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Intern@tion@l Dre@m.net |
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Intern@tion@l Dre@m.net |
Have you
ever wondered why you only get five-and-a-half days of diving in a seven-day
dive package? Sure, everyone knows that you aren't supposed to dive before you
fly, but how long are you supposed to wait? Is it 12 hours? 24 hours? The
answer isn't clear-cut, but one thing is: It all depends on the kind of diving
you've been doing and the amount of risk you're willing to take.
What's the Big Deal? | Pressurization in Airplane Cabins | How Long Must I Wait? | Losing An Entire Day? | Testing the Theories | A Conservative View | Extending the Test Group | 12 or 24 Hours? | Bottom Line on Bottom Time
Flying
after diving puts a diver at increased risk of decompression sickness (DCS) by
decreasing the pressure to which the body is exposed. Remember that nitrogen
dissolves in your blood and tissues during a dive and can bubble during ascent
to the surface? Likewise, residual nitrogen dissolved in your body at sea level
can also come out of solution on ascent in an airplane. The more nitrogen
dissolved in your tissues, the more likely this is to happen, particularly as
the result of diving.
Yes, they
are. Federal Aviation Administration guidelines stipulate that commercial
airline passengers not be exposed to altitudes greater than 8,000 feet. While
there are rare exceptions to this rule - the airport in Quito, Ecuador, through
which most divers pass on their way to and from the Galapagos, is at 9,200 feet
- most modern airplanes actually expose passengers to atmospheres equivalent to
an altitude of 5,000 to 7,000 feet. These altitudes correspond to a decrease in
the pressure to which the body is exposed of close to 25 percent, equivalent in
percentage change to an ascent from 99 to 66 feet. While this may not seem so
significant, it is important to keep in mind that the body's tissues are
saturated with nitrogen at sea level. And if you've been diving shortly before
flying, residual nitrogen may still be leaching out of your tissues, a process
that would be accelerated during flight and possibly causing symptoms of DCS.
The answer
to the question of how soon after diving a person can safely fly is still not
definitively known. Twelve hours is accepted by most as being a safe interval
for someone who has made a single dive, or a dive a day over several days.
Therefore, the real debate concerns what constitutes an appropriate time-to-fly
for a diver who has made multiple dives over multiple days, as is commonly done
at many resorts and on live-aboard dive boats.
The original guideline of
12 hours has its origins in the military, and was supposed to represent a time
after which the body was free of residual nitrogen. The National Oceanographic
and Atmospheric Administration (NOAA) guidelines stipulated that a person wait
until reverting back to being a Group B diver before flying. Neither the
military nor NOAA rules were arrived at using scientifically derived data, but
rather by empirical observation of what surface interval resulted in an
acceptably low incidence of decompression sickness (DCS). However, the
important point is that both of these organizations were willing to accept a
certain number of DCS cases under their guidelines. While this number was small
- at most 1 to 2 percent - these guidelines incorporated a degree of risk that
would not be acceptable to the general public.
In 1989, the Undersea and
Hyperbaric Medical Society held a workshop on the issue of flying after diving
and attempted to reach a consensus on what constituted a safe surface interval.
The problem was that there was no hard, scientific data on how long a diver
needed to wait before flying in order to minimize the risk of DCS. After much
discussion, workshop participants analyzed DAN accident data from 1987 and
looked at divers who experienced signs and symptoms of DCS during, or
immediately after, flying home. They found that almost all of the divers who
suffered onset of DCS while flying had flown within 24 hours of their last
dive. The vast majority of divers who experienced symptoms despite a greater
than 24-hour surface interval were felt, on retrospective study, to have had
some symptoms prior to flying.
Based upon this analysis,
the consensus of the workshop was that the recommended time between a diver's
last dive and his or her flight home - for someone who has made multiple dives
over several days - should be 24 hours.
While this
was only a recommendation, it rapidly became accepted as gospel among many
resorts and dive operators. This caused a conflict. Divers felt that they were
being denied a day of diving, and resorts feared that they would be exposed to
potential liability if they allowed guests to dive within 24 hours of their
scheduled departure. This resulted in significant pressure from the diving
industry either to produce data backing the 24-hour recommendation, or to
change the guidelines.
In 1991, DAN issued a
statement clarifying its position on flying after diving. It stated that there
should be a minimum surface interval of 12 hours before flying in a commercial
jet. The statement went on to say that divers who make multiple dives for
several days, or who make dives that require decompression stops, should
strongly consider extending their surface interval. This placed the responsibility
of deciding how long to wait between diving and flying squarely on the diver,
while acknowledging that there was currently no data to support any specific
surface interval prior to flying.
It was
around this time that DAN set out to obtain hard data on how long a diver
should wait after diving in order to minimize the risk of DCS triggered by
exposure to altitude. Test divers were subjected to a chamber dive, and then
exposed to hypobaric conditions and evaluated for symptoms. The first dive
profile tested was to 60 feet for 55 minutes. The results from this series of
tests showed that at surface intervals up to 10 hours divers still experienced
symptoms of DCS when exposed to simulated altitude conditions. However, no
symptoms of DCS were seen in subjects who had a surface interval of 12 hours or
longer. These results appear to support the recommendation of a 12-hour surface
interval for divers who do a single dive, or several single-dive days, during a
vacation.
Subsequent tests involved
an initial dive to 60 feet for 55 minutes, followed by a second 60-foot dive
for 20 minutes with a one-hour surface interval in between. Later profiles
added a third 60-foot dive for 20 minutes, again after a one-hour surface
interval. Divers exposed to hypobaric conditions mimicking a commercial
airliner at altitude after the two- and three-dive profiles experienced
symptoms of DCS at surface intervals up to 15 hours. This could be interpreted
as supporting DAN's recommendation of a surface interval longer than 12 hours
for divers who make multiple dives.
Michael
Emmerman, director of the Special Operations Support Group and an independent
researcher who has worked and published extensively on the subject of flying
and diving, feels that the recent DAN research is significant. While being
careful to acknowledge the limitations of the DAN data - these were chamber
dives and the profiles were very specific - Emmerman feels that it is not
unreasonable to make some assumptions from the data that would carry over into
real-life diving situations.
Most researchers agree that
chamber dives are less strenuous than open-water dives. Subjects in a chamber
are not exercising, are usually well-hydrated, and are not exposed to cold -
all factors thought to play a role in DCS. Therefore, Emmerman feels that the 16-hour
surface interval identified in the DAN multi-dive studies as being a safe
window prior to flying is probably too short for a diver who has completed
multiple open-water dives. If one were to add an additional 50 percent to this
interval, then we're back to the 24-hour recommended surface interval. Emmerman
says further research is needed in this area before definite answers are known
and accepted as scientifically valid, but in the interim he feels that the
current recommendations are reasonable, and supported by the emerging data.
That more
research is needed is not disputed by Bret Gilliam, the current chief executive
officer of UWATEC USA, president of Tech Divers International (TDI), and former
president of NAUI. But he disagrees with the validity of applying the results
of a relatively small number of chamber dives, which have square profiles, to
real-life, multi-level recreational diving.
According to Dr. Richard
Vann, DAN's research director, there have been a total of 540 chamber dives
conducted to date, with 22 suspected DCS hits. Gilliam asks why we should
utilize numbers derived from a few hundred subjects in chambers, when there are
hundreds of thousands of dives made annually that can provide more accurate and
meaningful data? To this end, UWATEC is one of the major suppliers of computers
and software being used to compile data for DAN's Project Dive Safety. Gilliam
argues that the data that will eventually result from Project Dive Safety -
data compiled from actual open-water dives - will be much more meaningful than
any chamber dives could be.
Vann agrees that Project
Dive Safety - and its European counterpart Project Safe Dive - will add useful
information. These ongoing projects aim to obtain information from over one
million actual dives. However, Vann points out that even though participants
are asked to record pre-flight surface intervals, the flying after diving
question is not what the project has been designed to address, and is unlikely
to provide any definitive answers.
Vann says that he is
looking forward to future studies in wet tanks that will incorporate all the
factors present in actual dives - depth, immersion, exercise and compressed air
- in a controlled laboratory setting.
So the
question remains for divers: How long should you wait before flying after
multiple dives? Both Emmerman and Vann, while citing the need for further
research, advocate a "better safe than sorry" attitude and feel that
the 24-hour recommendation is appropriate. In the absence of any hard data to
the contrary, Gilliam feels that a 12-hour surface interval prior to flying is
sufficient. He points out that advances such as computers, widespread use of
alternate air sources, and the 15-foot safety stop have all contributed to make
diving safer. Many divers are not waiting 24 hours before flying, and if this
is so dangerous then why aren't we seeing more cases of DCS? Gilliam asks.
So the
question still remains. How long should a diver wait after completing multiple
dives before flying? In many cases this is almost a moot point. Most
live-aboards only plan a half day of diving on the day before coming into port.
If divers rushed directly from the boat to the airport, it might be possible to
fly with a surface interval of less than 24 hours, but this would be a
challenge. Many resorts will likewise not schedule dives for guests in the 24
hours prior to their scheduled departure. Most guests are willing to abide by
this and spend their time at the bar or on the beach while giving their gear a
chance to dry before they pack.
But this is not always the
case. And given a situation where you have control over the surface interval
before flying after completing multiple dives, I would choose to wait 24 hours.
Based on the available data, I feel that this provides for the least amount of
risk, while not significantly inconveniencing the diver.
It is equally
understandable if a diver, looking at the same data, were to choose a shorter
surface interval prior to flying. This, after all, is what DAN's guidelines
were designed to address when they were written. The ultimate decision and
responsibility lie with you.
