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Site last updated
21 August, 2004

SCUBA Diving

SCUBA diver Jürgen Vigna - diving using Self Contained Underwater Breathing Apparatus (SCUBA) is an increasingly popular activity. Some commercial and military dives also make use of SCUBA equipment but recreational diving is by far the most common. Air from a tank is breathed using a regulator, which ensures that the diver receives it at the correct pressure to counteract the effect of the surrounding water.

The pressure within a diver's body tissues must equal the surrounding pressure. For example, at a depth of 80msw the pressure within the body must be equal to nine times normal atmospheric pressure (10msw = 2ata, therefore 80msw = 9ata).

There are certain risks associated with SCUBA diving. Although several organisations exist to promote safer practice the fact that there are so many divers ensures that the Hyperbaric Medicine Unit treats an increasing number with diving related problems.

Some potential problems are discussed below. For information on decompression sickness (DCS), including symptoms and first aid, refer to the section on decompression illness.

North Sea Saturation Diving

Diving Support Vessel and PlatformCommercial deep-sea divers usually reach the sea floor through a small hatch at the bottom of a diving bell, which is winched up and down from a ship called a diving support vessel (DSV). It is only possible to open the hatch if the pressure within the bell is equal to the water pressure outside. This compression is achieved by pumping a mixture of oxygen and an inert gas such as helium into a sealed chamber and can be completed in a few minutes.

Because commercial divers often work on the sea floor many times during their tours of duty, they must also spend their non-working time at working depth pressure. To do this, they must remain on the ship in chambers pressurised to the correct depth. These are known as deck decompression chambers (DDCs) and are linked together to enable movement between the diving bell, habitat and a hyperbaric lifeboat (HLB) capable of maintaining their current environmental pressure.

The time taken to safely return to surface pressure is based on a calculation of how long it will take for the body to excrete the inert gas. Similar calculations can be used to determine the decompression time for recreational divers and occupants of medical hyperbaric chambers. These can be illustrated as graphs of depth and time called a 'dive table' or 'dive profile' - the depth and duration of the dive dictating the table used. The dive tables most commonly used by the Hyperbaric Medicine Unit are explained in more detail in the dive tables section.

Nitrogen Narcosis

When a SCUBA diver is at pressure below the surface, the air that is breathed is also at increased pressure to equal that of the water. Because the air is under pressure, more molecules of nitrogen dissolve into the tissues and blood.

Too much can affect the brain because nitrogen is an anaesthetic at increased pressure. It causes a condition termed nitrogen narcosis - the "rapture of the deep" - which can lead to reduced manual dexterity, reduced cognitive and sensory ability, euphoria or even coma and death. The deeper a diver goes, the more likely they are to suffer these effects and the more severe they will be.

It only takes around three minutes for the narcotic effects to reach maximum. Onset becomes apparent at a pressure of 4ata (30msw) with symptoms increasing in severity with additional depth. At pressures greater than 10ata (90msw) unconsciousness may occur. Potentiating factors include:

  • Alcohol.
  • Carbon dioxide (CO2) retention.
  • Inspired O2 more than 100kPa, possibly due to the additive narcotic effect of oxygen.
  • Cold exposure if retaining carbon dioxide.

    There are conflicting reports of habituation to narcosis following repeated dives and experienced workers or those with high intelligence appear to be least affected.

    The effect of hyperbaric nitrogen (N2) on the brain is similar to the 'normobaric' effect of nitrous oxide (N2O), which is used as an anaesthetic agent.


The water temperature experienced by SCUBA divers is often just a few degrees above freezing and even in the summer time hypothermia can still develop. Genuine hypothermia is indicated by a core temperature of less than 35.5°c (normal is 37.1°c), cessation of shivering and reduced consciousness. Although it is a rare occurrence in diving, the intermediate effects of cold (mild hypothermia) can also severely interfere with normal function.

Thermal conduction from the body is 25 times greater in water than dry, still air. Divers can wear a 'wet suit' designed to trap a thin layer of water against the skin, providing an insulating barrier. In cold waters, such as those around Scotland, a 'dry suit' is used to insulate the skin by trapping air instead. As pressure rises, heat loss will increase leading to a greater risk of hypothermia.

Other problems can result from, or be exacerbated by, exposure to cold water. If cold water is allowed into one of the ear canals, the diver can suffer from severe vertigo, loss of balance and nausea. Symptoms should resolve quickly and continuing symptoms might indicate decompression sickness.

Oxygen Toxicity

Oxygen is essential for life, but exposure to increased pO2 levels can have a toxic effect on the lungs and brain. The pO2 in the lungs can be raised by either increasing the concentration as a percentage of the total gas volume or by increasing the atmospheric pressure. In SCUBA diving, O2 enriched gas mixes are often used in order to help flush out nitrogen from the body more rapidly, and divers also encounter raised atmospheric pressure.

Exposure to 100% oxygen at surface pressure over a prolonged period (8 - 12 hours) can lead to pulmonary toxicity in healthy adults. Symptoms of pulmonary oxygen toxicity include sub-sternal pain and coughing. Paradoxically, the lungs may become less able to supply the body with normal levels of oxygen due to damage to the gas exchanging lining of the air sacs within the lungs. Symptoms may last for several days. Although pulmonary toxicity is a result of prolonged exposure to elevated levels of oxygen, it is possible that SCUBA divers could become affected.

The greatest danger associated with oxygen in SCUBA diving is from the effect on the Central Nervous System. CNS toxicity occurs at oxygen pressures of around 1.5ata or higher. Symptoms include dizziness, nausea, fatigue, anxiety, confusion and lack of co-ordination. The affected diver may suffer from convulsions very similar to those of a tonic-clonic (grand mal) seizure. During a fit, the diver's mouth will open - losing the mouthpiece. Drowning then ensues.

In the Unit's hyperbaric chamber, patients are limited to a maximum oxygen partial pressure of 2.8ata (100% O2 at 18msw) with regular 'air breaks'. For treatment at greater depth, helium is used as a diluent gas.


Barotrauma is damage to tissues caused by pressure, or a difference in pressure. In diving there are several potential problems, including:

  • Ruptured eardrum due to unduly rapid compression (especially in chambers).
  • Damage to nasal sinuses due to blockage of the air passages (plural: ostia, singular: ostium).
  • Explosion of tooth due to sealed cavity.
  • Bloodshot eyes due to tight fitting mask.
  • Rupture of lung tissue due to blocked airways or breath holding while ascending.

Obviously, some of these examples are more serious than others. The rupture of lung tissue is likely to lead to a life threatening Cerebral Arterial Gas Embolism (CAGE), discussed in decompression illness section.

Breathing Gas Mixes

Although air is the most common SCUBA diving gas mix, it is possible to use other gases. Use of any mix other than standard air requires additional training and precautions. Any gas in addition to oxygen is a diluent but will have unique characteristics that might affect the body. The most commonly used diving gas mixes are explained below.

Nitrox (N2O2) is 'oxygen enriched air' produced by adding 100% oxygen to standard air (21% oxygen / 79% nitrogen) - also termed enriched air nitrox (EAN). This enables an increase in the oxygen and therefore a reduction in nitrogen content. Because Nitrox contains less nitrogen than air, the effects of narcosis are reduced. However, because the oxygen concentration is higher there is an increased risk of oxygen toxicity. The main use of Nitrox is to extend dive time rather than depth.

Accurate dive profiles depend on knowing the exact percentage of oxygen in Nitrox, which is produced as either 32% (Nitrox I) or 36% (Nitrox II). Establishing the O2 percentage requires specialised equipment and quality control measures.

Heliox (HeO2) is a mixture of oxygen and helium used primarily by commercial divers. The percentage of O2 can be adjusted to suit the type and depth of the dive. Using helium instead of nitrogen eliminates the problem of narcosis. However, using helium as a diluent carries with it other risks.

Helium has a higher rate of thermal conductivity than nitrogen therefore it cannot be used for replenishing a dry suit layer. Care must also be exercised when it is used in a hyperbaric chamber to prevent excessive heat loss from the body. The role helium plays in respiratory heat loss is less significant as this depends more on gas density and helium is less dense than nitrogen.

The use of heliox allows compression to depths that can result in a condition termed 'high pressure nervous syndrome' or 'high pressure neurological syndrome' (HPNS). This can occur at depths below about 160msw - although recreational divers rarely reach this depth. HPNS is thought to be caused by hyperexcitability of nerves in the brain and may result in tremors, dizziness, nausea and tiredness. Pressure can be thought of as a convulsant, like oxygen, in high doses.

Helium also has the effect of causing distorted speech. Its lower density makes it easier to breath than nitrogen at the same pressure - a fact that makes it useful in the treatment of certain obstructive airways conditions at surface pressure.

Click here for an example of Helium voice (requires mp3 compatible player).

Helium is also relatively expensive due to its rarity. Although most of Earth's helium can be found in the atmosphere, the economic cost of extracting it is extremely high due to its low concentration in the air. Instead, it is obtained from some natural gas fields during extraction of methane. Most helium-rich reserves are in the USA and Canada. The cost of helium is expected to rise dramatically in the next 25 years as sources are depleted. Hyperbaric chambers incorporate a reclamation system to recover up to 98% of the gas for re-use.

Trimix (HeN2O2) is a mix of oxygen, helium and nitrogen. The effects of nitrogen narcosis are limited by the addition of helium. Also, because of the counteracting effect of nitrogen narcosis, HPNS symptoms are reduced. The interacting effects of nitrogen and helium within decompressing tissues, and the role this plays in bubble formation, is not yet clearly understood.

Decompression Sickness

In order to understand what happens in decompression sickness (DCS) - i.e. the effects of rapid decrease in pressure - it is important to be aware of the effects that increased pressure has on the body. Refer to the section on gas laws and principles for a reminder.

For details on the treatment of decompression sickness, including symptoms and first aid, refer to the section on decompression illness.

Further Information