It is generally recognized that altitude exposure after diving
poses some increased risk of decompression sickness (DCS). Studies
underway at the Divers Alert Network and elsewhere have now confirmed
this increased risk. Lesser degrees of altitude exposure still
pose a risk but have not been as well studied. Guidelines for
altitude exposure after diving have been proposed by many different
agencies. Among those considered in the formulation of this document
are NOAA, USN, USAF, DAN, DCIEM, UHMS and the Australian Standard
for Occupational Diving. Any proposed standard must be flexible
enough to be modified as new data becomes available.
Scuba Diving is generally considered to be safe when practiced
in accordance with accepted limitations of depth and time. Nevertheless,
it is also known that decompression sickness is a "probabilistic"
event. It can occur at any time, and seemingly for "no reason"
(i.e., within the no-decompression limits of the table used).
As medical science advances, we learn more of previously unknown
risk factors that increase the likelihood of getting DCS. The
state of the body that describes the probability of dissolved
nitrogen coming out of solution to produce DCS is referred to
as "decompression stress." It is thought by many researchers
that disordering of the microcirculation can occur in some situations
such as multiple or deep dives, thus interfering with predictable
off-gassing of nitrogen. This increased risk and deviation from
predictability could be referred to as a state of increasing "decompression
stress." Altitude exposure after a dive also increases your
bodies "decompression stress."
ALTITUDE EXPOSURE RECOMMENDATIONS (waiting time in hours)
Single, Low Stress Dive
Multiple or Deco Dives
|1000 Ft. (300 m.)||none||
none (low stress)
6 hrs. (mod stress)
|2000 Ft. (600 m.)||none||
2 hrs. (low stress)
8 hrs. (mod stress)
|4000 Ft. (1200 m.)||8 hrs.||12 hrs.|
|6000 Ft. (1800 m.)||12 hrs. (F.A.D.)||24 hrs. (F.A.D.)|
F.A.D.: "Flying After Diving" surface
wait recommendations prior to air travel.
Low Stress: A dive performed well within the no-decompression limits, without significant risks such as high exertion, extremes of temperature, extreme fatigue etc.
Moderate Stress: Any dive performed to the no-decompression limit, decompression diving or dives involving one of the known risk factors for DCS.
High Stress: Extreme dive profiles, missed decompression stops or multiple risk factors. High Stress = High Risk. No predictions are possible due to the inherent alterations in off-gassing.
After Recompression, Status Post DCS: The highest risk of all is after recompression Hyperbaric Oxygen Therapy for even a mild case of decompression sickness. Many cases are on record documenting return of DCS symptoms with exposure to altitude. Recommendations are hopelessly inadequate except to suggest that an injured diver wait as long as is realistically possible before flying home, or driving over a high mountain pass to get home. One week is not too long to wait.
F.A.D. (Flying After Diving)
Studies presented to the Undersea and Hyperbaric Medical Society
at their Annual Meeting show that DCS risk while flying after
a single, low risk dive reaches a low level after approximately
10 hours. Allowing for the small number of divers studied it would
seem prudent to add a safety margin. When considering multiple
or decompression diving, DCS risk seems to reach a minimum at
18 hours, but again the "n" (number of subjects studied)
is small. The following are F.A.D. recommendations, supported
by experimental results and include a small safety
Flying After Diving Recommendations
|Type of Diving||Minimum Time to Wait|
|Single, Low Stress||
(Repetitive Factor = 1.0)
|Multiple or Deco||24 Hrs.|
*DCIEM recommendations are "your RF must be 1.0
before you fly." I no longer recommend using the USN/NOAA
"Group D" rule. Recent experimental evidence has identified
DCS risk while flying with this level of decompression stress.
Ron Nishi, M.A.Sc., P.Eng., Senior Scientist at DCIEM has more
than any other, contributed greatly to my understanding of DCS.
His input was invaluable in formulating the above recommendations.
Ron informs me that the original concept of a multi-tiered table
based on levels of decompression stress was first proposed at
DCIEM in unpublished work from the early 1980's. I strongly support
efforts to revisit this approach if divers can be convinced to
think in terms of variable risk for DCS.
These altitude exposure recommendations are submitted as guidelines
and not dogma. It is recognized that no two divers will have the
same degree of decompression stress after the same dive. Careful
decompression diving will often be less stressful than haphazard
no-deco diving. Therefore, relative decompression risk will vary
in a manner commensurate with overall decompression stress. A
diver should always travel after diving in the most conservative
way possible as it is impossible to design a totally risk free
table or altitude exposure guide which always works for everyone,
all the time. These guidelines are an attempt to introduce a surface
interval prior to altitude exposure in order to minimize decompression
stress without an unnecessary delay in travel.
FEEDBACK is encouraged and gratefully accepted.
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