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Divers face specific physical and health risks when they go underwater (e.g. with scuba or other diving equipment) or use high pressure breathing gases. Some of these conditions also affect people who work in raised pressure environments out of water, e.g. in caissons.
According to a North American 1972 analysis of calendar year 1970 data, diving was (on a man-hours based criteria) 96 times more dangerous than driving an automobile.[1] According to a 2000 Japanese study, every hour of recreational diving is 36 to 62 times riskier than automobile driving.[2]
Being unable to breathe fresh air naturally whilst submerged and relying on limited breathing gas supplies and fallible breathing equipment can have these effects. Click on the boldface links to find symptoms and more information for each topic.
| Type | Cause | How to avoid it |
|---|---|---|
| Drowning | Being unable to inhale anything but water | See under "anoxia" hereinunder |
| Secondary drowning | Can occur hours after a near drowning | Prompt medical treatment after near drowning |
| Oxygen toxicity | Breathing gas at too high a partial pressure of oxygen or at a partial pressure in excess of 0.5 atmospheres pressure for too much time; partial pressure depends upon proportion of oxygen and depth | Proper training before using a rebreather or oxygen enriched gases such as nitrox. |
| Hypoxia or anoxia occurs while having gas to breathe, but where the oxygen partial pressure is too low to sustain normal activity or consciousness. | A faulty or misused rebreather can provide the diver with hypoxic gas | Keep rebreathers properly maintained. Proper training before using a rebreather. |
| Some deep diving breathing gases such as trimix and heliox can be hypoxic at shallow depths | Don't breathe hypoxic gas in shallow water. Proper training before using mixed gases. Correct identification of cylinder gases and safe procedures for gas changes. |
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| A full cylinder standing for a long time while the inside of the cylinder rusts, using up oxygen in the contained air, before the diver uses the cylinder | Keep cylinders routinely checked and tested. If a cylinder has stood full for months, empty it and refill it. | |
| Anoxia due to having no air or gas to breathe | Equipment failure - particularly in rebreathers that monitor and maintain oxygen content | Keep equipment routinely checked and in good condition |
| Running out of air due to a number of factors, including poor dive discipline | Better training of divers. More disciplined attitude when underwater. | |
| Running out of air due to getting trapped by nets | Better awareness underwater. Carry a diver's net cutter, or dive tool/knife. | |
| Running out of air due to getting trapped or lost in enclosed spaces underwater (e.g. caves and shipwrecks) | Specific training and leadership for such types of diving. See cave diving and wreck diving. | |
| Running out of air due to getting lost in open water | Better training and leadership, including in using a compass underwater | |
| Salt water aspiration syndrome | Inhaling a mist of sea water from a faulty demand valve causing a reaction in the lungs | Keep equipment routinely checked and in good condition |
| Carbon monoxide poisoning | Air cylinder filled by a compressor which sucked in products of combustion, often its own engine's exhaust gas | Proper precautions when filling cylinders |
| Oil getting into the air feed and firing in the air compression cylinder, like in a diesel engine | Proper servicing of the compressor | |
| Emphysema caused by inhaling oil mist | This happens gradually over a long time. This is a particular risk with a pumped surface air feed. | Use proper filters in the air pump or air compressor. |
| Carbon dioxide poisoning: hypercapnia | Re-inhaling carbon dioxide-laden exhaled gas | Minimise the volume of any enclosed spaces which the diver breathes through. For example, this hazard can happen with diving with a large "bubblehead" helmet. |
| With a rebreather, the diver re-inhales carbon dioxide because the soda lime scrubber cannot absorb the exhaled carbon dioxide as fast as the diver produces it. See Rebreather#Carbon dioxide scrubber. | British naval divers called it shallow water blackout. Keep rebreathers properly maintained. Proper training before using a rebreather. | |
| Various effects of breathing a wrong gas | A wrong gas was put in a cylinder | Check conditions where you have your cylinders refilled. Put the proper gas identification markings on cylinders. |
See barotrauma and pressure for more information.
Air spaces within the body provide no support against greater outside pressure. This can happen from losing control of buoyancy causing excessive vertical speed during descent. Click on the boldface links to find symptoms and more information for each topic.
| Type | Cause | How to avoid it |
|---|---|---|
| Eardrum damage. Cold water in the middle ear chills the inner ear, causing dizziness and disorientation etc. | Failing to equalize the pressure in the middle ear with surrounding pressure. | Do not dive if the eustachian tube is congested, e.g. with the common cold. Proper diver training in clearing the ears. |
| The pressure in the outer ear not equalizing with surrounding pressure | Make sure that your hood does not make an airtight seal over the outside ear hole; never wear earplugs. | |
| Damage to other body air spaces, such as the paranasal sinuses. | Obstruction to the sinus ducts | Do not dive with conditions such as the common cold |
| Squeeze damage to blood vessels around the eyes | Caused by suction from the air space inside a mask ("mask squeeze") which is not a fullface mask | Let air into the mask through the nose. Do not dive with eyes-only goggles. |
| Squeeze damage to skin under folds in a drysuit | Suction into the space inside the fold | Modern drysuits have a tube connection to inflate the drysuit from the cylinder |
| Lung squeeze: blood in lungs | Extreme depth when snorkelling | Use an underwater breathing set |
| Helmet squeeze, with the old standard diving dress. This does not happen with scuba where there is no solid pressure-tight helmet | A non-return valve in the helmet failing, accompanied by a failure of the air compressor (on the surface) to pump enough air into the suit for the gas pressure inside the suit remaining equal to the outside pressure of the water. In severe cases much of the diver's body could be mangled and compacted inside the helmet, however, this requires substantial pressure difference caused by aforementioned failures in the air supply and the non-return valve (which was absent from the earliest models of this type of diving suit). | Keep equipment in good order and inspected. Proper training in its use. |
| Tooth squeeze[3] | Toothache, most often affects divers with preexisted pathology in the oral cavity[4] |
Air spaces within the body expand when the outside pressure decreases. This can happen from holding the breath on ascent, or from losing control of buoyancy causing excessive vertical speed during ascent. Click on the boldface links to find symptoms and more information for each topic.
| Type | Cause | How to avoid it |
|---|---|---|
| Pulmonary barotrauma: "burst lung" | Holding the breath while ascending | Never hold your breath while diving with breathing apparatus |
| This can cause: | ||
| Pneumothorax | Collapsed lung, air loose in the pleural cavity | |
| Interstitial emphysema | Gas trapped in the chest after burst lung | |
| Subcutaneous emphysema | Gas loose under the skin. | |
| Gas embolism | Air or other gas in the blood stream. Its effects can be very similar to decompression sickness. |
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| Pain in a sinus | Blockage of the sinus's duct | Do not dive with nasal congestion, e.g. the common cold. |
| Eardrum bursting outwards | Blocked Eustachian tube | |
Click on the boldface links to find symptoms and more information for each topic.
| Type | Cause | How to avoid it |
|---|---|---|
| Decompression sickness ("the bends") | Gas dissolves in tissues under pressure according to Henry's Law over time. After dive, ascending too quickly will cause gas to supersaturate and form bubbles in tissues depending on time and depth of the dive. | Plan your dive. Know how long you can stay at the planned depth and still make a normal ascent. If stops are necessary, do not miss or cut short decompression stops. Training in using diving tables and a dive computer. See decompression sickness for a detailed list of the symptoms. Use breathing gas mixtures with reduced inert gas fraction, eg Nitrox. Provide something for the diver to hold onto while ascending and decompressing to maintain accurate depth during stops and correct ascent rate. Avoid dehydration and hypothermia. Maintain cardiovascular fitness. |
| Bends in snorkellers. Uncommon but known. | Many deep dives in succession. See taravana. | Reduce the number of deep dives, increase surface interval or reduce dive depth. Use an underwater breathing apparatus and ascend at a rate determined by decompression tables or computer. |
| Nitrogen narcosis | Breathing a high partial pressure of nitrogen (or other gas, to varying degrees) | Dont dive deep on air. Limit the depth of the dive to limit the partial pressures of gases with narcotic effects to a level that you can safely manage. With mixed gas diving, use the correct breathing gas mixture to limit the equivalent narcotic depth to an acceptable level for the planned depth. |
| Oxygen toxicity | Breathing a high partial pressure of oxygen | This hazard is well known with closed circuit rebreathers when the control of the mixture fails. This can also happen when diving with open-circuit scuba and semi-closed circuit rebreathers if the maximum operating depth for the breathing gas is exceeded. |
| HPNS: High Pressure Nervous Syndrome or Helium Tremors | Breathing a high partial pressure of helium | Use another diving technique, such as an ROV; or add a little nitrogen as described at HPNS. |
The term dysbarism describes Decompression sickness, arterial gas embolism, and barotrauma.
Divers face specific physical and health risks when they go underwater (e.g. with scuba) or use high pressure breathing gases. Some of these conditions also affect people who work in raised pressure environments out of water, e.g. in caissons.
| Type | Cause | How to avoid it |
|---|---|---|
| Hypothermia | Losing body heat to the water. Water carries heat away far better than air. | In cool or cold water, wear an adequately warm diving suit for the conditions. Also, much heat can be lost from a head without a hood. |
| Cuts, sometimes with coral tissue left in them | Coral | Do not get too close to coral. Avoid diving with bare skin. |
| Cuts | Rock, metal, etc. | Avoid diving in bare skin, particularly in caves or shipwrecks. |
| Stings | Fire coral | It is yellow. Learn to identify it. |
| Stings, some dangerous | Some jellyfish | Learn about the dangerous species. Avoid diving with bare skin. |
| A deep cut which leaves poison in the wound | sting ray (its self-defence reaction) | Do not poke about in sand where they live. Care when wading. |
| Reef rash | A generic catch-all term that refers to the various cuts, scrapes, bruises and skin conditions that result from diving in tropical waters. This includes sunburn, jellyfish stings, sea lice bites, fire coral inflammation and other skin injuries that a diver may gain from using a shorty wetsuit or no diving suit. | Wear a full-body exposure suit to prevent direct skin to environment contact. |
| Poison-injecting spines | lionfish, stonefish, crown of thorns starfish, some sea urchins in warm seas | Learn to identify them. Keep away from them. Care when wading. |
| Poison injection | Blue ringed octopus, in parts of the Pacific Ocean | |
| Shark bites | Sharks, likelihood of risk is location dependent | Consult location-specific information to determine risk; never molest even seemingly-tame sharks underwater. |
| Crocodile attack | Crocodiles, in some tropical waters | Get proper information on them. Avoid waters known to be inhabited by crocodiles. |
| Attack by Titan Triggerfish | This tropical Indo-Pacific fish is very territorial during breeding season and will attack and bite divers | Keep a lookout for the fish and move away if they act aggressively |
| Attack by an unusually large grouper. | Epinephelus lanceolatus can grow very big in tropical waters, where protected from attack by sharks. There have been cases of very large groupers trying to swallow humans.[5][6][7][8][9] | Get proper information on them. |
| Electrocution | Electric eel, in some South American fresh water | Get proper information on them |
| Electric ray, in some warm seas | ||
| It is said that some naval anti-frogman defences use electric shock | Keep out of armed forces areas | |
| Powerful ultrasound | It is said that some naval anti-frogman defences use powerful ultrasound. Also used for long-range communication with submarines | Keep out of armed forces areas. Avoid large ships' ordinary sonar. See Underwater Port Security System. |
| Exposure to disease carried by in-water organisms | Weil's disease (in rat's urine) Bilharzia (in some warm fresh water) Various bacteria found in sewage |
In affected water, dive in watertight drysuit and full face diving mask |
| Exposure to harmful chemicals in the water | May be found in water polluted by industrial waste outfalls or by natural sources. For example hydrogen sulfide in some lakes and caves can be absorbed through the skin. | |
| Broken bones, bleeding wounds and other trauma | Colliding with a boat or its propeller. Wave action on the shore. |
Use Surface detection aids or a diving shot to mark surfacing position and aid searchers. Plan a safe exit point and check weather and tidal conditions. |
| Diver lost at sea after a boat dive | Separated from boat cover due to poor visibility at surface or strong underwater currents. Left behind due to inaccurate check by boat crew |
Ensure that boat uses a positive check system to identify each diver is on board after a dive. Carry a yellow flag or surface marker buoy to attract attention. Carry a personal submersible EPIRB or submersible vhf radio. Carry a signalling mirror and/or sound signalling device. |
| Diver lost at sea after a shore dive | Big waves made it unsafe to leave the water; currents moved the diver away from a safe exit; surface weather on the shore make the sea too rough to safely exit. | Local knowledge, good weather forecasts, plan alternative exits |
| Sudden loss of underwater visibility | Silt out: stirring up silt or other light loose material | Training in diving in zero visibility. Learn the frog kick. |
| Entrapment | Snagging on lines, nets, wrecks, debris or caves | Carry at least one line cutting implement. Dive with a buddy who is capable of helping to free you and will stay close enough to notice, Train in wreck diving and cave diving techniques, Use low snag equipment configurations (avoid dangling gear and snap hooks that can snag on lines) |
| Getting lost under an overhead | Losing your way in wrecks and caves where there is no direct route to the surface | Proper training and dive planning, Correct use of reels and lines, directional markers. Backup lights. |
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