Q:
While vacationing in St. Maarten, I plan to do
a day trip to Saba and do three dives there. I
would fly out in the morning and return in the
evening. The first dive would be between 90 and 130 feet
deep, and the two subsequent dives would be no deeper
than 80 feet. The dive operator assured me it would be
safe to fly back the same day, as the aircraft would not
exceed 2,000 feet during the 12-minute return flight.
Would it be safe to fly in this situation, and is 2,000 feet
the cutoff altitude for flying after diving?
A:
You are asking about a pattern of exposure for
which there is very limited empirical data. The
guidelines in this low altitude range are cobbled
together from a combination of expert opinion and
whatever research can reasonably be applied. Knowing that
there is a high likelihood of some degree of vertical travel
postdive (and no evidence of a huge problem resulting
from such activity) and appreciating the implications
of research data from Switzerland based on 1,700-foot
exposures, the 2002 Divers Alert Network® flying-after-
diving guidelines stipulated a cabin altitude within the
2,000- to 8,000-foot range to qualify for flying-after-diving
discussion. Lesser altitudes were excluded. Alternatively,
the U.S. Navy flying-after-diving Table 9-6 (2008, Revision
6) begins with an altitude table of 1,000 feet, although it
applies postdive surface-interval penalties only for high
repetitive groups.
While the above may seem promising, the dives you
propose to do could easily achieve
extremely high postdive repetitive
groups. The remaining wild cards are
the flight altitude and duration. Staying
below 2,000 feet may be possible, but it
is certainly not guaranteed. Weather and
traffic conditions might prolong flight
time or result in a higher altitude. At the
same time, low atmospheric pressure
could increase the decompression stress
of any given altitude. The person giving
you the advice was describing an optimal
case that may or may not reflect reality.
The overall risk in the day you
describe could be acceptable, but it is
certainly not a plan I would encourage or
choose to do for recreation. Three dives
to the depths described indicate that
this would not be a low-stress day under
the best conditions. The true net risk is
determined by multiple factors — dive profiles, thermal stress,
exertion level, between-dive surface interval, preflight surface
interval, environmental conditions (weather, atmospheric
pressure, geography), pilot discretion, other air traffic and
individual susceptibility. The plan entails enough risk that it
would not take too many factors going in the wrong direction
to produce a bad day.
Safety margins could be added through the use of nitrox
with a decompression algorithm set to air (but carefully
respecting the maximum depth limits to avoid oxygen
toxicity) and/or diving more conservative profiles and/or
delaying the return time. The problem with planning such an
ambitious day is that once it is set in motion it may be hard to
stop. A compromise here or there or in any number of ways
could easily increase the hazard.
Ultimately, you have to decide what risk you are willing to
assume. Unless you are diving alone, your partner or partners
would have to make similar decisions. In thinking of this,
remember that decompression injuries tend to be probabilistic
events. Getting away with something once, twice or 99 times
does not make it safe. My advice is to focus on all the ways
to increase safety margins to make diving as worry-free as an
enjoyable activity should be.
—Neal W. Pollock, Ph.D.
Q:
I read in a recent issue of Alert Diver (From
the Medical Line, Spring 2013) that coral
cannot grow under your skin, but can other sea
creatures? I scraped against something while diving;
58
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FALL 2013
RESEARCH, EDUCATION & MEDICINE
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F R O M T H E M E D I C A L L I N E
STEPHEN FRINK
