Diving hazards

Exterior of a deck decompression chamber

The prolonged exposure to breathing gases at high partial pressure will result in increased amounts of non-metabolic gases, usually nitrogen and/or helium, (referred to in this context as inert gases) dissolving in the bloodstream as it passes through the alveolar capillaries, and thence carried to the other tissues of the body, where they will accumulate until saturated. This saturation process has very little immediate effect on the diver. However, when the pressure is reduced during ascent, the amount of dissolved inert gas that can be held in stable solution in the tissues is reduced. This effect is described by Henry's Law.[5]

As a consequence of the reducing partial pressure of inert gases in the lungs during ascent, the dissolved gas will be diffused back from the bloodstream to the gas in the lungs and exhaled. The reduced gas concentration in the blood has a similar effect when it passes through tissues carrying a higher concentration, and that gas will diffuse back into the bloodsteam, reducing the loading of the tissues.[5] As long as this process is gradual, the tissue gas loading in the diver will reduce by diffusion and perfusion until it eventually re-stabilises at the current saturation pressure. The problem arises when the pressure is reduced more quickly than the gas can be removed by this mechanism, and the level of supersaturation rises sufficiently to become unstable. At this point, bubbles may form and grow in the tissues, and may cause damage either by distending the tissue locally, or blocking small blood vessels, shutting off blood supply to the downstream side, and resulting in hypoxia of those tissues.[5]

Divers inside a recompression chamber

This effect is called decompression sickness[4] or 'the bends', and must be avoided by reducing the pressure on the body slowly while ascending and allowing the inert gases dissolved in the tissues to be eliminated while still in solution. This process is known as "off-gassing", and is done by restricting the ascent (decompression) rate to one where the level of supersaturation is not sufficient for bubbles to form or grow. This level is only known statistically, and may vary for reasons which are not well understood. The level of supersaturation limited by controlling the speed of ascent and making periodic stops to allow gases to be eliminated by respiration. The procedure of making stops is called staged decompression, and the stops are called decompression stops. Decompression stops that are not computed as strictly necessary are called safety stops, and reduce the risk of bubble formation further at the cost of a longer ascent time, greater gas consumption and in many cases greater exposure to other hazards. Dive computers or decompression tables are used to determine a relatively safe ascent profile, but are not completely reliable. There remains a statistical possibility of decompression bubbles forming even when the guidance from tables or computer has been followed exactly.[5]

Decompression sickness must be treated as soon as practicable. Definitive treatment is usually recompression in a recompression chamber with hyperbaric oxygen treatment. Exact details will depend on severity and type of symptoms, response to treatment, and the dive history of the casualty. Administering enriched-oxygen breathing gas or pure oxygen to a decompression sickness stricken diver on the surface is the definitive form of first aid for decompression sickness, although death or permanent disability may still occur.[6]

Nitrogen narcosis or inert gas narcosis is a reversible alteration in consciousness producing a state similar to alcohol intoxication in divers who breathe high-pressure gas containing nitrogen or other potentially narcotic gas at raised partial pressures.[4] The mechanism is similar to that of nitrous oxide, or "laughing gas," administered as anaesthesia. Being "narced" can impair judgement and make diving considerably more dangerous. Narcosis starts to affect some divers at about 66 feet (20 m) on air. At this depth, narcosis often manifests itself as a slight giddiness. The effects increase with an increase in depth. Almost all divers will notice the effects by 132 feet (40 m). At this depth divers may feel euphoria, anxiety, loss of coordination and/or lack of concentration. At extreme depths, a hallucinogenic reaction, tunnel vision or unconsciousness can occur. Jacques Cousteau famously described it as the "rapture of the deep".[7] Nitrogen narcosis occurs quickly and the symptoms typically disappear equally quickly during the ascent, so that divers often fail to realise they were ever affected. It affects individual divers at varying depths and conditions, and can even vary from dive to dive under identical conditions. Diving with trimix or heliox reduces the effects, which are proportional to the partial pressure of nitrogen, and probably oxygen, in the breathing gas.

Oxygen toxicity occurs when the tissues are exposed to an excessive combination of partial pressure (PPO₂) and duration.[4] In acute cases it affects the central nervous system and causes a seizure, which can result in the diver losing consciousness, spitting out their regulator and drowning. While the exact limit is not reliably predictable, and is affected by carbon dioxide levels, it is generally recognised that central nervous system oxygen toxicity is preventable if one does not exceed an oxygen partial pressure of 1.4 bar.[8] For deep dives – generally past 180 feet (55 m), divers use "hypoxic blends" containing a lower percentage of oxygen than atmospheric air. A less immediately threatening form known as pulmonary oxygen toxicity occurs after exposures to lower oxygen partial pressures for much longer periods than generally encountered in scuba diving, but is a recognised problem in saturation diving.

High-pressure nervous syndrome (HPNS – also known as high-pressure neurological syndrome) is a neurological and physiological diving disorder that results when a diver descends below about 500 feet (150 m) using a breathing gas containing helium. The effects experienced, and the severity of those effects, depend on the rate of descent, the depth and percentage of helium.[4]

"Helium tremors" were first widely described in 1965 by Royal Navy physiologist Peter B. Bennett, who also founded the Divers Alert Network.[4][9] Russian scientist G. L. Zal'tsman also reported on helium tremors in his experiments from 1961. However, these reports were not available in the West until 1967.[10]

The term high-pressure nervous syndrome was first used by Brauer in 1968 to describe the combined symptoms of tremor, electroencephalography (EEG) changes, and somnolence that appeared during a 1,189-foot (362 m) chamber dive in Marseille.[11]

Most regulator malfunctions involve improper supply of breathing gas or water leaking into the gas supply. There are two main gas supply failure modes, where the regulator shuts off delivery, which is extremely rare, and free-flow, where the delivery will not stop and can quickly exhaust a scuba supply.[13]

• Loss of breathing gas - leaks, hose ruptures, free-flows, valve roll-off
• Malfunctions of demand valve - free-flow, freezing, leaks, blockages, sticking parts, excessive work of breathing, other

• Buoyancy and weighting
• Thermal insulation and heating
• Communications equipment
• Loss of mask
• Flooding of FFM or helmet
• Loss of fins
• Breaking of guideline
• Failure of instrumentation - Dive computer, timer, depth gauge, SPG
• Failure of lights

Dry suit for reducing exposure

Water conducts heat from the diver 25 times[14] more effectively than air, which can lead to hypothermia even in mild water temperatures.[4] Symptoms of hypothermia include impaired judgment and dexterity,[15] which can quickly become deadly in an aquatic environment. In all but the warmest waters, divers need the thermal insulation provided by wetsuits or drysuits.[16]

In the case of a wetsuit, the suit is designed to minimise heat loss. Wetsuits are usually made of foamed neoprene that has small closed bubbles, generally containing nitrogen, trapped in it during the manufacturing process. The poor thermal conductivity of this expanded cell neoprene means that wetsuits reduce loss of body heat by conduction to the surrounding water. The neoprene, and to a larger extent the nitrogen gas, function as an insulator. The effectiveness of the insulation is reduced when the suit is compressed due to depth, as the nitrogen filled bubbles are then smaller and the compressed gas conducts heat better. The second way in which wetsuits can reduce heat loss is to trap the water which leaks into the suit. Body heat then heats the trapped water, and provided the suit is reasonably well-sealed at all openings (neck, wrists, ankles, zippers and overlaps with other suit components), this water remains inside the suit and is not replaced by more cold water, which would also take up body heat, and this helps reduce the rate of heat loss. This principle is applied in the "Semi-Dry" wetsuit.[17]

Spring suit (short legs and sleeves) and steamer (full legs and sleeves)

A dry suit functions by keeping the diver dry. The suit is waterproof and sealed so that water cannot penetrate the suit. Special purpose undergarments are usually worn under a dry suit to keep a layer of air between the diver and the suit for thermal insulation. Some divers carry an extra gas bottle dedicated to filling the dry suit, which may contain argon gas, because it is a better insulator than air.[18] Dry suits should not be inflated with gases containing helium as it is a good thermal conductor.

Drysuits fall into two main categories:

• Membrane or Shell dry suits are usually a trilaminate or coated textile construction. The material is thin and not a very good insulator, so the insulation is provided by the air trapped in the undersuit.[19]
• Neoprene drysuits have a similar construction to wetsuits; these are often considerably thicker (7–8 mm) and have sufficient inherent insulation to allow a lighter-weight undersuit (or none at all); however on deeper dives the neoprene can compress to as little as 2 mm, losing some of its insulation. Compressed or crushed neoprene may also be used (where the neoprene is pre-compressed to 2–3 mm) which avoids the variation of insulating properties with depth. These drysuits function more like a membrane suit.[19]

Diving suits also help prevent the diver's skin being damaged by rough or sharp underwater objects, marine animals, coral, or metal debris commonly found on shipwrecks.

Some marine animals can be hazardous to divers. In most cases this is a defensive reaction to contact with, or molestation by the diver.

• Sharp hard coral skeleton edges can lacerate or abrade exposed skin, and contaminate the wound with coral tissue and pathogenic microorganisms.
• Stinging hydroids can cause skin rash, local swelling and inflammation by contact with bare skin.
• Stinging jellyfish can cause skin rash, local swelling and inflammation, sometimes extremely painful, occasionally dangerous or even fatal
• Stingrays have a sharp spine near the base of the tail which can produce a deep puncture or laceration that leaves venom in the wound as a result of a defensive reaction when disturbed or threatened.
• Some fish and invertebrates such as lionfish, stonefish, crown-of-thorns starfish, and some sea urchins have spines which can produce puncture wounds with venom injection. These are often extremely painful and may be fatal in rare cases. Usually caused by impact with the stationary animal.
• The venomous blue-ringed octopus may on rare occasions bite a diver.
• Lacerations by shark teeth can involve deep wounds, loss of tissue and amputation, with major blood loss. In extreme cases death may result.This may result from attack or investigation by a shark with bites. The risk depends on location, conditions, and species. Most sharks do not have suitable teeth for predation on large animals, but may bite in defence when startled or molested.
• Crocodiles can injure by lacerations and punctures by teeth, brute force tearing of tissues. and the possibility of drowning.
• The tropical Indo-Pacific Titan triggerfish is very territorial during breeding season and will attack and bite divers.
• Very large groupers have been known to bite divers, resulting in bite wounds, bruising and crushing injuries. This has been linked to divers feeding fish.[20][21][22][23]
• Electric shock is the defence mechanism of electric rays, in some tropical to warm temperate seas.
• Venomous sea snakes are a minor hazard in some regions.[24]

Scuba divers may get lost in wrecks and caves, under ice or inside complex structures where there is no direct route to the surface, and be unable to identify the way out, and may run out of breathing gas and drown. Getting lost is often a result of not using a distance line, or losing it in darkness or bad visibility, but sometimes due to the line breaking. Inappropriate response due to claustrophobia and panic is also possible.

Commonly referred to by professional divers as delta-p (δp or ΔP), these hazards are due to a pressure difference causing a flow, which if restricted, will result in a large force on the obstruction to the flow. The most dangerous pressure differentials are those causing outflow from the region occupied by a diver and any attached equipment, as the resultant forces will tend to force the diver into the outflow stream, which may carry the diver or equipment such as the umbilical into a confined space such as intake ducting, drain openings, sluice gates or penstocks, and which may be occupied by moving machinery such as impellers or turbines. When possible, a lockout-tagout system is used to disable the hazard during diving operations, or the divers umbilical is restrained to prevent the diver from getting into the danger zone. This method is used when it is not practicable to shut down equipment, like the bow thrusters on a dynamically positioned diving support vessel, which must be operating during the dive to keep the diver in the right place. Scuba divers are particularly vulnerable to delta-p hazards, and should generally not dive in areas where a delta-p hazard is suspected to exist.

Loss of visibility in itself is not harmful to the diver, but can increase the risk of an adverse incident due to other hazards if the diver cannot avoid or manage them effectively. The most obvious of these is the potential to get lost in an environment where the diver cannot simply ascend to the surface, such as the inside of a wreck or cave, or underneath a large ship. The risk is much greater for scuba divers as surface supplied divers have a secure breathing gas supply, and can follow the umbilical out of the overhead environment without extreme urgency. Loss of visibility can also allow the diver to approach other hazards such as pinch points and unexpected delta-p hazards. Scuba divers who enter overhead environments can take precautions to mitigate the effects of the two most common causes of loss of visibility, which are siltout and dive light failure. To compensate for dive light failure the standard procedure is to carry at least three lights, each of which is sufficient for the planned dive, and siltout can be managed by ensuring a continuous and correctly marked guideline to the exit, and staying close to it at all times.

Some physical and psychological conditions are known or suspected to increase the risk of injury or death in the underwater environment, or to increase the risk of a stressful incident developing into a serious incident culminating in injury or death. Conditions which significantly compromise the cardiovascular system, respiratory system or central nervous system may be considered absolute or relative contraindications for diving, as are psychological conditions which impair judgement or compromise the ability to deal calmly and systematically with deteriorating conditions which a competent diver should be able to manage.[25]

Safety of underwater diving operations can be improved by reducing the frequency of human error and the consequences when it does occur.[26] Human error can be defined as an individual's deviation from acceptable or desirable practice which culminates in undesirable or unexpected results.[27] 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.[26]

• 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.[28]
• 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.