Wednesday, August 10, 2005

Flying/Going to Altitude After Diving, Diving At Altitude

There is a recurring question that arises in one form or another about diving and then flying, or going to altitude and diving, going further to altitude and descending to place of abode or sea level. We have had a recent spate of queries about this culminating in a question that was forwarded me by Dr. Ed Kay for circulation among our consultants.

There are tables that address these issues in the NOAA textbook and the US Navy Diving Manual but these are not readily available to the general diving public. The US Navy made changes in it's recommendationsin their Revision 4 on the basis of research done by Divers Alert Network. [see below].

In addition, DAN has had a workshop in May, 2002, to sort out approaches to this problem and has made a review of the proceedings of the FAD Workshop Concensus and has come up with these recommendations:

1. For a single no-decompression dive, a minimum preflight surface interval of 12 hours is suggested.
2. For multiple dives per day or multiple days of diving, a minimum preflight surface interval of 18 hours is suggested.
3. For dives requiring decompression stops, there is little evidence on which to base a recommendation, but a preflight surface interval substantially longer than 18 hours appears prudent.
4. These recommendations apply to air dives followed by flights at cabin altitudes of 2,000 to 8,000 feet (610 to 2,438 meters).
5. These recommendations are for recreational divers who do not have symptoms of decompression sickness (DCS).

A question that we recently received is as follows:

I’ll soon be moving permanently to the Big Island of Hawaii.
I plan to dive regularly out of Kawaihae Harbor on the Island’s west side. However, I will be residing in Kamuela-Waimea about 15 miles away and at an elevation of about 2600 feet. I will spend an hour at sea-level after the second of my two dives before heading “up the hill” to home.

I will be diving NITROX, either EANx32 or EANx36, and would, under normal circumstances, remain within standard no-decompression time limits; to include remaining at sea-level for 24 hours after my last dive. But, of course, this latter precaution will NOT be the case for me.
Has anyone developed tables which would account for this sort of altitude factor such that I could further precisely limit time-on-bottom and then safely go home without feeling “that strange twang” in my elbow a few hours later?


A DAN researcher, Dr. Richard Vann has this to say about your query:
"See the attached FAD workshop proceedings [] that may be some help. The consensus guidelines probably won’t help much but look in the section by Ed Flynn about the USN guidelines that were based in part on the DAN trials. These procedures might work for you if you compute your EAD (Equivalent Air Depth) for your nitrox mix and estimate the USN Repet Dive Group for your dives. I believe that the Navy procedures may allow you to go home with little or no delay. Bear in mind, however, that other than the DAN trials, there has been little testing across the range of altitudes or for many dives. In fact, trials we are presently doing for the Navy suggest that our results might be different if our test subjects were immersed and exercising rather than dry and resting. We hope to test that idea in the coming year. Nothing is for sure, of course, but I wouldn’t be surprised if you can get away with it. Always a good idea to have 100% oxygen on hand at the dive site and in the car, too."

A highly respected PADI dive instructor has these remarks:
"Because people differ in their susceptibility to decompression sickness, no decompression table can guarantee that decompression sickness will never occur even though you dive within the table limits" is a common phrase to divers, and covers the real world variables. And most divers are aware that the waiting to fly after diving time is a function of the dive table used and types of dives made. Current minimum wait to fly times for PADI table users is in the 12 to 14 hour category, provided limits were not exceeded. Most every airline pilot I know on the Caribbean routes claims at least one passenger per flight returning to the USA after diving did not consider this advice and became a decompression sickness problem at altitude.
PADI has no specific time delay for those diving with the RDP before driving to altitude, but states divers should be conservative and allow as long a surface interval as possible, again trying to account for the myriad of diver profiles. So, we have concerns for after diving driving to altitude, have little data, and no table for this instance - only recommendations to wait.
I suggest that the diver contact the dive shops on Hawaii for information. There may be local knowledge on this matter which has not made it to the agencies."

Here are comments and caveats from a respected diving medical officer:
First, it depends on the amount of risk this diver wishes to assume. Some factors to consider are "can not wait", access to oxygen, use of experimental tables (not much data but appears reasonable, "tech diving" approach), access to recompression chamber etc.

Nitrox, surface oxygen all sound good coupled with conservative use of decompression procedures as well as use of flying after diving guidelines. Goes without saying the use of standard preventive measures are in effect; hydration, avoid questionable profiles, yo yo etc. There are dive computers that try to account for altitude exposure but again "experimental" probably due to every unique exposure.
This is a great illustration of the difficulty with the attempts at table investigation outside the "laboratory".

Ramifications of this "simple" question arise as can be seen in this query on the Scuba Clinic forum located at .
"Most of our little group of divers have 10 or less dives under our belts. We are planning a dive which takes us from our home at about 600 ft to the dive spot, which is 3205 ft. The drive is approximately 7 hours long, covering appx 430 miles. We will be driving to the location and arriving Thursday morning about 3:00 a.m.

We figured we'd plan two dives for Friday and two for Saturday, both no more than than 25 ft deep, as that is the max depth of the pool. We had planned to leave about noon on Sunday to drive back.

So herein is the issue ..... in the afternoons we'd like to visit some area attractions. However, these other places are about another 2000 to 3000 additional feet in altitude. Will it be safe for us to take these little excursions in the afternoon or should we plan on bringing lots of boardgames and staying in? "

The NOAA Diving Manual would say that you should consider that your dives would be in salt water [allowing you to leave out figuring in the difference in fresh water and salt water]. They would also allow you to subtract 600 feet from the 3205 foot projected altitude of the dive, a dive at about 2605 feet. This would round off to the nearest thousand, or 3000 feet.
In their Table 4.1 on page 4-25, a 25 foot dive at 3000 feet would be a sea level equivalent depth of 30 feet. However, correcting for pressure equivalents, it would closer to 27.75 fsw, placing you in Repetive group B on your dive tables. Rep dives then have to be figured as if the dives are from the sea level equivalent depth dives, possibly ending up as a group G [as an example].

NOAA also has a table [4.3] that gives the surface interval in hours:minutes before making a further ascent to altitude. The surface interval depends upon the highest repetitive group designator obtained in the previous 24 hour period at the altitude of the dive. Ascending to 6500 feet would be an increase of 3500 feet [rounded to 4000 feet], this entered on the table at rep Group G and the wait would be 1 hour, 23 minutes. There would be no wait with groups A-F.

Here is another question answered by Dr. Martin Quigley.
My query is about flying after a dive? Some say that the maximum altitude that one can go is 1000ft? others say it's 2000ft. justlast week my dive buddies had a discussion about the same topic. I'm trying to settle it once and for all. The question would be is what would be the maximum safe altitude that you can fly using a helicopter after making two dives with a maximum depth of 60 feet on the first and 40 feet on the second. between the dives you had a 90 minutes surface interval? after the second dive you have two fly out.

While your question might appear simple, the answer is not. I’ll try to pull together several different sources to give you an answer.

The current flying after diving guidelines from the Divers Alert Network (DAN) are:
Dives within the No-Decompression Limits
· A Single No-Decompression Dive: A minimum preflight surface interval of 12 hours is suggested.
· Multiple Dives per Day or Multiple Days of Diving: A minimum preflight surface interval of 18 hours is suggested.
Generally diving at an elevation of less than 1000 feet above sea level requires no adjustment of the No Decompression Limits (NDLs). [1000 feet altitude is approximately 0.95 ATA.]
Commercial airliners must be pressurized to no greater than 8000 feet (approximately 0.75 ATA) and are usually pressurized to a lower altitude.

Decompression theory is quite complex and I am going to oversimplify it. As an oversimplification, assume that the human body can stand a partial pressure of nitrogen of approximately 1.6 with getting decompression illness (DCI) symptoms. That means that as long as the partial pressure of nitrogen in any body tissue does not exceed 1.6 (approximately twice the nitrogen content at the surface) when surfacing, the DCI risk is minimal.

If you ascend to a higher altitude after diving, then the difference between the levels of absorbed nitrogen and the ambient pressure at altitude become the relevant measures. In my very crude example, if you ascended to 1000 feet altitude, you could only tolerate a maximum tissue nitrogen partial pressure of 1.55. At 8000 feet altitude, you could only tolerate 1.35.
In any case, decompression tables and dive computers only function to minimize the chances of getting DCS. The further you “push” the tables, the lower the margin of safety.

In your example, I don’t believe that a helicopter flight to 1000 feet altitude presents any significant increase in risk of DCS for the dive profiles you have described. There is no absolute “safe” altitude. 2000 feet elevation is riskier. At 8000 feet elevation (and this includes driving over mountains after diving), I’d recommend adherence to the DAN guidelines above.

Here is some information gleaned from the USN Diving Manual that is helpful in calculating the amount of time that should be spent waiting for safe ascent. One assumes that the diver has Repetitive Dive tables and is familiar with their use.

Equilibration at Altitude. Upon ascent to altitude, two things happen. The body off-gases excess nitrogen to come into equilibrium with the lower partial pressure of nitrogen in the atmosphere. It also begins a series of complicated adjustments to the lower partial pressure of oxygen. The first process is called equilibration; the second is called acclimatization. Twelve hours at altitude is required for equilibra-tion. A longer period is required for full acclimatization. If a diver begins a dive at altitude within 12 hours of arrival, the residual nitrogen left over from sea level must be taken into account. In effect, the initial dive at altitude can be considered a repetitive dive, with the first dive being the ascent from sea level to altitude. Table 9-4 gives the repetitive group associated with an initial ascent to altitude. Using this group and the time at altitude before diving, enter the Residual Nitrogen Timetable for Repetitive Air Dives (Table 9-7) to determine a new repetitive group designator associated with that period of equilibration. Determine sea level equivalent depth for your planned dive using Tabl e 9-3. From your new repetitive group and sea level equivalent depth, determine the residual nitrogen time associated with the dive. Add this time to the actual bottom time of the dive.
Example: A diver ascends rapidly to 6000 feet in a helicopter and begins a dive to 100 fsw 90 minutes later. How much residual nitrogen time should be added tothe dive? From Table 9-4, repetitive group upon arrival at 6000 feet is Group E. During 90 minutes at altitude, the diver will desaturate to Group D. From Tabl e 9-3, sea level equivalent depth for a 100 fsw dive is 130 fsw. From Tabl e 9-7, residual nitrogen time for a 130 fsw dive in Group D is 11 minutes. The diver should add 11 minutes to bottom time.

Table 9-4 can also be used when a diver who is fully equilibrated at one altitude ascends to and dives at a higher altitude. Enter Table 9-4 with the difference between the two altitudes to determine an initial repetitive group.

Example: Divers equilibrated at a base camp altitude of 6000 feet, fly by helicopter to the dive site at 10,000 feet. The difference between the altitudes is 4000 feet. From Table 9-4, the initial repetitive group to be used at 10,000 feet is Group C.

Table 9-4. Repetitive Groups Associated with Initial Ascent to Altitude.
Altitude (feet) Repetitive Group
1000 A
2000 B
3000 B
4000 C
5000 D
6000 E
7000 E
8000 F
9000 G
10000 H


Leaving the dive site may require temporary ascent to a higher altitude. For example, divers may drive over a mountain pass at higher altitude or leave the dive site by air. Ascent to altitude after diving increases the risk of decompression sick-ness because of the additional reduction in atmospheric pressure. The higher the altitude, the greater the risk. (Pressurized commercial airline flights are addressed in Note 3 of Table 9-5.)

Table 9-5 gives the surface interval (hours:minutes) required before making a further ascent to altitude. The surface interval depends on the planned increase in altitude and the highest repetitive group designator obtained in the previous 24- hour period. Enter the table with the highest repetitive group designator obtained in the previous 24-hour period. Read the required surface interval from the column for the planned change in altitude.

Example: A diver surfaces from a 60 fsw for 60 minutes no-decompression dive at sea level in Repetitive Group J. After a surface interval of 6 hours 10 minutes, the diver makes a second dive to 30 fsw for 20 minutes placing him in Repetitive Group C. He plans to fly home in a commercial aircraft in which the cabin pressure is controlled at 8000 feet. What is the required surface interval before flying? The planned increase in altitude is 8000 feet. Because the diver has made two dives in the previous 24-hour period, you must use the highest Repetitive Group Designator of the two dives. Enter Table 9-5 at 8000 feet and read down to Repetitive Group J. The diver must wait 17 hours and 35 minutes after completion of the second dive before flying.

Example: Upon completion of a dive at an altitude of 4000 feet, the diver plans to ascend to 7500 feet in order to cross a mountain pass. The diver's repetitive group upon surfacing is Group G. What is the required surface interval before crossing the pass? The planned increase in altitude is 3500 feet. Enter Table 9-5 at 4000 feet and read down to Repetitive Group G. The diver must delay 1 hour and 23 minutes before crossing the pass.

Example: Upon completion of a dive at 2000 feet, the diver plans to fly home in an unpressurized aircraft at 5000 feet. The diver's repetitive group designator upon surfacing is Group K. What is the required surface interval before flying?
The planned increase in altitude is 3000 feet. Enter Table 9-5 at 3000 feet and read down to Repetitive Group K. The diver must delay 6 hours and 25 minutes before taking the flight.

Table 9-5. Required Surface Interval Before Ascent to Altitude After Diving.

Table at this site

Exceptional Exposure Wait 48 hours before flying

NOTE 1 When using Table 9-5, use the highest repetitive group designator obtained in the previous 24-hour period.

NOTE 2 Table 9-5 may only be used when the maximum altitude achieved is 10,000 feet or less. For ascents above 10,000 feet, consult NAVSEA 00C for guidance.

NOTE 3 The cabin pressure in commercial aircraft is maintained at a constant value regardless of the actual altitude of the flight. Though cabin pressure varies somewhat with aircraft type, the nominal value is 8,000 feet. For commercial flights, use a final altitude of 8000 feet to compute the required surface interval before flying.

NOTE 4 No surface interval is required before taking a commercial flight if the dive site is at 8000 feet or higher. In this case, flying results in an increase in atmospheric pressure rather than a decrease.

NOTE 5 No repetitive group is given for air dives with surface decompression on oxygen or air. For these surface decompression dives, enter the standard air table with the sea level equivalent depth and bottom time of the dive to obtain the appropriate repetitive group designator to be used.

NOTE 6 For ascent to altitude following a non-staturation helium-oxygen dive, wait 12 hours if the dive was a no-decompression dive. Wait 24 hours if the dive was a decompression dive.

Another novel approach to this problem is offered by Ben Zwart, MD, PhD using his multi-level dive calculator developed by him in 1998.

You're ACTUALLY looking at a fairly complex, MULTIPLE level dive, with a STAGING HOLD at sea level (3 FSW equivalent) for your Surface Interval! Your equilibrium condition is actually at 2600 feet - - and your dive actually begins during the descent, on AIR to sea level. It is at this point that you begin to on-load nitrogen compared to your equilibrium status. Remember!! Haldanian mechanics is ONLY applicable to excursion diving - - NOT to saturation diving!!

Scubadoc has asked me to comment on your question. Please understand that the following comments are my own and in no way represent an official position of the United States Air Force, Navy, or any other official body. The information / opinion I am providing is correct to the best of my knowledge at the time of my response. Other conflicting information may exist that I am either unaware of, or simply disagree with, and so I may not have not addressed those issues in my reply to you. Answers to questions are offered as information only and not as medical diagnosis or advice and should always be used in conjunction with advice from your personal diving physician. Diving is an inherently dangerous sport, and even if all the rules are correctly followed, significant injury and even death may result. I assume no liability for the use of information freely given without intent for willful or wanton negligence. Caveat Emptor.

The Multi-level diving calculator I designed back in 1998 using the Microsoft Excel spreadsheet (copy attached in PK-Zip format, including white paper in Adobe Acrobat PDF format) is the only thing I can think of that might help you out. Since you dive Nitrox, I assume you have a bit of technical expertise regarding use of mixed gases, and will be able to read through the Nobendem derivation to gain a better understanding of how the calculator will work for you. You may also download it from the USAF web site at (this may be required if your Juno account does not allow attachments).

You will need to make a few adjustments!! You will have to set the LOCAL BARO settings to 692 Torr in the initial "Multi M1" segment (equivalent to -3 FSW), which represents your actual equilibrium condition at 2600 feet. I will assume that you make a relatively linear descent from your home to sea level over a period of 1 hour, you take an hour at sea level to park, get into your wet suit and begin the dive. I will limit your depth to 60 FSW, assume square dives, and will only give you an hour at depth (if you're using AL-80's, you have 74.8 SCF of Nitrox available at 3000 PSI, and if you breathe 1 liter tidal volume at 10 breaths per minute, this will leave you 500 PSI for ascent after 1 hour - assuming a 2 minute descent). You will have to remember to ADD those extra 3 feet to your depth below the surface of the water during calculations where the surface Baro is set to 692! Because the surface interval is spent at 0 FSW, I will set the second table with a Baro of 760, and the dive will actually END back at -3 FSW = 2600 feet altitude.

You don't specify anything regarding your planned dive profiles, so I'll have to make something up. I will select EANx 36, will use my standard (and fairly conservative) Safety Enhancement of 20 (for comparison, this would recommend 60 feet for 45 minutes instead of 60 for 60 using AIR!! for a diver equilibrated at 1 ATA)

If you are going to make TWO, one-tank dives, because of your use of EANx, you could actually spend more than an hour at 60 FSW on your first dive if you had sufficient tank volume, before you would need a decompression stop - - however, as indicated above, you'll probably run out of gas before then.

I'll assume a 1-hour SI between dives, and will use either the no-deco times at 60 FSW, or 1 hour, whichever is shorter.

So, here goes:

You have a linear transition from your "surface" at 2600 feet to a depth of 3 FSW over a 1-hour period on 0.21 "nitrox" = air :-)
You then spend 1 hour at 3 FSW (sea level compared to your starting point) parking your car and getting into your gear.
You spend a 2 minute descent to 63 FSW (60 FSW below the surface, but 63 FSW equivalent relative to your starting point) on EANx 36.
You stay at 63 FSW for 60 min again on Nitrox, and then ascend to 3 FSW (water's surface) over 2 minutes.

You need NO deco time for this dive, although when you emerge from the water, your 40 minute tissue group is close to the safety threshold with a buffer of 0.32. This is perfectly acceptable, as the safety factor is currently set at 20 (calculated in the Multi M1 spreadsheet).

You then spend 1 hour at the surface. Since the RNT is calculated via reverse engineering, I have elected to set this segment's calculation (Multi M2) with a baseline of 760 mm Hg, and will end the dive with your eventual return to 2600 feet (= -3 FSW equivalent). The calculator suggests your RNT back to 60 FSW at the end of your 60 minute SI is 35 minutes.

Since the "surface" is now at 760 Torr, your descent is to 60 FSW over 2 minutes, but you can only spend 32 minutes at 60 FSW on EANx 36, assuming a 2 minute, no deco ascent.

If you now jumped immediately into your car, and made a linear ascent back home, taking NO LESS than 9 minutes, you will not break the Nobendem (Navy) model decompression tables with a Safety Enhancement of 20.

Lets say you wanted to spend a little more time at depth on your second dive - - maybe maximize your time at depth (TAD 60). You could spend 50 minutes at 60 FSW on EANx 36, take a 2 minute ascent to 5 FSW, spend 4 minutes at 5 FSW, then ascend to the surface, within the current model limits.

If you now jumped immediately into your car, and made a linear ascent back home, taking NO LESS than 10 minutes, you will not break the Nobendem (Navy) model decompression tables with a Safety Enhancement of 20.

With either of these dives, the longer you spend on the surface before hopping into your car and going home, the safer you should be.

Please remember! Calculation of diving tables, particularly in a new, untested application such as this, can only be an approximation based on the extrapolation of currently accepted values. Although I believe these recommendations are valid, they have not been tested - hence my disclaimer above. In the end, the diver must be responsible for his selection of diving tables and any possible consequences thereof.

I hope this has been helpful.

Benton P. Zwart, MD, MPH