Although the use of NITROX can increase the diver’s bottom time and reduce the
risk of nitrogen narcosis, using a NITROX mixture raises the concern for oxygen
toxicity. For example, using air as the breathing medium, an oxygen partial pressure
(ppO2) of 1.6 ata is reached at a depth of 218 fsw. In contrast, when using the
NITROX mixture containing 60 percent nitrogen and 40 percent oxygen, a ppO2
of 1.6 ata is reached at 99 fsw. Therefore, oxygen toxicity must be considered
when diving a NITROX mixture and is a limiting factor when considering depth
and duration of a NITROX dive.
Generally speaking, there are two types of oxygen toxicity—central nervous
system (CNS) oxygen and pulmonary oxygen toxicity. CNS oxygen toxicity is
usually not encountered unless the partial pressure of oxygen approaches or
exceeds 1.6 ata, but it can result in serious symptoms (see paragraph 3-10.2.2),
including potentially life-threatening convulsions. Pulmonary oxygen toxicity
may result from conducting long-duration dives at oxygen partial pressures in
excess of 1.0 ata. For example, a dive longer than 240 minutes at 1.3 ata or a dive longer than 320 minutes at 1.1 ata may place the diver at risk if the exposure is on
a daily basis. Pulmonary oxygen toxicity under these conditions can result in
decrements of pulmonary function, but is not life threatening.
The NITROX Equivalent Air Depth (EAD) Decompression Selection Table (Table
10-1) was developed considering both CNS and pulmonary oxygen toxicity.
Normal working dives that exceed a ppO2 of 1.4 ata are not permitted, principally
to avoid the risk of CNS oxygen toxicity. Dives with a ppO2 less than 1.4 ata,
however, can be conducted using the full range of bottom times allowed by the air
tables without concern for CNS or pulmonary oxygen toxicity.
Supervisors must keep in mind that pulmonary oxygen toxicity may become an
issue with frequent, repetitive diving. The effects of pulmonary oxygen toxicity
can be cumulative and can reduce the underwater work performance of susceptible
individuals after a long series of repetitive daily exposures. Fatigue, headache, flulike
symptoms, and numbness of the fingers and toes may also be experienced
with repetitive exposures. Table 10-1 takes these repetitive exposures into account,
and therefore problems with oxygen toxicity should not be encountered with its
use. If symptoms are experienced, the diver should stop diving NITROX until they
resolve.
Selecting the Proper NITROX Mixture. Considerable caution must be used when
selecting the proper NITROX mixture for a dive. The maximum depth of the dive
must be known as well as the planned bottom time. Once the maximum depth is
known, the various NITROX mixtures can be evaluated to determine which one
will provide the least amount of decompression while also allowing for a
maximum bottom time. If a diver’s depth exceeds that allowed for a certain
NITROX mixture, the diver is at great risk of life-threatening oxygen toxicity.
