The onset of seizure depends upon the partial pressure of oxygen in the breathing gas and exposure duration. However, exposure time before onset is unpredictable, as tests have shown a wide variation, both amongst individuals, and in the same individual from day to day. In addition, many external factors, such as underwater immersion, exposure to cold, and exercise will decrease the time to onset of central nervous system symptoms. Decrease of tolerance is closely linked to retention of carbon dioxide. Other factors, such as darkness and caffeine, increase tolerance in test animals, but these effects have not been proven in humans. Lungs
Pulmonary toxicity symptoms result from an inflammation that starts in the airways leading to the lungs and then spreads into the lungs (tracheobronchial tree). The symptoms appear in the upper chest region (substernal and carinal regions). This begins as a mild tickle on inhalation and progresses to frequent coughing. If breathing increased partial pressures of oxygen continues, patients experience a mild burning on inhalation along with uncontrollable coughing and occasional shortness of breath (dyspnea). Physical findings related to pulmonary toxicity have included bubbling sounds heard through a stethoscope (bubbling rales), fever, and increased blood flow to the lining of the nose (hyperaemia of the nasal mucosa). X-rays of the lungs show little change in the short term, but extended exposure leads to increasing diffuse shadowing throughout both lungs.
However, helium is not narcotic, and a usable mixture may be blended either by completely replacing nitrogen with helium (the resulting mix is called heliox), or by replacing part of the nitrogen with helium, producing a trimix.Pulmonary oxygen toxicity is an entirely avoidable event while diving. The limited duration and naturally intermittent nature of most diving makes this a relatively rare (and even then, reversible) complication for divers. Established guidelines enable divers to calculate when they are at risk of pulmonary toxicity. In saturation diving it can be avoided by limiting the oxygen content of gas in living areas to below 0.4 bar. Hyperbaric setting
The presence of a fever or a history of seizure is a relative contraindication to hyperbaric oxygen treatment. The schedules used for treatment of decompression illness allow for periods of breathing air rather than 100% oxygen (oxygen breaks) to reduce the chance of seizure or lung damage. The U.S. Navy uses treatment tables based on periods alternating between 100% oxygen and air. For example, USN table 6 requires 75 minutes (three periods of 20 minutes oxygen/5 minutes air) at an ambient pressure of 2.8 standard atmospheres (280 kPa), equivalent to a depth of 18 metres (60 ft). This is followed by a slow reduction in pressure to 1.9 atm (190 kPa) over 30 minutes on oxygen.