| Hazard |
Consequences |
Cause |
Avoidance and prevention |
| Oxygen partial pressure in the breathing gas is too low to sustain normal activity or consciousness. |
Hypoxia:
Reduced level of consciousness, seizures, coma, death. Severe hypoxia induces a blue discoloration of the skin, called cyanosis, but this may also be present in a diver due to peripheral vasoconstriction resulting from exposure to cold. There is typically no warning of onset or development. The extreme case, anoxia, implies an absence of oxygen and is rapidly fatal. |
Equipment failure: A faulty or misused rebreather can provide the diver with hypoxic gas. |
- Correct maintenance, preparation and pre-use procedures and checks.
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- Correct use of recommended procedures and checklists when preparing for use.
- Adequate and redundant instrumentation for monitoring gas quality during use.
- Constant vigilance during use.
- Adequate bailout facilities in case of failure.
- Adequate training in the use of rebreathers in general and the specific model.
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| Some breathing gas mixtures for deep diving such as trimix and heliox are hypoxic at shallow depths, and do not contain enough oxygen to maintain consciousness, or sometimes life, at or near the surface. |
- Gas requirements planned to suit the intended dive profile.
- 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.
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- Gas switches planned and executed at appropriate depths.
- Depth and ascent rate accurately monitored and controlled.
- Clear and unambiguous identification of cylinder gases.
- Adequate training in the use of mixed gases.
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| Internal corrosion of full cylinder standing for a long time can potentially use up some of the oxygen in the contained gas before the diver uses the cylinder. |
- Routine periodical inspection and testing of cylinders.
- Analysis of oxygen fraction of gas before use, particularly if cylinder has been stored for a long time.
|
| Loss of breathing gas supply. |
May result in drowning, occasionally asphyxia without water aspiration. |
Equipment failure: Several modes are possible.
- Closing and jamming of the cylinder valve by rolloff on an overhead.
- Rupture of a bursting disc overpressure protection on a cylinder valve.
- Rupture of a regulator hose or loss of the end component, leaving an open hose end.
- Unrecoverable free flow of a second stage.
- 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.
- Inspection of the external condition, and testing of the function of equipment before use.
- Use only of equipment in good working condition.
- 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.
- Use of bailout gas supply.
- The buddy system, when correctly followed, allows the diver's buddy to supply breathing gas in an emergency.
- "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.
- Dual independent cylinders ensure that if one cylinder supply fails there is another available.
- Use of DIN connections can reduce the risk of catastrophic O-ring failure.
- Emergency free ascent may be possible, and is generally more survivable than drowning.
|
| Running out of breathing gas because of poor gas monitoring discipline |
- Adequate training of divers.
- Disciplined attitude and situational awareness during dives.
- Use of reserve valve.
- Use of surface supplied diving equipment.
- Use of bailout gas supply.
- The buddy system, when correctly followed, allows the diver's buddy to supply breathing gas in an emergency.
|
| Running out of breathing gas because of being trapped by nets or lines |
- 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.
|
| Running out of breathing gas because of being trapped or lost in enclosed spaces underwater, such as caves or shipwrecks. |
- Appropriate safety equipment and procedures to avoid getting lost (cave lines).
- Specific training for overhead diving. See cave diving and wreck diving.
- Assess stability of underwater structures and avoid entry if a structure is unstable.
|
| Inhalation of salt spray |
Salt water aspiration syndrome: a reaction to salt in the lungs. Early symptoms are:
- Post-dive cough, with or without sputum, usually suppressed during the dive.
- In serious cases the sputum may be bloodstained, frothy and copious.
- Over time further symptoms may develop, including:
-
- rigors, tremors or shivering;
- anorexia, nausea or vomiting;
- hot or cold sensations;
- dyspnoea; cough; sputum;
- headaches; malaise; and generalised aches.
|
Inhaling a mist of sea water from a faulty demand valve |
- Appropriate maintenance and servicing of equipment.
- 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).
- Use equipment only if it is in good working condition.
- 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.
|
| Carbon monoxide contamination of breathing gas |
Carbon monoxide poisoning.
- Initial symptoms include: headache, nausea, malaise, and fatigue.
-
- Headache is the most common symptom; it is often described as dull, frontal, and continuous.
- Increasing exposure produces cardiac abnormalities including fast heart rate, low blood pressure, and cardiac arrhythmia; central nervous system symptoms include delirium, hallucinations, dizziness, unsteady gait, confusion, seizures, central nervous system depression, unconsciousness, respiratory arrest, and death.
|
Contaminated air supplied by a compressor which sucked in products of combustion, often its own engine's exhaust gas. Aggravated by increased partial pressure due to depth. |
- Adequate precautions to ensure that intake is in uncontaminated air when operating breathing air compressors.
- 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 which have a smell or taste. Air which smells or tastes of exhaust fumes should not be breathed.
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| Oil getting into the air and partially oxidising in the compressor cylinder, like in a diesel engine, due to worn seals and use of unsuitable oils, or an overheated compressor. |
- Adequate maintenance of the compressor.
- Use of correct oil rated for breathing air compressor lubrication.
- 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.
|
| Hydrocarbon (oil) contamination of air supply. |
Emphysema or lipid pneumonia (more to be added) |
Caused by inhaling oil mist. This may happen gradually over a long time and is a particular risk with a surface supplied air feed. |
- Use of a suitable separators and air filter after compression.
-
- 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
|
| Excessive carbon dioxide in breathing gas |
Carbon dioxide poisoning or hypercapnia.
- Symptoms and signs of early hypercapnia include: flushed skin, full pulse, increased breathing rate, shortness of breath, muscle twitches, hand flaps, reduced neural activity, headache, confusion, lethargy, increased cardiac output, an elevation in arterial blood pressure, and a tendency for cardiac arrhythmias.
- In severe hypercapnia (generally partial pressure of CO2 greater than 10 kPa), symptoms progress to disorientation, panic, hyperventilation, convulsions, unconsciousness, and eventually death.
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- 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.
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- Minimise the volume of any enclosed spaces which the diver breathes through. For example, this can happen with diving with a large "bubblehead" helmet.
- Avoiding breathing shallow (low volume) breaths.
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| The scrubber of a diving rebreather, fails to absorb enough of the carbon dioxide in recirculated breathing gas. This can be due to the scrubber absorbent being exhausted, the scrubber being too small, or the absorbent being badly packed or loose, causing "tunneling" and "scrubber breakthrough" when the gas emerging from the scrubber contains excessive carbon dioxide. |
- Adequate maintenance of rebreathers.
- Correct packing and assembly of scrubber canisters.
- 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.
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| Filling of cylinders with compressed air taken from an area of raised concentration of carbon dioxide. |
- 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.
|
| Breathing the wrong gas |
Consequences depend on the circumstances, but may include oxygen toxicity, hypoxia, nitrogen narcosis, anoxia, and toxic effects of gases not intended for breathing. Death or serious injury is likely. |
- 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.
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- Cylinders should be filled by competent people.
- 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.
- 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.
- Procedures designed to positively identify the gas may be used when switching mixes.
- Valves which 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.
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| Displacement of demand valve (DV) from the diver's mouth. |
Inability to breathe until demand valve is replaced. This should not normally be a major problem as techniques for DV recovery are part of basic training. Nevertheless it is an urgent problem and may be exacerbated by loss of the mask and/or disorientation. |
- Unconscious diver releases grip on mouthpiece.
- DV is forcibly knocked or pulled from the diver's mouth by impact with surroundings or another diver.
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- 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.
- 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.
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