Skydiving and scubadiving are dangerous activities. The information provided here only serves as a means to provide skydivers with more background information. Do not just experiment with the information provided here, but ask for good instruction and guiding by qualified instructors.
Nowadays more and more skydivers can afford to not only jump in summer, but also in wintertime during skydive holidays. Often these winter skydive places are in a more tropical environment than where we live and frequently also offer an excellent opportunity for some scuba diving near coral reefs with colorful tropical fish. Certified divers get informed during their training about the risks involved in scuba diving after skydiving, but someone who is up for just an introduction dive rarely hears anything about this. Tricky, cause mixing skydiving and scuba diving can be a lethal tropical cocktail.
The coke bottle story.
For the certified scuba divers among us the following is maybe well known, but for those who have never been diving before just summarize this: the air you inhale into your lungs contains about 21% oxygen ( O2) and 79% nitrogen (N2). Oxygen reacts in your body with fuels like fats and carbohydrates and the result of this process is energy for movement and body functions and the release of carbon dioxide and water. Nitrogen is also absorbed into your body but does not react. It's what we call an inert gas. However it does have a narcotic effect when diving deeper than approximately 30 meters (100ft). During scuba diving the pressure increases quickly when you go deeper, cause water is much heavier than air. Due to the higher pressure nitrogen and oxygen get absorbed by your body much easier. It's being “pushed” into your body so to speak. The oxygen is partially used by your body tissues and otherwise exhaled again, but nitrogen is not used by your body tissues and it builds up in solution until the partial pressure of nitrogen inside your body is the same as outside your body. When the partial pressures of inside and outside your body are equal we say that your body tissues are saturated. In scuba diving going deep quickly is never a problem concerning gas absorption. Going up quickly is a different story however. During ascend the nitrogen has to release the body because the reduction of pressure inhibits some nitrogen to stay in solution. This process of exhaling excess nitrogen takes place in the lungs. So this is the only area where nitrogen can come out of its solution back into gas. Not all body tissues can release nitrogen equally fast. Fat tissue for example is a “slow” tissue. Because of these differences it is possible that during a fast ascend the decrease in pressure is that big that the difference of nitrogen concentration inside and outside the body exceeds a certain limit. This maximum limit is called the “M-value”. If this limit is exceeded gas comes out of its solution and becomes gas while it is still in the body tissues or the bloodstream. These gas bubbles can block bloodstream and oxygen support to tissues and organs and result in a complex of symptoms called “decompression sickness” or the “bends”. To prevent this it is critical to reduce the pressure slowly by ascending slowly so the excess nitrogen can be exhaled by the lungs. That's also why dive depth has been limited by dive times to control the uptake of nitrogen. These maximum dive times are called decompression limits (see figure 1). If you exceed these limits you have to make a decompression stop at a shallower depth to prevent exceeding the M-value resulting in gas bubble formation in your tissues.
Figure 1. Dive time and decompression limit. Source: Physiology of Sport and Exercise by Wilmore & Costill p.284.
Decompression theory.
When you reach the surface after a dive you still have a rest amount of nitrogen into your body which will be released from the body tissues slowly over time. The speed of nitrogen uptake and release of body tissues differs. Until recently it wasn't possible to exactly figure out what happened into the body and experiments were based on very much on a black box kind of research: you do something, you can't see the process inside and afterwards you try to figure out what happened according to the results that came out of the black box. Therefore nitrogen uptake and release were estimated by mathematical models as made by John Haldane for the first time in 1908. Important elements in these models are the compartment halftimes: the speed factor for nitrogen uptake and release.
An important factor in the classification of a “slow” or a “fast” tissue is the blood flow through the tissue. Body tissues that have a good blood flow can release nitrogen more easily. The nitrogen will be transported quickly trough the veins to the lungs where it can be exhaled. For body tissues with a less good blood flow the opposite applies.
In figure 2 you can see that with a halftime of 5 minutes tissue absorbs nitrogen much faster (after only half an hour the tissue is almost completely saturated) than with a 30 minutes halftime, then it nearly takes 3 hours to be saturated.
In 1965 the US Navy developed the concept of M-values as a way of calculating the maximum allowable amount of nitrogen for each compartment more easily. Fast tissues have a high M-value, which means that you can reach the surface with a relative high amount of nitrogen for that particular compartment. Slow tissues have a low M-value.
For diving this means that slow tissues are the limiting factor for deep dives, while fast tissues are the limiting factor for relative shallow dives. During deep dives the fast tissues are saturated quickly, but during the ascent they also release the nitrogen quickly, making it no problem when you reach the surface.
From this point of view it's better to make a short deep dive before flying or diving compared to a long shallow dive. The fast tissues release the nitrogen quickly and the slow tissues do not absorb that much.
In practice it does not work this way as we will see.
By using the above theories and models, the more practical dive tables have been developed (figure 3) which tell the diver how long he can maximal stay at a certain depth before ascending without problems. These tables also tell divers how long it will take before the nitrogen has been released from the body once they are on the surface. Disadvantage of these tables is that they are mathematical models, representing an average person. They do not exactly measure or predict the behavior of biological body tissues in reality.
What if I do get DCS?
If you do get DCS pure oxygen has to be administered. This has to prevent the blockage of blood flow to essential organs by nitrogen bubbles so these organs can receive oxygen.
Oxygen also helps in transporting nitrogen to the lungs because more nitrogen can be dissolved in the bloodstream if there is no nitrogen inhaled.
However administering just oxygen is not enough. Therefore you have to be evacuated to a recompression chamber. In a recompression chamber the pressure will be increased again so the nitrogen gas bubbles go back into solution. Slowly the pressure can be decreased then so the nitrogen can be exhaled in the normal way trough the lungs and leave the body. Divers alert network (DAN) is a non-profit organization supplying emergency services for divers and the maintenance and staffing of recompression chambers. The also try to make divers more aware of dive safety and train them in First Aid techniques after diving accidents and conduct and finance dive medical research.
Diving and flying.
The surface intervals provided by most dive tables for residual nitrogen consider a 60 minutes half time to be the slowest compartment. So after 6 hours you should hardly have any residual nitrogen left. To have a bigger safety margin it is recommended to have a minimal surface interval of 12 hours before flying. If you are making multiple dives a day or doing a single dive requiring a decompression stop it is recommended not to fly within 18 hours. When a decompression has been forgotten or ignored you are not allowed to reenter the water within 24 hours, but no strict guidelines exist as when it is safe again to fly in this case. That is dependant on how severe the violation is.
When you get higher into the atmosphere the air pressure decreases (figure 4) which can result in decompression sickness, just like when you get out of the water to the surface, because there is still a residual amount of nitrogen into your body. So if, during a day off at a boogie, and you come out of the water at 17:00 hours after your 2 nd dive, you violate the safety guidelines of diving when you are boarding the airplane at 9:00 hours the next morning. In this case you have been only at the surface for 16 hours. So when you have 2 days off at a boogie, dive on the first day and relax on the second as long as this is not mountain climbing. Above 600 meters above sea level the reduced air pressure might trigger DCS already.
Diving and skydiving.
Because the amount of people that combine skydiving and scuba diving is very limited to say the least, there is hardly any research time spent on this topic. There are however some worrying aspects:
Most research studying the relationship between flying and diving and responsible for the guidelines in recreational diving limit their studies to altitudes up to 8000ft. This is the normal cabin pressure in commercial airliners once they are at altitude. During skydiving it is not uncommon to go up to 15.000ft. This creates an extra air pressure difference of 210 millibar and a higher risk for decompression sickness.
Research using Doppler technique, with which there is literally listened to gas bubble formation in the body tissues, has shown that some tissue compartments have much longer halftimes than 60 minutes, even up to 480 minutes. These results make a “no fly time” of 12 tot 18 hours after diving maybe a bit short. Nowadays this knowledge is used in diving computers, which create a customized dive table for the diver and tell him exactly how much nitrogen is absorb according to the time and depth dive profile (this is a mathematical calculated amount of nitrogen absorption, which does not necessarily has to be the same as the actual biological tissue nitrogen absorption!). These computers also keep track of your no fly time after the dive (figure 5). The other side of these dive computers is the fact that many recreational divers simply follow their computer display, without making a proper dive plan in advance. The result is they stay the maximum limited no decompression time at each depth so all compartments get nearly saturated with nitrogen. The reason is probably that people want to get the most out of their dive since they cannot dive every day and their holiday time reserved for diving is limited. Coincidentally often the best things and colors can be seen in the first 10 meters . At these depths the air consumption is relatively small compared to greater depths so it is possible to stay under water longer. This way the slow tissues can absorb a large amount of nitrogen. The advantage of the short deep dive as mentioned earlier is neglected this way. Especially during repetitive dives at the same day nitrogen can build up significantly in the slow tissue compartments and therefore a dive computer recommends quickly more than 12 to 18 hours of no flying after diving.
The average skydiver on holiday is not reluctant of some alcohol. This can easily lead to dehydration which increases the risk for DCS greatly, even if it is mildly, because it diminishes the transportation capability of nitrogen by the blood.
Maybe there is a difference between performing DCS tests in a recompression chamber (as usual) or diving for real.
How seriously do we have to consider the above points?
On average 300.000 to 400.000 people fly within 12 to 24 hours after their last dive each year. 0,004% of them suffer from DCS. About 14 of every 350.000 people, about 10 of every 10.000 divers suffer from DCS before boarding the airplane. So it seems like the added risk of flying is small compared to the inherent risk of diving. However in most research conducted a no fly time of 12 hours seems to be enough for altitudes up to 8000ft, but the number of test subjects is very small so it is risky to draw general conclusions. And even then the risk is slightly more than 1%. The few studies researching aircrews show 1 in 35 people get DCS after a no fly time of 12 to 18 hours. After 24 hours nobody suffers from DCS. During these studies oxygen was used to prevent for hypoxia to reach altitudes of 25.000ft.
Most studies perform tests after a single dive. Representative numbers of repetitive diving at the same day is not available yet. This way nitrogen build up in slow tissue compartment is ignored.
Almost 1 in 3 DCS patients suffer from mild to severe dehydration. Together with a warm environment (as normal in the tropics), packing in the heat and sitting in a warm airplane before jumping this can be an important contribution to DCS when skydiving. Also some diving physiological mechanisms are responsible for an increased body fluid excretion, creating dehydration already.
Since testing in recompression chambers is not the same as real diving in water it can be doubted if these studies come to the right conclusions. Some studies comparing these different testing circumstances indeed discovered different test results. However these differences are not big and not consecutive, so till proven otherwise recompression chamber testing can be considered correct.
John Scott Haldane
Born May 3, 1860 in Edinburgh , Scotland . Deceased May 14, 1936 in Oxford , Oxfordshire England .
John Haldane was a respiratory physiologist. In 1905 he discovered that it is actually carbon dioxide that triggers respiration and not oxygen.
He also developed the foundation of decompression theory by creating a model to predict how long construction workers could work in pressurized dry docks. Even today all dive tables in use find their origin in his basis of decompression theory as he published in his 1908 article “The Prevention of Compressed Air Illness”. Due to his formulas and concept of tissue compartments with different halftimes it became possible to calculate the expected amount of absorbed nitrogen:
In this formula n is the number of halftimes. Halftime is the time necessary to reduce a certain amount to half it's original amount. A more general formula is:
In this formula T is the total time in minutes and ht the compartment halftime. With this formula the amount of nitrogen can be calculated in a compartment at a specific time.
Other than some people think DCS can never be avoided for a 100%, not even when diving within the limits of the dive tables. Also there are a number of factors influencing the likelihood of getting DCS like:
Dehydration
Illness
Age
Fatigue
Body fat percentage
Physical and Mental Fitness
Gender
And much more…
Figure 2. When tissue gets more saturated the speed of nitrogen absorption slows down. With nitrogen release it is the same: the speed slows down when time goes on.
Figure 3. The Padi recreational diveplanner. With this tool divers can determine how long they can stay at a certain depth without getting DCS when they reach the surface again and they can figure out how long they have to stay at the surface before the nitrogen has left the body.
Figure 4. The lower part of the atmosphere, the first 7 km, contains most air and airpressure decreases almost linear with increasing altitude. Above 7 km airpressure decreases more slowly with increasing altitude.
Figure 5.
After a dive with a maximal depth of 20 meters the divecomputer tells us not to fly for at least 11 more hours.
Figure 6. Skydiving above coral reefs. It can be very tempting to mix scubadiving and skydiving, but be carefull with this tropical cocktail!
Conclusions:
The decreasing risk for DCS between a no fly time of 12, 18 and 24 hours cannot be ignored. For skydivers jumping from altitudes higher than 8000ft it is recommended to wait at least 24 hours after scuba diving before skydiving again. If possible keep one day in between and drink enough water or other non alcohol containing drinks.
About the author:
Kjeld van Druten has studied Exercise Physiology at the Free University of Amsterdam ( Netherlands ) and Physical Education at The Hogeschool of Amsterdam. He is also a Dive Master and worked as a dive guide in Thailand. In summer Kjeld works as an instructor, cameraman and tandemmaster at Skydive Texel and has about 2000 jumps at the moment of publication of this article. Click here for more info.
Sources
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Freiberger JJ, Denoble PJ, Pieper CF, Uguccioni DM, Pollock
NW, Vann RD. The relative risk of decompression sickness during and after
air travel following diving.
Aviat Space Environ Med. 2002 Oct;73(10):980-4.
Pollock NW, Natoli MJ, Gerth WA, Thalmann ED, Vann RD. Risk of decompression sickness during exposure to high cabin
altitude after diving.
Aviat Space Environ Med. 2003 Nov;74(11):1163-8.
Richardson D. The Encyclopedia for recreational diving.
International Padi Inc. ISBN 1-878663-02-X
Richardson D. Padi Divemaster Manual International Padi Inc. ISBN 1-878663-07-0
Srinivasan RS, Gerth WA, Powell MR. Mathematical models of diffusion-limited gas bubble dynamics
in tissue.
J Appl Physiol. 1999 Feb;86(2):732-41.
Figure 2. When tissue gets more saturated the speed of nitrogen absorption slows down. With nitrogen release it is the same: the speed slows down when time goes on. 
