Oxygen toxicity or oxygen toxicity syndrome is severe hyperoxia caused by breathing oxygen at elevated partial pressures. The high concentration of oxygen damages cells.

Types of oxygen toxicity

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In humans, there are several types of oxygen toxicity:

• Central nervous system (CNS) oxygen toxicity is manifested as dizziness, nausea and twitching, especially on the face, leading to convulsions, which although not lethal themselves, can cause drowning of divers or lethal pressure damage during a rapid ascent to the surface.

The likelihood of this type of accident is directly proportional to the partial pressure of oxygen (ppO₂) in the breathing gas and to the duration of exposure.

• Pulmonary oxygen toxicity is caused by exposure over 16 hours to partial pressures of 0.5 bar or more causing breathing difficulty and pain resulting in lung damage which may be irreversible. This is a rare complication for divers, but may be of concern in intensive care patients needing high inspired oxygen concentrations.
• Retinopathic oxygen toxicity causes damage to the retina. Oxygen may be a contributing factor for the disorder called retinopathy of prematurity.

Hyperoxia

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Hyperoxia is excess oxygen in body tissues or higher than normal partial pressure of oxygen. Hyperoxia is caused by breathing gas at pressures greater than normal atmospheric pressure or by breathing oxygen-rich gases at normal atmospheric pressure for a prolonged period of time.

Common causes

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The oxygen toxicity syndrome may occur

• as a diving disorder, when divers breathe any breathing gas deeper than its maximum operating depth,
• as a potential complication of mechanical ventilation with pure oxygen, where it is called the respiratory lung syndrome.

Oxygen toxicity is not a major factor in hyperventilating, as some people believe. The problems caused by hyperventilating are due to decreased carbon dioxide within the blood. With or without hyperventilating, it is impossible to develop oxygen toxicity breathing air at typical surface atmospheric pressure.

Avoiding oxygen toxicity while diving

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There is an increased risk of CNS oxygen toxicity on deep dives, long dives or dives where oxygen-rich breathing gases are used.

Divers are taught, in some diver training courses for these types of diving, to plan and monitor what is called the "oxygen clock" of their dives. This clock is a notional alarm clock, which "ticks" more quickly at increased ppO₂ and is set to activate at these maximum single exposure limits recommended in the NOAA Diving Manual: 45 minutes at 1.6 bar, 120 minutes at 1.5 bar, 150 minutes at 1.4 bar, 180 minutes at 1.3 bar and 210 minutes at 1.2 bar.

The aim is to avoid activating the alarm by reducing the ppO₂ of the breathing gas or the length of time breathing gas of higher ppO₂. As the ppO₂ depends on the oxygen concentration in the breathing gas and the depth of the dive, the diver can obtain more time on the oxygen clock by diving at a shallower depth, by breathing a less oxygen-rich gas or by shortening the exposure to oxygen-rich gases.

See also

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• Maximum operating depth
• Breathing gas

References

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• Scubadoc's Diving Medicine Online^*

• The Diving Emergency Handbook, John Lippmann and Stan Bugg, ISBN0946020183

Pulmonology | Diving medicine | Intensive care medicine

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