3
Chapter 3 The Physiology of Diving and Nitrox 43
time because they are effectively holding their breath.
Because the convulsion was precipitated by breathing
a high partial pressure of oxygen, and oxygen tensions
in the body are therefore high, the person remains well
oxygenated during the convulsion, and hypoxia is not
a problem. Carbon dioxide levels will also become very
high because the muscles are exercising heavily while
the victim is not breathing. When the post-convulsive,
resting phase begins, the muscles relax, and the victim
remains unconscious. At this point, the victim can be
taken to the surface and first aid care begun.
Managing Oxygen Exposure
The best way to avoid oxygen toxicity problems is to
stay within correct oxygen exposure limits. As stated
above, the generally accepted limit for diving is 1.4 ata
PO2, with 1.6 ata PO2 as a contingency. Many divers
have dived to 61 meters (200 feet) and even considerably
deeper on air, and most have returned none the worse for
wear. Prudence should be part of all dive planning, and if
you have reason to dive beyond recreational limits, you
should be preparing for and taking a NAUI Technical
Diving training course. In deep diving, trimix reduces
not only your oxygen exposure but also your nitrogen
exposure. (If you want to dive deeper in a safer manner,
get training in helium-based gas mixtures.)
The oxygen exposure limits described in this book
carry an extremely low risk of oxygen toxicity. They are
well below any levels that might reasonably be expected
to cause problems.
In addition to a general PO2 limit, NOAA, in the
third edition of its Diving Manual, introduced oxygen
exposure time limits for a range of oxygen partial
pressures from 0.6 ata to 1.6 ata (Figure 3-9). These
limits were set to address the risks of pulmonary oxygen
toxicity. The table shows allowable time for a single
dive at any PO2 as well as the maximum accumulated
exposure time over any 24-hour period. For example, for
a PO2 of 1.0 ata (equivalent to 38 meters/124 feet on air),
the maximum dive time for a single dive is 300 minutes
(and is the same for any 24-hour period). For a PO2 of
1.4 ata (33 meters/111 feet on EAN32) the time limit
for a single dive is 150 minutes (and 180 minutes in any
24-hour period).
In all cases, a recreational nitrox diver’s single dive
time will be limited by the no-decompression limits as
well as the diver’s gas supply. These will be less than
the NOAA single dive oxygen exposure limit, and the
controlling limit, if it arises at all, would be the 24-hour
oxygen exposure limit. Even then, only the most
dedicated and determined nitrox diver would possibly
exceed the 24-hour limit. (A diver using EAN36 and
diving to the maximum dive times allowed by the NAUI
EAN36 Dive Tables would have to perform seven squareprofile
NOAA Oxygen Exposure Limits
PO2 (atm)
Maximum
Single
Dive Limit
(minutes)
Maximum
24-Hour
Limit
(minutes)
1.60 45 150
1.55 83 165
1.50 120 180
1.45 135 180
1.40 150 180
1.35 165 195
1.30 180 210
1.25 195 225
1.20 210 240
1.10 240 270
1.00 300 300
0.90 360 360
0.80 450 450
0.70 570 570
0.60 720 720
Figure 3-9 NOAA Oxygen Exposure Time Limits.
The table gives the limits for a single
dive exposure to various PO2 levels and
for exposure during any 24-hour period.
(NOAA Diving Manual, 4th edition, 2001).
Oxygen partial pressures between 1.4 ata
and 1.6 ata are shaded.
dives to 27 meters (90 feet) with dive times of
50 minutes for the first dive and 24 minutes for each
repetitive dive with a minimum surface interval of 2
hours 39 minutes between each dive.)