• Asphyxia by drowning.
• Near drowning is the survival of a drowning event involving unconsciousness or water inhalation and can lead to serious secondary complications, including death, after the event.^[3][4]

• Avoid out of air emergencies underwater.
• Use of a redundant emergency breathing gas supply^[5]
• Provide appropriate buoyancy.
• Avoid or prevent accidents resulting in unconsciousness.
• Use of a full face mask or diving helmet to protect the airway.^[6]
• Use of surface-supplied diving equipment with voice communications.^[7][5]
• Adequate swimming skills and fitness for the circumstances.^[8]
• Use of snorkel when appropriate.^[8]
• Lifejackets that hold the wearer's face above the water may be worn when appropriate.
• Availability of a stand-by diver or reliable dive buddy to assist.^[5]
• Competence in recovering and clearing a dislodged scuba regulator.^[8]
• Use of an alternative regulator which can be easily reached if the dislodged primary is difficult to reach.^[9]

• Exudation of liquid into the lungs (pulmonary edema) over the hours following aspiration of liquid, which reduces the ability to exchange air and can lead to a person "drowning in their own body fluid".
• Aspiration of vomit can have a similar effect.

• Correct maintenance, preparation and pre-use procedures and checks.^[10]

• Correct use of recommended procedures and checklists when preparing for use.^[11]
• Calibration of oxygen monitoring instruments^[11][12]

• Adequate and redundant instrumentation for monitoring gas quality during use.^[12]
• Constant vigilance during use.^[12]
• Adequate bailout facilities in case of failure.^[12]
• Adequate training in the use of rebreathers in general and the specific model.^[12]

• Gas requirements planned to suit the intended dive profile.^[9]
• Use of a travel mix for descent and a decompression mix for ascent through the depth range where the bottom gas is hypoxic.
• Safe procedures used for gas changes.^[14]

• Gas switches planned and executed at appropriate depths.^[13]
• Depth and ascent rate accurately monitored and controlled.
• Clear and unambiguous identification of cylinder gases.^[9]

• Adequate training in the use of mixed gases.^[9]

• Routine periodical inspection and testing of cylinders.^[17][18]
• Analysis of oxygen fraction of gas before use, particularly if cylinder has been stored for a long time.

• Closing and jamming of the cylinder valve by rolloff on something overhead (rotation of the knob to close the valve by friction when dragged along in contact with a surface), or by kelp when pushing through dense kelp.^[9]
• Rupture of a bursting disc overpressure protection on a cylinder valve (a thin metal membrane calibrated to fail if the pressure exceeds a safe value for the cylinder).^[18]
• Rupture of a regulator hose or loss of the end component, leaving an open hose end.^[18]
• Unrecoverable free flow of a second stage (valve jammed open, allowing gas to escape even when not needed by the diver).
• Freezing of a first stage regulator, locking the valve mechanism open, and consequent free flow of the demand valve due to excessive interstage pressure.
• O-ring failure at the connection of a regulator to a cylinder valve.

• Appropriate maintenance and servicing of equipment.^[5]
• Inspection of the external condition, and testing of the function of equipment before use.^[5]
• Use only of equipment in good working condition.^[5]
• Connection and mounting of equipment to minimise risk of damage.
• Avoidance of damage to equipment during dives.
• Use of two fully independent breathing gas supplies.^[19]
• Use of bailout gas supply.^[5]
• The buddy system, when correctly followed, allows the diver's buddy to supply breathing gas in an emergency.^[9]
• "H" or "Y" type cylinder valves or manifolded twin cylinders with two cylinder valves allow the dysfunctional supply to be closed to prevent total loss, and the other regulator to be used for the remaining gas supply. (frequently used to mitigate regulator freezing in cold water)
• Dual independent cylinders ensure that if one cylinder supply fails there is another available.^[5]
• Use of DIN connections can reduce the risk of catastrophic O-ring failure.^[20]
• Emergency free ascent may be possible, and is generally more survivable than drowning.

• Adequate training of divers.^[9]
• Disciplined attitude and situational awareness during dives.^[9]
• Use of reserve valve.^[22]
• Use of surface supplied diving equipment.^[7]
• Use of bailout gas supply.^[5][7]
• The buddy system, when correctly followed, allows the diver's buddy to supply breathing gas in an emergency.^[9]

• Situational awareness underwater.
• Use of a diver's net cutter, or dive tool/knife to cut free of entanglement.
• Carrying sufficient gas in reserve to allow a reasonable amount of time to deal with emergencies.
• Use of surface supplied breathing equipment.^[7]

• Appropriate safety equipment and procedures to avoid getting lost (cave lines).^[23]
• Specific training for overhead diving. See cave diving and wreck diving.^[23]
• Assess stability of underwater structures and avoid entry if a structure is unstable.

• Appropriate maintenance and servicing of equipment.^[18]
• Inspect external condition and test function before use. (specifically test the seal of exhaust valves and possible leaks in the second stage casing and mouthpiece before opening the cylinder valve).^[24]
• Use equipment only if it is in good working condition.^[5]
• Use of alternative air source if DV breathes wet during dive.
• The technique of inhaling slowly and using the tongue to deflect spray particles may be effective as a temporary mitigation.

• Adequate precautions to ensure that intake is in uncontaminated air when operating breathing air compressors.^[25]
• Periodical air quality testing of compressors.
• Use of compressor output filter containing "Hopcalite" catalyst to convert possible carbon monoxide contamination to less hazardous carbon dioxide.
• Test air quality before use (portable carbon monoxide analysers are available and may be worth using in places where air quality is questionable).
• Air contaminated with carbon monoxide is often contaminated by substances having a smell or taste. Air smelling or tasting of exhaust fumes should not be breathed.

• Adequate maintenance of the compressor.
• Use of correct oil rated for breathing air compressor lubrication.^[25]
• Ensure compressor running temperature is within manufacturer's specifications.
• Ensure adequate supply of cooling air to compressor.
• Compressor should not be run when ambient temperatures exceed manufacturer's limits.

• Use of a suitable separators and air filter after compression.^[25]

• Monitor and drain separators and change filters as necessary.

• Periodic testing of delivered air quality.

• Smell and taste can distinguish oil contamination in many cases.
• Passing a metered quantity of the air through an absorbent filter paper may show up an oil deposit.
• Directing air flow onto a clean mirror surface or glass sheet may show gross contamination.

• Re-inhaling carbon dioxide-laden exhaled gas due to excessive dead space in breathing apparatus.
• Shallow breathing—not exchanging sufficient air during a breathing cycle.

• Minimise the volume of any enclosed spaces through which the diver breathes. For example, this can happen with diving with a large "bubblehead" helmet.
• Avoiding breathing shallow (low volume) breaths.

• Adequate maintenance of rebreathers.
• Correct packing and assembly of scrubber canisters.^[29]
• Pre-use inspection and testing of rebreathers using an appropriate checklist.
• Use of correct scrubber absorbent material.
• Use of absorbent that is of good working quality.
• Discard absorbent after use.
• Use of carbon dioxide monitoring instruments.
• Adequate training in the recognition of hypercapnia before using a rebreather.
• Bail-out to open circuit if carbon dioxide levels get too high.

• Siting the compressor air intake in an area of fresh air and ducting it to the compressor.
• Passing intake air through a carbon dioxide scrubber element before compression.
• Periodical air quality testing of compressors.

• The wrong gas was put in a cylinder.
• A cylinder was marked or labelled incorrectly.
• A correctly labelled cylinder was mistaken by the user.
• The diver unintentionally switches to the wrong gas during a dive.

• Cylinders should be filled by competent people.^[17]
• Clear instructions, preferably written, for the composition of the gas to be mixed will reduce the risk of filling with the wrong gas.
• Clear, unambiguous and legible labels indicating maximum operating depth and cylinder contents, applied in a way that the user will be able to positively identify the gas at the time when it is to be used can prevent confusion and inadvertent use of the wrong gas.^[9]
• Analysing gas after filling, before accepting delivery, and before use (before the dive) may detect errors in labelling or composition in time to take corrective action.^[17]
• Procedures designed to positively identify the gas may be used when switching mixes.^[9]
• Valves that change gas mixes may be fitted with a positive interlock preventing accidental or inadvertent switching, and may include a method of confirming the gas connected by feel.

• Unconscious diver releases grip on mouthpiece.^[30]
• DV is forcibly knocked or pulled from the diver's mouth by impact with surroundings or another diver.

• Use of full face mask reduces risk of loss of DV as it is strapped to the head and can not be dropped if the diver loses consciousness.^[6]
• Adequate training and practice of DV recovery skills.
• Use of an alternative air source such as octopus DV or bailout cylinder, which can be used if the primary DV is not immediately accessible.
• Mounting the alternative air source and DV so that it is easily accessible in an emergency and protected from damage when not in use.

• Disruption of breathing by watery suspension/solution containing scrubber absorbent medium.
• Aspiration of water contaminated by scrubber medium.

• Prevent ingress of water to rebreather by:
  • checking before use that the unit does not leak,
  • closing the dive/surface valve when the mouthpiece is not in the diver's mouth.
• Prevent creation of caustic cocktail by:
  • use of less soluble and less alkaline scubber media,
  • design using water traps and drainage arrangements (on some rebreather designs),
  • introducing a semi-permeable membrane to block water from the scrubber.
• Avoid aspiration of water from loop by recognising the characteristic gurgling sounds and increased breathing resistance, and taking appropriate action by bailing out or draining the set if possible.
• In the event of caustic cocktail reaching the mouth, bail out to alternative gas supply and rinse mouth with ambient water.

• The hood should not make an airtight seal over the outside ear opening.
• Sealed earplugs should never be worn while diving.^[32]

• Mask squeeze can be avoided by allowing air into the mask through the nose whenever the pressure difference is noticeable.
• A fullface mask will automatically equalise through the demand valve.
• Air filled eyes-only goggles can not be equalised and are not suitable for diving.

• Loss of buoyancy.
• Suit squeeze injury (usually restricted to bruising and minor abrasion) to skin.^[34]

• Volume of air in a drysuit reduces as pressure increases with depth.
• Skin may be pinched by folds in a drysuit as the air inside is compressed.

• The squeeze due to depth changes was more likely when the air supply was powered by men. Motorised compressors are usually able to supply air much faster, so an adequate air reservoir on the compressor should prevent this problem.
• The diver may be prevented from sinking too deep by minimizing slack in the lifeline or umbilical.
• The diver may work at neutral buoyancy when there is a risk of falling off a structure, or may clip on to the structure, but this presents a hazard of entrapment.

• Uncontrolled descent.
• Inability to achieve neutral buoyancy.
• Inability to surface due to insufficient buoyancy.
• Difficulty in controlling depth and ascent rate. This can be critical when decompression is required, and oxygen-rich breathing gases are used.

• Use of buoyancy compensator with volume appropriate to expected buoyancy variation during dive.
• Use of appropriate ballast weight for dive profile and equipment in use.
• Use of inflation system for replacing lost volume in drysuits.
• Excessive weighting makes buoyancy control more difficult and loss of control more likely.
• Adequate training and practice of buoyancy control skills.

• Pneumothorax: Free air in the pleural cavity, leading to collapsed lung.
• Interstitial emphysema: Gas trapped in the spaces between tissues.

• Mediastinal emphysema: Gas trapped around the heart.

• Subcutaneous emphysema: Free gas under the skin.
• Arterial Gas embolism: Air or other breathing gas in the blood stream, causing blockage of small blood vessels.

• The best option is to breathe normally while ascending when possible, and passively exhale during free ascent.^[38]
• Forced exhalation before starting an emergency free ascent may increase risk of lung overpressure injury.^[38]

• Not diving with nasal congestion, e.g. Hay fever, or the common cold.
• Checking before a dive to ensure that sinuses and middle ears will equalise without undue effort.
• Systemic decongestants have been used successfully, but may have undesirable side-effects, and there is a risk that they will wear off before surfacing. Topical decongestants do not usually have sufficient lasting effect.

• Tooth pain, loss of fillings, cracking of teeth.

• Automatic dump valves in dry suits.
• Monitoring of buoyancy on a continuous basis when in mid-water, and manually adjusting volume of buoyancy compensator when necessary.
• Appropriate training and practice to develop good buoyancy control skills to suit the equipment in use.
• Ability to recover from inversion in dry suit.
• Maintaining the minimum air volume for adequate liner bulk maintenance in a dry suit, as this prevents excessive buoyancy shifts. This implies use of the buoyancy compensator for buoyancy control, not the suit.
• Minimizing weighting to what is actually necessary, so compensatory air volume is minimized. This reduces the magnitude and rate of buoyancy change with pressure change.

• Decompress to suit the dive profile and gas mixtures used.
• Use appropriate ascent rates and decompression stops.
• Oxygen-rich gas mixtures may be used to accelerate decompression.
• Use depth control aids to maintain correct decompression depth.
• Avoid dehydration and hypothermia.
• Maintain cardiovascular fitness.

• A reversible alteration in consciousness that occurs while diving at depth.
• A state similar to alcohol intoxication or nitrous oxide inhalation.
• The most dangerous aspects of narcosis are the loss of decision-making ability and focus, and impaired judgement, multi-tasking and coordination.
• Other effects include vertigo, and visual or auditory disturbances, exhilaration, giddiness, extreme anxiety, depression, or paranoia, depending on the individual diver.

• Related to the increased solubility of gases in body tissues at elevated pressure.^[39]: 308
• Inert gases dissolving in the lipid bilayer of cell membranes may cause narcosis.^[40]
• Other possibilities include neurotransmitter receptor protein mechanisms.^[41]

• Use of less narcotic gases to dilute the breathing gas, or
• Limit the partial pressure of narcotic gases at maximum depth by limiting the depth of the dive.

• Convulsions similar to epileptic seizure. Loss of consciousness may occur with no warning, or may be preceded by any of the following symptoms:

• Muscle twitching,
• Tinnitus,
• Blurred or tunnel vision,
• Disorientation,
• Aphasia,
• Nystagmus,
• Incoordination.

• Appropriate training before using a rebreather or oxygen enriched gases such as nitrox.
• Correct labeling of cylinders containing mixed breathing gases, specifying oxygen fraction and maximum operating depth.
• Accurate monitoring of dive depth to ensure that gases are not used below the appropriate maximum operating depth for the mixture.

• Signs of pulmonary toxicity begin with inflammation of the upper airways.
• Temporarily reduced lung capacity.
• Acute respiratory distress syndrome.

• Not normally a risk for recreational divers due to short exposures.
• Limit use of rich nitrox mixtures and pure oxygen for accelerated decompression.
• Limit exposure by calculating Oxygen Toxicity Units for pre-existing and planned exposures and keeping below recommended limits.
• Most likely to be encountered in recompression treatment for decompression illness.

• The compression effects may occur when descending below 500 feet (150 m) at rates greater than a few metres per minute, but reduce within a few hours once the pressure has stabilised.
• The effects from depth become significant at depths exceeding 1,000 feet (300 m) and remain regardless of the time spent at that depth.^[42]

• The susceptibility of divers to HPNS varies over a wide range depending on the individual, but has little variation between different dives by the same diver.^[42]
• Use another diving technique, such as an atmospheric diving suit, submersible or ROV.
• Including other gases in the mix, such as nitrogen (creating trimix) or hydrogen to make (hydreliox) suppresses the neurological effects.^[43][44][45]

• Diving suits are available that are suited to a wide range of water temperature down to freezing.^[47] The appropriate level of insulation for the conditions will reduce heat loss.
• In extreme conditions and when helium-based mixtures are in use as breathing gas, heated suits may be necessary.^[46]
• On the surface, wind chill can be avoided by staying out of the wind, staying dry, and suitable protective clothing.^[46]
• Some parts of the body, particularly the head,^[47] are more prone to heat loss and insulation of these areas is correspondingly important.

• Fully Functional 18 °C (64.4 °F) Non Freezing Cold Injury Threshold < Week.
• 12 °C (54 °F) approximately 3 hours.
• 8 °C (46.4 °F) for maximum of 30 min.
• 6 °C (42.8 °F) immediate rewarming required.

Protection in order of effectiveness:

• Dry gloves attached to drysuit without wrist seal.^[48]
• Dry gloves with wrist seal.
• Wet suit (neoprene) gloves.
• Rubberised cloth or leather gloves.

• Adequate insulation of the diving suit, particularly the gloves and boots.
• Prevention of wind chill by use of shelters and additional layers of clothing when out of the water.

• Inadequate insulation.
• Reduced perfusion to the legs and feet (occasionally hands).

• Coral cuts may be prevented by avoiding contact of unprotected skin with coral.^[46]
• Protective clothing such as wet-suit, dry suit, skin/lycra suit or overalls are effective.^[46]

• Most cuts may be avoided by wearing protective clothing such as wet-suit, dry suit, skin/lycra suit or overalls.^[46]
• Avoiding high risk areas such as shipwrecks during strong water movements such as surge or currents is also effective.
• Strength and skill in avoiding contact with sharp edges will help, but does not eliminate the risk when water movement is strong.

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