3
42 NAUI Nitrox Diver
Maximum and Contingency
Operating Depths
EAN32 EAN36
Maximum
Depth
33 msw
(110 fsw)
27 msw
(90 fsw)
PO2 1.39 1.34
Contingency
Depth
40 msw
(130 fsw)
33 msw
(110 fsw)
PO2 1.58 1.56
in, you could win, or you could lose a little, or you could
lose a lot, but in the long run the house always wins.
For recreational nitrox diving, the generally accepted
PO2 exposure limit is 1.4 atmospheres absolute, with
1.6 ata reserved for contingencies (Figure 3-7). With
appropriate selection of your enriched air nitrox mixture,
1.4 ata PO2 is more than adequate for 99.9% of the dives
you may want to accomplish.
Among the many factors that can increase your
susceptibility to CNS oxygen toxicity (an “Ox-Tox Hit”)
are heavy exercise, increased carbon dioxide build-up
from whatever cause, chilling or hypothermia, and water
immersion (as opposed to “chamber diving”). One
cannot predict oxygen toxicity.
It is also impossible to predict a reliable sequence of
toxicity signs and symptoms. The first noticeable sign
may be the epilepsy-like convulsions. This may not be
serious in itself, but it is most certainly a problem if it
occurs at a depth of 30 meters (100 feet) while breathing
out of a scuba regulator. Drowning is a very likely result.
Many preliminary manifestations of CNS oxygen toxicity
have been reported, either singly or in combination.
The mnemonic acronym “ConVENTID” is useful for
remembering the most obvious of them. ConVENTID
stands for: Convulsions, Visual disturbances, Ears,
Nausea, Twitching or Tingling, Irritability, and Dizziness
or Dyspnea.
• Convulsions are the most obvious and most
serious signs. Possible precursors to convulsions
are:
• Visual disturbances, tunnel vision, dazzle or
seeing “fireflies.”
• Ear ringing, tinnitus, or sounds like an
approaching train in a tunnel.
• Nausea, including vomiting.
• Twitching, especially of the lips and small facial
muscles or the hands, or tingling (paresthesia)
especially in the fingers.
• Irritability restlessness, euphoria, dysphoria
(uneasiness or feelings of impending doom),
anxiety, or general confusion.
• Dizziness and vertigo or dyspnea (difficult or
labored breathing).
Other signs can include facial pallor, slowed
heart rate (bradycardia), pupil dilation, hiccups, and
hallucinations.
Appearance of any sign or symptom of oxygen
toxicity is reason to terminate the dive (Figure 3-8). But,
because precursor symptoms are highly variable – as
well as subjective – and they may not occur, it is doubly
important that divers keep their PO2 exposure within
an acceptable limit. In one study, convulsions were
the first noticed manifestation in 40% of the subjects
studied. In another study, nausea was the most common
first manifestation, followed by muscular twitching and
vertigo.
If a convulsion were to occur underwater, there is
little that can be done until the active phase of the seizure
is over and the muscles relax. Muscle contraction may
cause the diver to lose the regulator, but the victim also
ceases to breathe during the active phase as the vigorous,
uncontrolled muscle contractions interrupt breathing
and the tongue blocks the airway. No attempt should
be made to surface victims of an “Ox-Tox Hit” at this
Figure 3-7 Maximum operating depths and maximum
contingency depths for standard mixes,
based on a normal maximum PO2 of
1.4 ata, and a contingency limit PO2 of
1.6 ata.
Figure 3-8 Appearance of any sign or symptom of
oxygen toxicity is reason to terminate the
dive.