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.

Contents

1 The ordinary features of diving 2 Effects of relying on breathing equipment while underwater 3 Effects of barotrauma or pressure damage 3.1 On descent 3.2 On ascent 4 Effects of breathing gas at high pressure 5 Other risks encountered by people in water

The ordinary features of diving

Click on the boldface links for more information about each topic. Also see Diving equipment.

Need to:-	Remedy	Comments
Breathe underwater	Open-circuit scuba set	Easy to refill. Safer to use than a rebreather.
	rebreather	Longer duration per weight of set, but needs more training to be safe.
	Surface supplied diving	Mostly for work diving
	Standard diving dress	Mostly for work diving. Mostly merely of historical interest now.
	(Liquid breathing system)	(In the real world this is at the early experiment stage. It should allow very deep diving when it is developed.)
See underwater. Protect the eyes. (Man's eyes cannot focus accurately when directly submerged.)	Eyes-and-nose diving mask	Easy to clear if flooded. Easily knocked off.
	Full face mask	Allows talking if no mouthpiece inside. Difficult or impossible to go to snorkel on running out of air.
	Diving helmet	As fullface mask. Protects the head well.
	Special contact lenses	Occasionally used by commando frogmen instead, to avoid seachlight beams reflecting off a mask window.
Move faster than with unequipped swimming	Fins on feet	Cheap, easy to use. Even without a diving suit, wetsuit bootees let the diver wear bigger fins.
	Diver Propulsion Vehicles	Faster but expensive, and heavy and bulky to carry about on land.
Avoid hypothermia (losing body heat to the water)	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.
Control buoyancy	Diving weighting systems	Usually a weight belt. Some breathing sets have built-in weight pouches.
	diving suits	The buoyancy of most drysuits can be changed during the dive.
	buoyancy compensators	These make diver's buoyancy control much easier.
Protect the skin from cuts and stings and grazes	Diving suits and diving gloves and diving suit bootees serve this purpose.	In very warm water try diving in a boiler suit, or other strong clothes with long sleeves and legs.
Breathe from atmosphere to save air when on surface	snorkel	Advisable, despite some naval divers' opinions.

Effects of relying on breathing equipment while underwater

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 each type to find its symptoms.

Types of this sort of diving disorder, and how to avoid them

Type	Cause	How to avoid it
Drowning	Being unable to inhale anything but water	See under "anoxia"
Secondary drowning	Can occur hours after a near drowning	Prompt medical treatment after near drowning
Oxygen poisoning	Breathing gas with too much oxygen in	Proper training before using a rebreather.
Hypoxia or anoxia while having gas to breathe, but with too little oxygen to support normal activities or consciousness.	This is a risk when using a rebreather	Keep rebreathers properly maintained. Proper training before using a rebreather.
	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 gas to breathe	Equipment failure	Keep equipment routinely checked and in good condition
	Running out of air due to 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 .
	Running out of air due to getting trapped or lost in enclosed spaces underwater (e.g. caves and shipwrecks)	Better 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 its engine's (or another engine's) exhaust	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	The diver producing carbon dioxide faster than his rebreather's soda lime can absorb it	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.

Effects of barotrauma or pressure damage

On descent

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 each type to find its symptoms.

Types of this sort of diving disorder, and how to avoid them

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.
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 bood vessels around the eyes	Caused by suction from the air space inside a mask 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 valve in the helmet failing. In severe cases much of the diver's body could be mangled and compacted inside the helmet	Keep equipment in good order and inspected. Proper training in its use.

On ascent

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 each type to find its symptoms.

Types of this sort of diving disorder, and how to avoid them

Type	Cause	How to avoid it
Pulmonary barotrauma: "burst lung"	Holding the breath while ascending	Never hold the 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.
Pain in a sinus	Blockage of the sinus's duct	Do not dive with nasal congestion
Eardrum bursting outwards	Blocked Eustachian tube	Do not dive with nasal congestion

Effects of breathing gas at high pressure

Click on each type to find its symptoms.

Types of this sort of diving disorder, and how to avoid them

Type	Cause	How to avoid it
Decompression sickness ("the bends")	Ascending too quickly to be able to eliminate from the body tissues excess nitrogen or helium that had been adsorbed at depth.	Do not miss or skimp decompression stops. Training in using diving tables and a dive computer. See decompression sickness for a detailed list of the symptoms. Provide something for the diver to hold onto while decompressing.
Bends in snorkellers. Uncommon but known.	Many deep dives in succession. See taravana.	Use an underwater breathing set
Nitrogen narcosis	Breathing a high partial pressure of nitrogen (or other inert gas, to varying degrees)	Do not dive too deep on ordinary air. With mixture diving, use the correct breathing gas.
Oxygen toxicity	Breathing a high partial pressure of oxygen	This can happen in very deep diving with open-circuit scuba
HPNS: High Pressure Nervous Syndrome or Helium Tremors	Breathing a high partial pressure of helium	Use another diving technique, such as an ROV

The term dysbarism describes Decompression sickness, arterial gas embolism, and barotrauma.

Other risks encountered by people in water

Types of this class of diving disorder, and how to avoid them. Click on each type to find its symptoms.

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.
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	Learn to identify them. Keep away from them.
Shark bites	Sharks, in warm seas	Get proper information on them
Crocodile attack	Crocodiles, in some tropical waters	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	Get proper information on them
	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.
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	In affected water, dive in watertight drysuit and full face diving mask
Broken bones, bleeding wounds and other trauma	Colliding with a boat or its propellor. Wave action on the shore.	Use Surface detection aids or a diving shot to mark position. Plan a safe exit point and check weather and tidal conditions.