Oxygen may cause damage to cells at elevated partial pressures and a condition called oxygen toxicity syndrome can therefore occur whenever the partial pressure of oxygen in the breathing gas is substantially elevated for a prolonged period of time.

The oxygen toxicity syndrome may occur

• as a diving disorder, when divers breathe any breathing gas at the high pressure of 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.

Types of oxygen toxicity

In humans, there are several types of oxygen toxicity:

• Central nervous system (CNS) oxygen toxicity is manifested as convulsions, which although not lethal themselves, can cause drowning of divers or lethal pressure damage during a rapid ascent.

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. The damage to the lungs may be irreversible. This is rare complication in divers, but may be of concern in intensive care patients needing high-inspired oxygen concentrations.
• Rethinopathic oxygen toxicity causes damage to the retina. Oxygen may be a contributing factor for the disorder called retinopathy of prematurity.

Avoiding oxygen toxicity while diving

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, of 120 minutes at 1.5 bar, of 150 minutes at 1.4 bar and of 180 minutes at 1.3 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.