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Diving safety

The safety of underwater diving depends on four factors: the environment, the equipment, behaviour of the individual diver and performance of the dive team. The underwater environment can impose severe physical and psychological stress on a diver, and is mostly beyond the diver's control. Equipment is used to operate underwater for anything beyond very short periods, and the reliable function of some of the equipment is critical to even short term survival. Other equipment allows the diver to operate in relative comfort and efficiency. The performance of the individual diver depends on learned skills, many of which are not intuitive, and the performance of the team depends on communication and common goals.[1]

There is a large range of hazards to which the diver may be exposed. These each have associated consequences and risks, which should be taken into account during dive planning. Where risks are marginally acceptable it may be possible to mitigate the consequences by setting contingency and emergency plans in place, so that damage can be minimised where reasonably practicable. The acceptable level of risk varies depending on legislation, codes of practice and personal choice, with recreational divers having a greater freedom of choice.[2]


Environmental factorsEdit

The underwater environment is alien to humans. When not actively hostile, it is unforgiving of errors, and some errors can escalate rapidly to a fatal conclusion. Many aspects of the underwater environment are static or predictable, others vary and may not be easily or reliably predictable, and must be managed as found. The reasonably predictable factors can be allowed for in the dive planning. Suitable equipment can be selected, personnel can be trained in its use and support provided to manage the foreseeable contingencies. When conditions are found to be other than predicted, plans may have to be changed. Sometimes conditions are better than expected, but other times they may be worse, and may deteriorate during the course of a dive to the extent that recovery becomes an emergency.

  • Predictable/static environmental factors - conditions which should be considered in the dive plan
  • Variable environmental factors - conditions can change during a dive - dive contingency plans should take into account the reasonably foreseeable variations based on forecasts and local knowledge. When there is no reliable local knowledge, a wider range of contingencies should be considered.

Equipment safetyEdit

Two basic classes of equipment are used by divers: Equipment necessary to do the planned dive, and equipment required to do the task for which the dive is necessary. Recreational divers may not require equipment for a task, but it is quite common for them to use a camera, and some will survey a dive site, or use a small lift bag to recover an anchor or diving shot. There are no particularly significant risks associated with tools commonly used by recreational divers. Commercial divers usually use tools of some kind while diving, and some of these tools can be very dangerous if used incorrectly, such as high-pressure water-jets, explosive bolts, oxy-arc cutting and welding and heavy lifting equipment and rigging.


Sidemount scuba is useful in tight restrictions

Open circuit scuba is mechanically robust and reliable, but can malfunction when damaged, misused, poorly maintained, or occasionally due to unplanned circumstances. Provision of a completely independent emergency supply capable of providing sufficient breathing gas to allow the diver to surface safely from any point on the planned dive profile reduces the risk of a non-survivable out of gas incident to an extremely low level. This remains valid only as long as the emergency gas supply is within immediate reach of the diver, which is more reliably achieved by the diver carrying a bailout cylinder than by relying on a buddy or standby diver, who may not be where needed in an emergency.[citation needed]

Rebreathers have an intrinsically much higher risk[3] of mechanical failure than open circuit scuba because of their structural and functional complexity, but this can be mitigated by good design which provides redundancy of critical items and by carrying sufficient alternative breathing gas supplies for bailout including any required decompression in case of failure. Designs that minimize risk of human-machine interface errors, and adequate training in procedures that deal with this area may help reduce the fatality rate. Two thirds of fatalities were associated with high risk behaviour of a high risk dive profile.[4]

Surface-supplied equipmentEdit

Surface-supplied divers riding a stage to the underwater workplace

The essential aspect of surface-supplied diving is that breathing gas is supplied from the surface, either from a specialized diving compressor, high-pressure cylinders, or both. In commercial and military surface-supplied diving, a backup source of breathing gas should always be present in case the primary supply fails. The diver may also wear a cylinder called a "bail-out bottle," which can provide self-contained breathing gas in an emergency. Thus, the surface-supplied diver is much less likely to have an "out-of-air" emergency than a scuba diver as there are normally two alternative air sources available. Surface-supplied diving equipment usually includes communication capability with the surface, which adds to the safety and efficiency of the working diver.[5]

Surface-supplied equipment is required under the US Navy operational guidance for diving in harsh contaminated environments which was drawn up by the Navy Experimental Diving Unit.[6] Surface-supplied diving equipment is required for a large proportion of the commercial diving operations conducted in many countries, either by direct legislation, or by authorised codes of practice, as in the case of IMCA operations.[7]

Human factorsEdit

Human factors are the physical or cognitive properties of individuals, or social behavior which is specific to humans, and influence functioning of technological systems as well as human-environment equilibria. The safety of underwater diving operations can be improved by reducing the frequency of human error and the consequences when it does occur.[1] Human error can be defined as an individual's deviation from acceptable or desirable practice which culminates in undesirable or unexpected results.[8]

Human error is inevitable and everyone makes mistakes at some time. The consequences of these errors are varied and depend on many factors. Most errors are minor and do not cause significant harm, but others can have catastrophic consequences. Examples of human error leading to accidents are available in vast numbers, as it is the direct cause of 60% to 80% of all accidents.[9] In a high risk environment, as is the case in diving, human error is more likely to have catastrophic consequences. A study by William P. Morgan indicates that over half of all divers in the survey had experienced panic underwater at some time during their diving career.[10] These findings were independently corroborated by a survey[11] that suggested 65% of recreational divers have panicked under water. Panic frequently leads to errors in a diver's judgment or performance, and may result in an accident. Human error and panic are considered to be the leading causes of dive accidents and fatalities.[10][12][13][14][15][16]

Only 4.46% of the recreational diving fatalities in a 1997 study[17] were attributable to a single contributory cause. The remaining fatalities probably arose as a result of a progressive sequence of events involving two or more procedural errors or equipment failures, and since procedural errors are generally avoidable by a well-trained, intelligent and alert diver, working in an organised structure, and not under excessive stress, it was concluded that the low accident rate in commercial scuba diving is due to this factor.[18] The study also concluded that it would be impossible to eliminate absolutely all minor contraindications of scuba diving, as this would result in overwhelming bureaucracy and would bring all diving to a halt.[17]

Humans function underwater by virtue of technology, as our physiology is poorly adapted to the environment. Human factors are significant in diving because of this harsh and alien environment, and because diver life support systems and other equipment that may be required to perform specific tasks depend on technology that is designed, operated and maintained by humans, and because human factors are cited as significant contributors to diving accidents in most accident investigations[18]

Professional diving is a means to accomplish a wide range of activities underwater in a normally inaccessible and potentially hazardous environment. While working underwater, divers are subjected to high levels of physical and psychological stress due to environmental conditions and the limitations of the life support systems, as well as the rigours of the task at hand.

Recreational, or sport divers, including technical divers, dive for entertainment, and are usually motivated by a desire to explore and witness, though there is no distinct division between the underwater activities of recreational and professional divers. The primary distinction is that legal obligations and protection are significantly different,[19][20] and this is reflected in organisational structure and procedures.

Recreational diving has been rated more risky than snow skiing, but less risky than other adventure sports such as rock climbing, bungee jumping, motorcycle racing and sky diving. Improvements in training standards and equipment design and configuration, and increased awareness of the risks of diving, have not eliminated fatal incidents, which occur every year in what is generally a reasonably safe recreational activity.[16]

Both categories of diver are usually trained and certified, but recreational diving equipment is typically limited to freediving and scuba, whereas professional divers may be trained to use a greater variety of diving systems, from scuba to surface supplied mixed gas, saturation systems and atmospheric diving suits. A recreational diver may use some ancillary equipment to enhance the diving experience, but the professional will almost always use tools to perform a specific task.

Since the goal of recreational diving is personal enjoyment, a decision to abort a dive, for whatever reason, normally only affects the diver and his companions. A working diver faced with the same decision, must disappoint a client who needs and expects the diver's services, often with significant financial consequences. Therefore, the working diver often faces greater pressure to provide the service at the cost of reduced personal safety. An understanding of the human factors associated with diving may help the diving team to strike an appropriate balance between service delivery and safety.[1]

Human factors are the influences on human behavior, and the resulting effects of human performance on a process or system. Safety can be improved by reducing the frequency of human error and the consequences when it does occur.[1] Human error can be defined[8] as an individual's deviation from acceptable or desirable practice which culminates in undesirable or unexpected results.

Diver performanceEdit

Safety of underwater diving operations can be improved by reducing the frequency of human error and the consequences when it does occur.[1] Human error can be defined as an individual's deviation from acceptable or desirable practice which culminates in undesirable or unexpected results.[8] Human error is inevitable and everyone makes mistakes at some time. The consequences of these errors are varied and depend on many factors. Most errors are minor and do not cause significant harm, but others can have catastrophic consequences. Human error and panic are considered to be the leading causes of dive accidents and fatalities.[1]

  • Inadequate learning or practice of critical safety skills may result in the inability to deal with minor incidents, which consequently may develop into major incidents.
  • Overconfidence can result in diving in conditions beyond the diver's competence, with high risk of accident due to inability to deal with known environmental hazards.
  • Inadequate strength or fitness for the conditions can result in inability to compensate for difficult conditions even though the diver may be well versed at the required skills, and could lead to over-exertion, overtiredness, stress injuries or exhaustion.
  • Peer pressure can cause a diver to dive in conditions where they may be unable to deal with reasonably predictable incidents.
  • Diving with an incompetent buddy can result in injury or death while attempting to deal with a problem caused by the buddy.
  • Overweighting can cause difficulty in neutralising and controlling buoyancy, and this can lead to uncontrolled descent, inability to establish neutral buoyancy, inefficient swimming, high gas consumption, poor trim, kicking up silt, difficulty in ascent and inability to control depth accurately for decompression.
  • Underweighting can cause difficulty in neutralising and controlling buoyancy, and consequent inability to achieve neutral buoyancy, particularly at decompression stops.
  • Diving under the influence of drugs or alcohol, or with a hangover may result in inappropriate or delayed response to contingencies, reduced ability to deal timeously with problems, leading to greater risk of developing into an accident, increased risk of hypothermia and increased risk of decompression sickness.[21]
  • Use of inappropriate equipment and/or configuration can lead to a whole range of complications, depending on the details.
  • High task loading due to a combination of these factors can result in a dive that goes well enough until something goes wrong, and the diver's residual capacity is not enough to cope with the changed circumstances. This can be followed by a cascade of failures, as each problem loads the diver more and triggers the next. In such cases the diver is lucky to survive, even with the assistance of a buddy or team, and there is a significant risk of others becoming part of the accident.

Dive team performanceEdit

A dive team can vary from a recreational buddy pair to a professional saturation diving team working 24 hours per day with dive and habitat support personnel on a dynamically positioned vessel.[7] The primary purpose of a professional diving team is to improve safety for the working diver by providing backup and support, and to manage the surface equipment required for the operation.[20][19][22] A buddy pair is also intended to improve the safety of recreational divers, and in some circumstances succeeds in this aim, depending on the skills, situational awareness and compliance with procedures of the divers. Technical diving teams can vary between the recreational buddy pair at its worst to expedition teams with structure, competence and planning similar to professional teams.

For many applications, the minimum personnel requirement for a professional diving operation is a working diver, to do the job, a diver's tender to assist the diver and manage the umbilical or airline, a standby diver, competent and ready to go to the assistance of the working diver, and a supervisor, to co-ordinate the team, ensure that the plan is acceptably safe in terms of the organisational policies coded of practice and applicable legislation, ensure that the operation follows the plan as far as possible, and to manage any contingencies or emergencies that may come up during the operation. The primary responsibility of the supervisor of a professional diving team is the health and safety of the diving team.[20][19][22]


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.[23][24]

Professional divers may be exposed to a wider range of hazards, some of which are inherent in the equipment used to reduce the risk of other hazards. Saturation diving is intended to reduce a relatively high risk of decompression sickness, but introduces other health and safety hazards of living at a high ambient pressure for extended periods, and transfer between pressurised spaces. Failure of a saturation system can be catastrophic and fatal to the occupants and bystanders. Such failures are seldom engineering failures, they are more often ergonomic design and operation failures, and usually systems are corrected after analysis of such failures.


Mask squeeze barotrauma of descent

Diving related medical conditions, are conditions associated with underwater diving, and include both conditions unique to underwater diving, and those that also occur during other activities. This second group further divides into conditions caused by exposure to ambient pressures significantly different from surface atmospheric pressure, and a range of conditions caused by general environment and equipment associated with diving activities.

Disorders particularly associated with diving include those caused by variations in ambient pressure, such as barotraumas of descent and ascent, decompression sickness and those caused by exposure to elevated ambient pressure, such as some types of gas toxicity. There are also non-dysbaric disorders associated with diving, which include the effects of the aquatic environment, such as drowning, which also are common to other water users, and disorders caused by the equipment or associated factors, such as carbon dioxide and carbon monoxide poisoning. General environmental conditions can lead to another group of disorders, which include hypothermia and motion sickness, injuries by marine and aquatic organisms, contaminated waters, man-made hazards, and ergonomic problems with equipment. Finally there are pre-existing medical and psychological conditions which increase the risk of being affected by a diving disorder, which may be aggravated by adverse side effects of medications and other drug use.

Treatment depends on the specific disorder, but often includes oxygen therapy, which is standard first aid for most diving accidents, and is hardly ever contra-indicated for a person medically fit to dive, and hyperbaric therapy is the definitive treatment for decompression sickness. Screening for medical fitness to dive can reduce some of the risk for some of the disorders.


The labels used to classify dives are not sufficiently precise for analysing risk. Terms like "recreational", "technical", "commercial", "military", "scientific" and "professional" are used but are not precisely defined, particularly for risk analysis as they do not identify specific contributors to diving risk. Categorisation by depth and obligation for decompression stops are also insufficient to classify risk.[25]

The diving mode has a large influence on risk, and choice of diving mode is commonly based on the outcome of a risk assessment for the diving operation.

  • Hazard Identification and Risk Assessment: HIRA is a procedure applied to a project, and the results would be used to inform the planners on safety related issues such as choosing the appropriate diving mode, selection of equipment and dive team members, specialised training that may be required, and contingency and emergency planning.[26]
  • Job Safety Analysis: A (JSA) is a procedure which helps integrate accepted safety and health principles and practices into a particular task or job operation. In a JSA, each basic step of the analysis is to identify potential hazards and to recommend the safest way to do the job. In professional diving a JSA would be done for the planned task for a specific dive, and the result would be included in the dive briefing.

Risk managementEdit

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[27]

Hazard controlEdit

The classic methods of hazard control are applied when reasonably practicable: The modes of diving can be considered levels of hazard control. An alternative mode of diving may include hazard elimination or substitution, engineering controls, administrative controls and personal protective equipment to reduce risk for a given activity, usually at considerable logistical cost, and often reducing operational flexibility.

Hazards to divers can be completely eliminated when a machine can do the job. There are a growing number of commercial, military and scientific applications where a remotely operated or autonomous underwater vehicle can produce satisfactory results. To a lesser extent this applies to atmospheric pressure diving, where the diver is not exposed to the environment as long as the suit integrity is maintained, but some of the hazards and risks remain. Saturation diving is a technique that allows divers to reduce the risk of decompression sickness ("the bends") when they work at great depths for long periods of time.[28][29]


freediving in the Red Sea
Competition freediving with safety divers

Freediving, or breath-hold diving, is the original mode of diving, and was used for centuries in spite of limitations as it was the only option available. It is simple and inexpensive, but severely limited in the time available to do useful work at depth. The risk of drowning is relatively high, as the diver is limited to the oxygen supplied by a single breath, and the risk of hypoxic blackout underwater, followed by drowning, is significant.

Hypoxic blackout during freediving is a loss of consciousness caused by cerebral hypoxia towards the end of a breath-hold dive, when the swimmer does not necessarily experience an urgent need to breathe and has no other obvious medical condition that might have caused it. It can be provoked by hyperventilating just before a dive, or as a consequence of the pressure reduction on ascent, or a combination of these. Victims are often established practitioners of breath-hold diving, are fit, strong swimmers and have not experienced problems before.[30][31][32]

Divers and swimmers who blackout or grey out underwater during a dive will usually drown unless rescued and resuscitated within a short time.[33] Freediving blackout has a high fatality rate but is generally avoidable. The risk cannot be quantified, but is clearly increased by any level of hyperventilation.[34]

Freediving blackout can occur on any dive profile: at constant depth, on an ascent from depth, or at the surface following ascent from depth and may be described by a number of terms depending on the dive profile and depth at which consciousness is lost. Blackout during a shallow dive differs from blackout during ascent from a deep dive in that deep water blackout is precipitated by depressurisation on ascent from depth while shallow water blackout is a consequence of hypocapnia following hyperventilation.[32][35]

Trained freedivers are well aware of this and competitions must be held under strict supervision and with competent first-aiders on standby.[36] However this does not eliminate the risk of blackout. Freedivers are recommended to only dive with a 'buddy' who accompanies them, observing from in the water at the surface, and ready to dive to the rescue if the diver loses consciousness during the ascent.[citation needed]

Scuba divingEdit

Diving using self-contained underwater breathing apparatus was developed after surface supplied diving, and was intended as a method of improving the mobility and horizontal range of the diver who is not restricted by a physical connection to a surface gas supply. The diver has a larger gas supply than the freediver, and this allows a greatly extended underwater endurance, and lower risk of drowning, but at the cost of higher risk from decompression sickness, lung over-pressure barotrauma, nitrogen narcosis, oxygen toxicity and hypothermia, all of which must be limited by procedural and engineering controls, and personal protective equipment.

For acceptable safety the diver must be able to survive any reasonably foreseeable single point of failure. For scuba equipment this implies that the failure of any single item of equipment should not put the diver out of reach of a breathing gas supply.[37]

Open circuitEdit

In the case of a single cylinder scuba set with a single first stage, and a single second stage, each of these items has a low but non-zero probability of failure. The components work in series - if any one of them fails, the system fails. It is equivalent to a single chain in which if any link fails, the chain breaks. When the dive is very shallow, the diver can safely escape to the surface, and when there is another diver right there with spare gas at the time of failure, they can share gas. At other times, a failure of a single item can kill the diver.[37]

Assuming independence of failure events, each item that can cause failure of the combined system is a critical point of failure and increases the probability of system. For the system not to fail, all items must not fail according to the formula:



  •   – number of components
  •   – probability of component i failing
  •   – the probability of all components failing (system failure)

As a purely illustrative example, if there is a 1 in 100 probability of a regulator failure, and a 1 in 1000 probability of a scuba cylinder failure then

 , and  



Substituting values:

     which is close to the sum of the two probabilities.

The example shows that each critical point of failure increases the probability of system failure by approximately that item's probability of failure.

If there are two completely independent scuba sets at the diver's disposal, either one of which is sufficient to allow the diver a safe return, then both sets must fail during the same dive to cause a fatal outcome. These items work in parallel - all must fail for the system to fail. The probability of this happening is extremely low for reliable equipment.[37]

Assuming independence of failure events, each duplicate redundant item added to the system decreases the probability of system failure according to the formula:-



  •   – number of components
  •   – probability of component i failing
  •   – the probability of all components failing (system failure)

Taking two independent sets with the same probability of failure calculated in the example above:

 , and  



Substituting values:


It is clear from the example that redundancy reduces the risk of system failure very rapidly, and conversely, that disregarding a failure of a redundant item increases the probability of system failure equally rapidly.

Closed circuitEdit

Open circuit scuba has a small number of fairly rugged and reliable components, each with a small number of failure modes and a low probability of failure. Most of these components remain present in closed circuit scuba, but there are also a number of additional items which could fail. Therefore, the rebreather architecture is inherently more likely to fail, and it is necessary to provide redundancy of critical components to provide reliability even approaching that of open circuit scuba. It is also more important to provide full redundancy of breathing gas supply as some rebreather failure modes do not allow safe ascent. Bailout to open circuit is the simplest and most robust option, but for dives where a long return under an overhead, or long decompression are necessary, open circuit can be impractically bulky. There is a point at which closed circuit bailout becomes a more manageable option, and the requirement for ability to return safely from any point on the planned dive profile makes it necessary for the breathing loop and gas supplies to be fully independent, though the ability to make use of the primary gas supply in the bailout rebreather can considerably extend the range for a small added complexity, using highly reliable components, but adding to the task loading of the diver.[37]

A hazard specific to closed circuit rebreathers is failure of the oxygen partial pressure control system. The breathing gas mixture in a diving rebreather loop is usually measured using electro-galvanic oxygen sensors, and the output of the cells is used by either the diver or an electronic control system to control addition of oxygen to increase partial pressure when it is below the chosen lower set-point, or to flush with diluent gas when it is above the upper set-point. When the partial pressure is between the upper and lower set-points, it is suitable for breathing at that depth and is left until it changes as a result of consumption by the diver, or a change in ambient pressure as a result of a depth change.[citation needed]

Accuracy and reliability of measurement is important in this application for two basic reasons. Firstly, if the oxygen content is too low, the diver will lose consciousness due to hypoxia and probably die,[38] or if the oxygen content is too high, the risk of central nervous system oxygen toxicity causing convulsions and loss of consciousness, with a high risk of drowning becomes unacceptable.[38] Secondly, decompression obligations cannot be accurately or reliably calculated if the breathing gas composition is not known.[38] Pre-dive calibration of the cells can only check response to partial pressures up to 100% at atmospheric pressure, or 1 bar. As the set points are commonly in the range of 1.2 to 1.6 bar,[38] special hyperbaric calibration equipment would be required to reliably test the response at the set-points. This equipment is available, but is expensive and not in common use, and requires the cells to be removed from the rebreather and installed in the test unit. To compensate for the possibility of a cell failure during a dive, three cells are generally fitted, on the principle that failure of one cell at a time is most likely, and that if two cells indicate the same PO2, they are more likely to be correct than the single cell with a different reading. Voting logic allows the control system to control the circuit for the rest of the dive according to the two cells assumed to be correct. This is not entirely reliable, as it is possible for two cells to fail on the same dive.[38]

Surface oriented surface supplied divingEdit

Surface-supplied diving is diving using equipment supplied with breathing gas using a diver's umbilical from the surface, either from the shore or from a diving support vessel, sometimes indirectly via a diving bell.[39]

The copper helmeted free-flow standard diving dress is the version which made commercial diving a viable occupation, and although still used in some regions, this heavy equipment has been superseded by lighter free-flow helmets, and to a large extent, lightweight demand helmets, band masks and full-face diving masks. Breathing gases used include air, heliox, nitrox, oxygen and trimix. Gases with raised oxygen fraction are used to reduce decompression obligation and accelerate decompression, and gases containing helium are used to reduce nitrogen narcosis. Both applications reduce the risk to the diver when applicable.[28]

The primary advantages of conventional surface supplied diving over scuba are lower risk of drowning and considerably larger breathing gas supply than scuba, allowing longer working periods and safer decompression.

Surface supplied diving systems improve safety by virtually eliminating the risk of a lost diver, as the diver is physically connected to the surface control point by the breathing gas supply hose, and other components of the umbilical cable system. They also significantly reduce the risk of running out of breathing gas during the dive, and allow multiple redundancy of gas supply, with main and secondary surface supply, and a scuba bailout emergency gas system. Use of helmets and full-face masks help protect the diver's airway in case of loss of consciousness. These can be considered engineering controls of the hazards.

Saturation divingEdit

Saturation divers are transferred under pressure between the living quarters and the work site in a closed bell

Decompression sickness occurs when a diver with a large amount of inert gas dissolved in the body tissues is decompressed to a pressure where the gas forms bubbles which may block blood vessels or physically damage surrounding cells. This is a risk on every decompression, and limiting the number of decompressions can reduce the risk.[28][29]

"Saturation" refers to the fact that the diver's tissues have absorbed the maximum partial pressure of gas possible for that depth due to the diver being exposed to breathing gas at that pressure for prolonged periods. This is significant because once the tissues become saturated, the time to ascend from depth, to decompress safely, will not increase with further exposure.[28][29]

In saturation diving, the divers live in a pressurized environment, which can be a saturation system - a hyperbaric environment on the surface - or an ambient pressure underwater habitat. This may continue for up to several weeks, usually with the divers living at the same or very similar ambient pressure to the work site, and they are decompressed to surface pressure only once, at the end of their tour of duty. By limiting the number of decompressions in this way, the risk of decompression sickness is significantly reduced at the cost of exposing the diver to other hazards associated with living under high pressure for prolonged periods.[28][29] Saturation diving is an example of substitution of a hazard expected to present a lower risk than surface oriented diving for the same set of operations.

Atmospheric pressure divingEdit

The Newtsuit has fully articulated, rotary joints in the arms and legs. These provide great mobility, while remaining largely unaffected by high pressures.

Atmospheric pressure diving isolates the diver from the ambient pressure of the environment by using an atmospheric diving suit (ADS), which is a small one-person articulated submersible of anthropomorphic form which resembles a suit of armour, with elaborate pressure joints to allow articulation while maintaining an internal pressure of one atmosphere. The ADS can be used for very deep dives of up to 2,300 feet (700 m) for many hours, and eliminates the majority of physiological dangers associated with deep diving; the occupant need not decompress, there is no need for special gas mixtures, and there is no danger of decompression sickness or nitrogen narcosis, and a drastically reduced risk of oxygen toxicity.[40] Hard suit divers do not even need to be skilled swimmers, as swimming is not yet possible in atmospheric suits. The current generation of atmospheric suits are more ergonomically flexible than earlier versions, but are still very limited in personal mobility and dexterity compared to an ambient pressure diver. Use of an atmospheric suit may be considered as substituting a relatively low risk of crushing for a higher risk of decompression sickness and barotrauma, by using the suit as an engineered barrier between the diver and the hazards.

Remotely operated underwater vehiclesEdit

ROV at work in an underwater oil and gas field. The ROV is operating a subsea torque tool (wrench) on a valve on the subsea structure.

A remotely operated underwater vehicle (ROV) is an unoccupied, highly maneuverable, tethered mobile underwater device operated by a crew aboard a base platform. They are linked to the base platform by a neutrally buoyant tether or, often when working in rough conditions or in deeper water, a load-carrying umbilical cable is used along with a tether management system (TMS). The purpose of the TMS is to lengthen and shorten the tether so the effect of cable drag where there are underwater currents is minimized. The umbilical cable is an armored cable that contains a group of electrical conductors and fiber optics that carry electric power, video, and data signals between the operator and the TMS. Where used, the TMS then relays the signals and power for the ROV down the tether cable. Most ROVs are equipped with at least a video camera and lights. Additional equipment is commonly added to expand the vehicle’s capabilities. These may include sonars, magnetometers, a still camera, a manipulator or cutting arm, water samplers, and instruments that measure water clarity, water temperature, water density, sound velocity, light penetration, and temperature.[41] ROVs are commonly used in deep water industries such as offshore hydrocarbon extraction, where they can carry out many tasks previously requiring diver intervention. ROVs may be used together with divers, or without a diver in the water, in which case the risk to the diver associated with the dive is eliminated altogether.

Administrative controlsEdit

Administrative controls include medical screening, planning and preparation for diving and training in essential skills.

Legislation, codes of practice and organisational proceduresEdit

  • Exemptions from regulations for emergency public safety diving - applicable in some jurisdictions only where there is a possibility of rescuing a survivor.

Medical screeningEdit

Hand held spirometer used to test lung function, one of the standard medical screening tests for divers.
PC based spirometer output

Fitness to dive, (also medical fitness to dive), is the medical and physical suitability of a diver to function safely in the underwater environment using underwater diving equipment and procedures. Depending on the circumstances it may be established by a signed statement by the diver that he or she does not suffer from any of the listed disqualifying conditions and is able to manage the ordinary physical requirements of diving, to a detailed medical examination by a physician registered as a medical examiner of divers following a procedural checklist, and a legal document of fitness to dive issued by the medical examiner.

The most important medical is the one before starting diving, as the diver can be screened to prevent exposure when a dangerous condition exists. The other important medicals are after some significant illness, where medical intervention is needed there and has to be done by a doctor who is competent in diving medicine, and can not be done by prescriptive rules.[42]

Psychological factors can affect fitness to dive, particularly where they affect response to emergencies, or risk taking behaviour. The use of medical and recreational drugs, can also influence fitness to dive, both for physiological and behavioural reasons. In some cases prescription drug use may have a net positive effect, when effectively treating an underlying condition, but frequently the side effects of effective medication may have undesirable influences on the fitness of diver, and most cases of recreational drug use result in an impaired fitness to dive, and a significantly increased risk of sub-optimal or inappropriate response to emergencies.

Pre-dive preparation and planningEdit

Dive planning is the process of planning an underwater diving operation. The purpose of dive planning is to increase the probability that a dive will be completed safely and the goals achieved.[43] Some form of planning is done for most underwater dives, but the complexity and detail considered may vary enormously.[44]

Professional diving operations are usually formally planned and the plan documented as a legal record that due diligence has been done for health and safety purposes.[19][20] Recreational dive planning may be less formal, but for complex technical dives, can be as formal, detailed and extensive as most professional dive plans. A professional diving contractor will be constrained by the code of practice, standing orders or regulatory legislation covering a project or specific operations within a project, and is responsible for ensuring that the scope of work to be done is within the scope of the rules relevant to that work.[19] A recreational (including technical) diver or dive group is generally less constrained, but nevertheless is almost always restricted by some legislation, and often also the rules of the organisations to which the divers are affiliated.[44]

The planning of a diving operation may be simple or complex. In some cases the processes may have to be repeated several times before a satisfactory plan is achieved, and even then the plan may have to be modified on site to suit changed circumstances. The final product of the planning process may be formally documented or, in the case of recreational divers, an agreement on how the dive will be conducted. A diving project may consist of a number of related diving operations.

A hazard identification and risk assessment procedure is the basis of a large part of dive planning. The hazards to which the divers will be exposed are identified, and the level of risk associated with each is evaluated. If the risk is deemed to be excessive, control methods will be applied to reduce the risk to an acceptable level, and where appropriate, further controls will be set in place to mitigate the effects if an incident does occur.[19][26]

A documented dive plan may contain elements from the following list:[43]

  • Overview of Diving Activities
  • Schedule of Diving Operations
  • Specific Dive Plan Information
  • Budget

Following the planEdit

A basic strategy of risk management is to plan an operation and then conduct it, as far as reasonably practicable, according to the plan. If this is done, the risks will have been assessed and the equipment chosen will be suitable. Deviation from the plan brings in unassessed factors. In professional diving where a diving operation plan must be drawn up, variation from the plan generally requires reassessment of risk and recording of the deviation and any measures that were found necessary to manage the changed circumstances.[19][7] In recreational diving, the diver is free to plan or not, and to change the plan on whim, but technical diving certification agencies generally encourage divers to "plan the dive and dive the plan", as this is considered good practice for safety, and is the same strategy used by professionals.[45][46]

Standard operating procedures and codes of practice are used to reduce the amount of detail required in dive planning. These documents provide much of the necessary detail of how frequently encountered tasks should be performed, using methods which have been tested and found to be effective, efficient and acceptably safe. When standard procedures are used, it is not necessary to detail those procedures in the dive plan, as the team members should be familiar with them already.

Standard operating procedures are the procedures identified by the diving contractor as the recommended or required way of performing a range of routine activities and codified in a document. Following SOPs is generally a condition of employment for the diving team, and the provision of SOPs may be a requirement of health and safety regulations. The document is often called the operations manual, diving manual or something similar. For example, the US Navy Diving Manual,[47] NOAA Diving Manual,[43]

Codes of Practice are procedures identified by a larger population as preferred methods for a similar range of activities. They may be a set of industry best practice recommendations, such as the IMCA Code of Practice for offshore diving,[48] a government regulated set of recommendations,[26] or a regulated set of requirements which must be followed.[49]

Training, practice and experienceEdit

To make effective use of standard procedures, the diving team must be competent in the procedures, particularly the diving and emergency skills. These skill sets are the basis of the standard operating procedures, and have themselves been standardised to a degree where they are largely internationally accepted, and are portable between organisations without requiring much re-learning.[50] A large part of the variation is connected to different equipment and equipment configurations, and operators need to become familiar with new equipment under controlled conditions before operating in the field. This is the realm of formal training for diving certification, which is normally done by registered diving schools and instructors, and equipment rating and familiarisation, which may be done by the employer or by diver training schools, depending on the risks and complexity of the training, and how much unfamiliar equipment is involved. For example, basic operation of an unfamiliar mode of life support equipment like surface supplied diving or a rebreather is likely to be learned at a school, while the details of operating a different model of non-diving equipment, like a hydraulic bolt-tensioner is likely to be learned from a skilled operator of that equipment, or at a manufacturer's familiarisation workshop. It is common practice to record such training and the associated assessment in the diver's logbook, as well as any certificate which may be issued.[19][49]

Appropriate response to minor life-support equipment malfunctions which can be corrected by the diver is very important for diving safety. The diver is expected to deal with a number of small problems promptly and correctly before the situation escalates. Dealing with such problems as a dislodged or flooded mask, or free-flowing regulator, or correctable buoyancy fault should be done before the situation deteriorates to an emergency. A basic understanding of the physics and physiology of diving should give the diver the ability to predict the consequences of possible responses to unfamiliar contingencies. A diver with inadequate understanding may respond inappropriately to an emergency outside of their training and experience, which though unlikely remains possible. Repeated practice beyond initial competence of standard responses to the more likely contingencies develops a "muscle memory" response, which helps the diver perform the correct response under stress, and when more than one problem occurs simultaneously. It is possible to never experience one of these problems, and some divers may never need the skills in practice, but divers who do not practice the skills are more likely to be overtaken by circumstances if something does go wrong. The practice of stress-training in benign conditions, where the diver is task loaded with an increasing level of simulated problems and must deal with them, is thought to develop the diver's confidence in their ability to manage an emergency effectively, which may give them the ability to avoid panic and continue to respond usefully to the situation, giving a better chance of survival.[citation needed]

Continued occasional practice of emergency procedures after initial training ensures that the skills are not lost due to lack of use. Divers who have not practiced their skills for several months or years are at higher risk of accidents when first returning to the water, and refresher courses and checkout dives in benign conditions are available to get the skills back to standard and thereby reduce risk of an accident.[citation needed]

Personal protective equipmentEdit

Surface-supplied commercial diving equipment on display at a trade show

A large part of personal diving equipment can be classified as personal protective equipment.

  • Breathing apparatus
  • Exposure suits - Wetsuits, Dry suits, and hot-water suits provide thermal protection to the diver. Where thermal protection is not necessary, divers may wear overalls as protection against stings, cuts and abrasions which could be caused by contact with the environment.
  • Diving helmets provide thermal protection and impact protection for the diver's head. Neoprene hoods provide protection against high volume sound, often produced by the breathing apparatus, but also from other sources.
  • Gloves and boots serve similar functions underwater to those they provide at the surface.


It is not usually possible to entirely eliminate risk to a diver, and where there is sufficient residual risk it is necessary to provide mitigation for the foreseeable consequences of an incident occurring.

Emergency planningEdit

Professional divers may be legally obliged to make plans and provide equipment and personnel to manage reasonably foreseeable accidents.[20][26][22] This can include a requirement for the contractor's operations manual to include instructions for the members of a dive team in the event of any of several classes of emergency, which may include managing an injured or unconscious diver underwater or at the surface, recovery of such diver from the water, provision of first aid, provision of recompression therapy in the case of decompression illness, communication with emergency services and the contracted diving medical practitioner on standby, decontaminating divers and emergency evacuation of the worksite. Specific checklists or flowcharts may be provided with emergency plans where they may be useful to ensure correct sequencing and that no critical stage is omitted.[26]

Recreational dive leaders such as divemasters and instructors may also be required to produce emergency plans for a dive site or area The contents may vary depending on location and access to assistance, and would contain the information necessary to handle reasonably foreseeable emergencies. Content may include contact details for local emergency medical care, a casualty evacuation plan, how to arrange emergency recompression and other diving specific emergencies, and what assistance can be expected from the local emergency services.[51]

Recreational, and particularly technical divers are recommended by certification agencies to have some form of emergency plan in case something goes wrong. The international organisation Divers Alert Network provides a hotline service giving advice on diving emergencies, and in the case of members, authorising and arranging emergency medical assistance and evacuation.[52]

Training to manage foreseeable incidentsEdit

A large part of diver training is in the emergency procedures known to be effective at managing the most common incidents which could be life-threatening if not manages promptly and appropriately. The amount of overlearning and the level of skill required for certification varies considerably with the training standard for different certifications, but minimum standards for recreational diver and instructors have been established by the International Standards Organisation (ISO),[53] and national and international standards for professional divers have been published by various controlling bodies. All of these standards include management of the most frequent diving emergencies by application of well established techniques, though not always by identical procedures.

Emergency and rescue: procedures, personnel and equipmentEdit

The diver should be able to manage a reasonably foreseeable and immediately life-threatening emergency unaided as there can be no guarantee that someone else will be near enough to help, will notice and will respond appropriately in time. Lower priority threats can be managed by teamwork and resource sharing. Since most of the critical safety skills for diving are not intuitive, nor associated with activities the diver is likely to have learned for other purposes, diver safety is enhanced by comprehensive training and frequent exercise of safety critical skills.

One of the standard ways to help the diver to manage an emergency is to provide another diver ready to assist. In professional diving this is known as the stand-by diver, and in the case of bell diving, the bellman. In recreational diving, buddy diving and team diving procedures are intended to provide similar benefits, where each diver in a pair or team is stand-by diver to the other or others. This system can be effective when the divers are all adequately skilled, fit and dedicated to the task, as has been shown in many deep dives and cave penetrations. The buddy diver is less effective when insufficiently skilled, inattentive, or unfit. Buddy and team diving procedures impose a significant additional task loading on the divers, particularly in adverse conditions, such as darkness, low visibility, confined spaces, strong currents, cold water and unfamiliarity with each other's equipment and habits. Nevertheless, many recreational training agencies maintain that buddy diving is intrinsically safer than solo diving.

Stand-by diverEdit

The stand by diver's job is to wait until something goes wrong, and then be sent in to sort it out. For this reason a stand by diver should be one of the best divers on the team regarding diving skills and strength, but does not have to be expert at the work skills for the specific job. The standby diver is usually required to remain ready for deployment at very short notice during the entire working dive, and will usually be fully dressed ready to deploy, except for helmet or mask. When deployed, the standby diver will normally follow the umbilical of the diver who is in trouble, as unless it has been severed, it will reliably lead to the correct diver. The standby diver must maintain communications with the supervisor throughout the dive and is expected to give a running commentary of progress so that the supervisor and surface crew know as much as possible what is happening and can plan accordingly, and must take the necessary steps to resolve incidents, which may involve supply of emergency air or locating and rescuing an injured or unconscious diver. In bell diving, the bellman is the standby diver, and may have to recover a distressed diver to the bell and give first aid if necessary and possible.[54][7] The standby diver and working diver are generally interchangeable, unless specialised skills are required for the task of the specific dive, and professional divers are trained in rescue procedures appropriate to the equipment they are qualified to use.[50] Rescue skills are not included in the minimum training standards for entry level recreational divers according to RSTC and ISO publications.[55][53]

Buddy or team diversEdit

A Navy buddy diver team checking their gauges together

A buddy or team diver is simultaneously the diver and the standby diver for the buddy or other members of the team. Since it is increasingly difficult to keep track of a larger number of divers, and the benefits of larger groups are small, teams are usually of three divers. Larger groups are generally split up into three diver teams and pairs.

When using the buddy system, members of the group dive together and co-operate with each other, so that they can help or rescue each other in the event of an emergency.[56] This is most effective if the divers are both competent in all the relevant skills and are sufficiently aware of the situation to be able to respond in time, which is a matter of both attitude and competence.[57]

In recreational diving, a pair of divers is usually the best combination in buddy diving; with threesomes, one of the divers can easily lose the attention of the other two. The system is likely to be effective in mitigating out-of-air emergencies, non-diving medical emergencies and entrapment in ropes or nets. When used with the buddy check it can help avoid the omission, misuse and failure of diving equipment.

In technical diving activities such as cave diving, threesomes are considered an acceptable practice.[58] This is usually referred to as team diving to distinguish it from buddy diving in pairs.

When professional divers dive as buddy pairs their responsibility to each other is specified as part of the standard operating procedures, code of practice or governing legislation.

Analysis of incidentsEdit

The incidents that are documented and analysed are usually those which lead to serious injury or death. Valuable understanding of the risks of diving can be derived from analysis of such incidents, but they are a small fraction of the potential learning opportunities because for each documented accident there are estimated to be possibly hundreds of undocumented near-misses.[25]

Cardiac events account for approximately 28% of diving fatalities. Approximately 60% of these had signs or symptoms that could later be identified as cardiac related before or during the dive, but chose to continue to dive.[59]

DAN data suggest that limited experience is associated with diving fatalities, with entry level divers and divers and divers certified for less than a year or with limited experience at the highest risk. Divers with very few dives in the previous year, and divers with a very large number of dives (>300) in the previous year are also high risk groups. In the first case due to lack of practice, and in the second case due to overconfidence in their proficiency.[59]

A high body mass index may correlate to the risk of a diving emergency becoming a fatality, which may indicate a lack of exercise tolerance that may reduce the ability to successfully manage an emergency.[59]

Root cause analysis of incidents shows four phases commonly present during the sequence of events leading to a fatality. These are: the trigger, the disabling agent, the disabling injury and the cause of death. Triggering events in decreasing order of frequency include running out of breathing gas, entrapment, equipment problems, rough conditions, trauma, buoyancy problems and breathing an inappropriate gas.[59]

Very few out-of gas incidents are a consequence of equipment malfunctions. Most divers might have survived if they managed their gas supply correctly. Ineffective gas management puts the divers, their buddies and any other diver in the vicinity at risk. Most entrapment fatalities involved an overhead environment, where the diver was unable to make a direct ascent to the surface. While in some circumstances it is possible to enter an overhead environment by accident, it is almost always intentional or due to lack of attention to the surroundings.[59]

The majority of the equipment failures leading to fatalities were not due to faults inherent in the equipment, but to improper use, incorrect configuration, poor maintenance or unfamiliarity with the equipment. It is more often the diver’s response to equipment malfunction than the malfunction itself which results in injury or death.[59][60]

The triggering event, if not managed effectively, leads to a harmful action that exacerbates the situation, and the most commonly identified harmful action is an emergency ascent, which shows that most of the divers chose to try to escape to the surface instead of dealing with the problem underwater. The next stage of the cascade/sequence is an incapacitating injury, which prevents the diver from further efforts to control the incident, most commonly asphyxia, followed by the official cause of death as the final stage. This is usually found to be drowning.[59]

In an analysis of recreational closed circuit rebreather deaths between 1998 and 2010, a somewhat arbitrary risk rating for each dive was allocated:[4]

  • low risk, for open water dives to depths not exceeding 40 metres (130 ft), where all checks and tests were done
  • moderate risk, for penetration dives to depths not exceeding 40 metres (130 ft), where all checks and tests were done,
  • intermediate risk, open water dives to depths between 40 metres (130 ft) and 150 metres (490 ft), where all checks and tests were done
  • high risk, for penetration dives to depths between 40 metres (130 ft) and 150 metres (490 ft), where all checks and tests were done,
  • extreme risk, for all dives to depths exceeding 150 metres (490 ft), or where checks and tests were not done, or alarms were ignored.

When applied to the database this indicated that about two thirds of the deaths appear to be associated with high risk behaviour.[4]

The annual rebreather death rate of approximately 4 per 10 000 dives is approximately 10 times the rate for non-technical recreational scuba diving.[4]


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