training and experience.^[102]

Risk management has three major aspects besides equipment and training: Risk assessment, emergency planning and insurance cover. The risk assessment for a dive is primarily a planning activity, and may range in formality from a part of the pre-dive buddy check for recreational divers, to a safety file with professional risk assessment and detailed emergency plans for professional diving projects. Some form of pre-dive briefing is customary with organised recreational dives, and this generally includes a recitation by the divemaster of the known and predicted hazards, the risk associated with the significant ones, and the procedures to be followed in case of the reasonably foreseeable emergencies associated with them. Insurance cover for diving accidents may not be included in standard policies. There are a few organisations which focus specifically on diver safety and insurance cover, such as the international Divers Alert Network^[103]

Recreational

Scuba diving education levels as used by ISO, PADI, CMAS, SSI and NAUI

Basic diving skills training in a swimming pool

Recreational diver training is the process of developing knowledge and understanding of the basic principles, and the skills and procedures for the use of scuba equipment so that the diver is able to dive for recreational purposes with acceptable risk using the type of equipment and in similar conditions to those experienced during training. Recreational (including technical) scuba diving does not have a centralised certifying or regulatory agency, and is mostly self-regulated. There are, however, several international organisations of varying size and market share that train and certify divers and dive instructors, and many diving related sales and rental outlets require proof of diver certification from one of these organisations prior to selling or renting certain diving products or services.^[105][106]

Not only is the underwater environment hazardous but the diving equipment itself can be dangerous. There are problems that divers must learn to avoid and manage when they do occur. Divers need repeated practice and a gradual increase in challenge to develop and internalise the skills needed to control the equipment, to respond effectively if they encounter difficulties, and to build confidence in their equipment and themselves. Diver practical training starts with simple but essential procedures, and builds on them until complex procedures can be managed effectively. This may be broken up into several short training programmes, with certification issued for each stage,^[107] or combined into a few more substantial programmes with certification issued when all the skills have been mastered.^[108][109]

Many organizations exist, throughout the world, offering diver training leading to certification: the issuing of a "Diving Certification Card," also known as a "C-card," or qualification card. This diving certification model originated at Scripps Institution of Oceanography in 1952 after two divers died while using university-owned equipment and the SIO instituted a system where a card was issued after training as evidence of competence.^[110][111] Diving instructors affiliated to a diving certification agency may work independently or through a university, a dive club, a dive school or a dive shop. They will offer courses that should meet, or exceed, the standards of the certification organization that will certify the divers attending the course. Certification of the diver is done by the certification organisation on application by the registered instructor.^[107]

The International Organization for Standardization has approved six recreational diving standards that may be implemented worldwide, and some of the standards developed by the World Recreational Scuba Training Council are consistent with the applicable ISO Standards,^[72][112][6] as are equivalent standards published by the Confédération Mondiale des Activités Subaquatiques and the European Underwater Federation^[113][114]

The initial open water training for a person who is medically fit to dive and a reasonably competent swimmer is relatively short. Many dive shops in popular holiday locations offer courses intended to teach a novice to dive in a few days, which can be combined with diving on the vacation.^[107] Other instructors and dive schools will provide more thorough training, which generally takes longer.^[109] Dive operators, dive shops, and cylinder filling stations may refuse to allow uncertified people to dive with them, hire diving equipment or have their diving cylinders filled. This may be an agency standard, company policy, or specified by legislation.^[115]

Professional

Class IV scientific divers assembling a structure during a training exercise

It is fairly common for a national standard for commercial diver training and registration to apply within a country. These standards may be set by national government departments and empowered by national legislation, for example, in the case of the United Kingdom, where the standards are set by the Health and Safety Executive,^[43] and South Africa where they are published by the Department of Labour.^[70] Many national training standards and the associated diver registrations are recognised internationally among the countries which are members of the International Diving Regulators and Certifiers Forum (IDRCF). A similar arrangement exists for state-legislated standards, as in the case of Canada and Australia.^[104] Registration of professional divers trained to these standards may be directly administered by government, as in the case of South Africa, where diver registration is done by the Department of Labour,^[70] or by an approved external agent, as in the case of the Australian Diver Accreditation Scheme (ADAS)^[116]

The following countries and organisations are members of the European Diving Technology committee, which publishes minimum standards for commercial diver training and competence accepted by these and some other countries through membership of the IDRCF and IDSA: Austria, Belgium, Croatia, Czech Republic, Denmark, Estonia, Finland, France, Germany, Italy, Latvia, Romania, The Netherlands, Norway, Poland, Portugal, Spain, Slovak republic, Sweden, Switzerland, Turkey, United Kingdom, International Marine Contractors Association (IMCA), International Oil and Gas Producers (IOGP), International Transport Workers’ Federation (ITF), International Diving Schools Association (IDSA), European Underwater Federation, and International Diving Regulators and Certifiers Forum (IDRCF).^[117]:2 These standards include Commercial SCUBA Diver.^[117]:8

An example of a widely accepted training standard – EDTC 2017 Commercial SCUBA Diver – requires the professional scuba diver to be certified as medically fit to dive, and competent in skills covering the scope of:^[117]:8–9

• Administrative procedures relating to statutory requirements, employment conditions, health and safety at the workplace, and the basic theoretical grounding in physics, physiology and medicine that are relevant to their work as a diver.
• The skills required for routine diving operations, including working as part of the diving team, planning of diving operations, and diving in open water, exposed to the normal hazards of the diving environment, decompression procedures, serving as attendant to another diver, communications and the safe use of the tools appropriate to the work.
• The skills in emergency procedures for management of reasonably foreseeable emergencies, including standby diver skills for diver assistance and rescue, management of emergencies unaided where appropriate, and team procedures for handling emergencies.
• Preparation of diving and task-related equipment for use
• Provision of first aid and basic life support procedures in a diving emergency, and assistance, under supervision, in the treatment of diving disorders
• Competence to assist under supervision with chamber operations, including acting as inside attendant to an afflicted diver.

International Diving Schools Association (IDSA) provides a Table of Equivalence of various national commercial diver training standards.^[118]

Military scuba training is usually provided by the armed force's internal diver training facilities, to their specific requirements and standards, and generally involves basic scuba training, specific training related to the equipment used by the unit, and associated skills related to the particular unit. The general scope of requirements is generally similar to that for commercial divers, though standards of fitness and assessment may differ considerably.^[1]