alt=Closeup of a diving cylinder with a band reading "NITROX". A hand-printed label at the neck reads "MOD 28m 36% O2", with the 28 in much larger size. The prevention of<unk>  depends entirely on the setting. Both underwater and in space, proper precautions can eliminate the most pernicious effects. Premature infants commonly require supplemental<unk>  to treat complications of preterm birth. In this case prevention of bronchopulmonary dysplasia and retinopathy of prematurity must be carried out without compromising a supply of<unk>  adequate to preserve the infant's life. Underwater<unk>  is a catastrophic hazard in scuba diving, because a seizure results in near certain death by drowning. The seizure may occur suddenly and with no warning symptoms. The effects are sudden convulsions and unconsciousness, during which victims can lose their regulator and drown. One of the advantages of a full-face diving mask is prevention of regulator loss in the event of a seizure. As there is an increased risk of central nervous system<unk>  on deep dives, long dives and dives where<unk> -rich breathing gases are used, divers are taught to calculate a maximum operating depth for<unk> -rich breathing gases, and cylinders containing such mixtures should be clearly marked with that depth. In some diver training courses for these types of diving, divers are taught to plan and monitor what is called the<unk>  clock' of their dives. This is a notional alarm clock, which ticks more quickly at increased<unk>  pressure and is set to activate at the maximum single exposure limit recommended in the National Oceanic and Atmospheric Administration Diving Manual. For the following partial pressures of<unk>  the limits are: 45 minutes at , 120 minutes at , 150 minutes at , 180 minutes at and 210 minutes at , but it is impossible to predict with any reliability whether or when toxicity symptoms will occur. Many nitrox-capable dive computers calculate an<unk>  loading and can track it across multiple dives. The aim is to avoid activating the alarm by reducing the partial pressure of<unk>  in the breathing gas or by reducing the time spent breathing gas of greater<unk>  partial pressure. As the partial pressure of<unk>  increases with the fraction of<unk>  in the breathing gas and the depth of the dive, the diver obtains more time on the<unk>  clock by diving at a shallower depth, by breathing a less<unk> -rich gas, or by shortening the duration of exposure to<unk> -rich gases. Diving below on air would expose a diver to increasing danger of<unk>  as the partial pressure of<unk>  exceeds , so a gas mixture must be used which contains less than 21%<unk>  (a hypoxic mixture). Increasing the proportion of nitrogen is not viable, since it would produce a strongly narcotic mixture. 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<unk>  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<unk>  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<unk>  treatment. The schedules used for treatment of decompression illness allow for periods of breathing air rather than 100%<unk> <unk>  breaks) to reduce the chance of seizure or lung damage. The U.S. Navy uses treatment tables based on periods alternating between 100%<unk>  and air. For example, USN table 6 requires 75 minutes (three periods of 20 minutes<unk> /5 minutes air) at an ambient pressure of , equivalent to a depth of . This is followed by a slow reduction in pressure to over 30 minutes on<unk> . The patient then remains at that pressure for a further 150 minutes, consisting of two periods of 15 minutes air/60 minutes<unk> , before the pressure is reduced to atmospheric over 30 minutes on<unk> . Vitamin E and selenium were proposed and later rejected as a potential method of protection against pulmonary<unk> . There is however some experimental evidence in rats that vitamin E and selenium aid in preventing in vivo lipid peroxidation and free radical damage, and therefore prevent retinal changes following repetitive hyperbaric<unk>  exposures. Normobaric setting Bronchopulmonary dysplasia is reversible in the early stages by use of break periods on lower pressures of<unk> , but it may eventually result in irreversible lung injury if allowed to progress to severe damage. One or two days of exposure without<unk>  breaks are needed to cause such damage. Retinopathy of prematurity is largely preventable by screening. Current guidelines require that all babies of less than 32 weeks gestational age or having a birth weight less than should be screened for retinopathy of prematurity at least every two weeks. The National Cooperative Study in 1954 showed a causal link between supplemental<unk>  and retinopathy of prematurity, but subsequent curtailment of supplemental<unk>  caused an increase in infant mortality. To balance the risks of hypoxia and retinopathy of prematurity, modern protocols now require monitoring of blood<unk>  levels in premature infants receiving<unk> . Hypobaric setting In low-pressure environments<unk>  may be avoided since the toxicity is caused by high partial pressure of<unk> , not merely by high<unk>  fraction. This is illustrated by modern pure<unk>  use in spacesuits, which must operate at low pressure (also historically, very high percentage<unk>  and lower than normal atmospheric pressure was used in early spacecraft, for example, the Gemini and Apollo spacecraft). In such applications as extra-vehicular activity, high-fraction<unk>  is non-toxic, even at breathing mixture fractions approaching 100%, because the<unk>  partial pressure is not allowed to chronically exceed .
Oxygen toxicity
Cerebral hypoxia
Nitrogen dioxide poisoning
Chromium toxicity
Histotoxic hypoxia
Fluoride toxicity
Chlorine gas poisoning
Silent hypoxia