Divers face specific physical and health risks when they go underwater with scuba or other diving equipment, or use high pressure breathing gas. Some of these factors also affect people who work in raised pressure environments out of water, for example in caissons. This article lists hazards that a diver may be exposed to during a dive, and possible consequences of these hazards, with some details of the proximate causes of the listed consequences. A listing is also given of precautions that may be taken to reduce vulnerability, either by reducing the risk or mitigating the consequences. A hazard that is understood and acknowledged may present a lower risk if appropriate precautions are taken, and the consequences may be less severe if mitigation procedures are planned and in place.
A hazard is any agent or situation that poses a level of threat to life, health, property, or environment. Most hazards remain dormant or potential, with only a theoretical risk of harm, and when a hazard becomes active, and produces undesirable consequences, it is called an incident and may culminate in an emergency or accident. Hazard and vulnerability interact with likelihood of occurrence to create risk, which can be the probability of a specific undesirable consequence of a specific hazard, or the combined probability of undesirable consequences of all the hazards of a specific activity. The presence of a combination of several hazards simultaneously is common in diving, and the effect is generally increased risk to the diver, particularly where the occurrence of an incident due to one hazard triggers other hazards with a resulting cascade of incidents. Many diving fatalities are the result of a cascade of incidents overwhelming the diver, who should be able to manage any single reasonably foreseeable incident. The assessed risk of a dive would generally be considered unacceptable if the diver is not expected to cope with any single reasonably foreseeable incident with a significant probability of occurrence during that dive. Precisely where the line is drawn depends on circumstances. Commercial diving operations tend to be less tolerant of risk than recreational, particularly technical divers, who are less constrained by occupational health and safety legislation.
Decompression sickness and arterial gas embolism in recreational diving are associated with certain demographic, environmental, and dive style factors. A statistical study published in 2005 tested potential risk factors: age, gender, body mass index, smoking, asthma, diabetes, cardiovascular disease, previous decompression illness, years since certification, dives in last year, number of diving days, number of dives in a repetitive series, last dive depth, nitrox use, and drysuit use. No significant associations with decompression sickness or arterial gas embolism were found for asthma, diabetes, cardiovascular disease, smoking, or body mass index. Increased depth, previous DCI, days diving, and being male were associated with higher risk for decompression sickness and arterial gas embolism. Nitrox and drysuit use, greater frequency of diving in the past year, increasing age, and years since certification were associated with lower risk, possibly as indicators of more extensive training and experience.
Statistics show diving fatalities comparable to motor vehicle accidents of 16.4 per 100,000 divers and 16 per 100,000 drivers. Divers Alert Network 2014 data shows there are 3.174 million recreational scuba divers in America, of which 2.351 million dive 1 to 7 times per year and 823,000 dive 8 or more times per year. It is reasonable to say that the average would be in the neighbourhood of 5 dives per year.
The aquatic environment
Hazard Consequences Cause Avoidance and preventionAny liquid environment. 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.Inhalation of liquid (water), usually causing laryngospasm and suffocation caused by water entering the lungs and preventing the absorption of oxygen leading to cerebral hypoxia. Avoid out of air emergencies underwater. Use of a redundant emergency breathing gas supply Provide appropriate buoyancy. Avoid or prevent accidents resulting in unconsciousness. Use of a full face mask or diving helmet to protect the airway. Use of surface-supplied diving equipment with voice communications. Adequate swimming skills and fitness for the circumstances. Use of snorkel when appropriate. 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. Competence in recovering and clearing a dislodged scuba regulator. Use of an alternative regulator which can be easily reached if the dislodged primary is difficult to reach.Complications can occur up to 72 hours after a non-fatal drowning incident, and may lead to a serious condition or death.Physiological responses to contaminants in the lung due to inhalation of liquid. 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.Prompt and appropriate medical treatment after near drowning, including a medical observation period.
Use of breathing equipment in an underwater environment Hazard Consequences Cause Avoidance and preventionOxygen 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. Equipment failure: A faulty or misused rebreather can provide the diver with hypoxic gas. Correct maintenance, preparation and pre-use procedures and checks. Correct use of recommended procedures and checklists when preparing for use. Calibration of oxygen monitoring instruments 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.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 gas mix for descent and a decompression gas mix for ascent through the depth range where the bottom gas is hypoxic. Safe procedures used for gas switching. 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.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 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. 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). Rupture of a regulator hose or loss of the end component, leaving an open hose end. 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. 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. (frequently used to mitigate regulator freezing in cold water) 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. Use of surface supplied breathing equipment.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 spraySalt water aspiration syndrome: a reaction to salt in the lungs.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 gasCarbon monoxide poisoning.Contaminated air supplied by a compressor that 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).