Divers should follow the limits set by their dive tables and computers. This is because there is lesser risk of decompression sickness when divers properly used tables and computers. However, safety stops and slower ascent rates are recommended for repetitive divers.
When I sat down to put my thoughts together, I realized what a difficult undertaking this is. Before beginning any discussion of safe decompression, there are certain ground rules that must be established. Most importantly, all divers must understand there is no such thing as perfectly safe diving (or for that matter perfectly safe anything). If you want to be 100 percent sure you never get decompression sickness (DCS), don’t dive. There are no tables, no computers, no algorithms that are 100 percent free from decompression sickness. Thus, the question is not what type of diving behavior eliminates the risk of DCS completely but, rather, what behavior reduces its incidence to acceptable levels.
An acceptable risk of DCS may be different under different circumstances. For example, if you are diving in an environment with a chamber immediately available, you can tolerate a higher incidence of decompression sickness than if you are in a very remote location with help hours or even days away. We must all realize each time we dive we are accepting a definite risk of decompression sickness. Were one to make a no stop dive to 60 fsw (feet of salt water) for 30 minutes, this risk would be negligible. On the other hand, this same 30 minute no stop dive to 160 fsw would produce DCS in nearly everyone. What we must attempt to do is find a happy medium where we can enjoy diving and have an acceptable incidence of DCS.
The occurrence of decompression sickness is a statistical reality. If you were to dive to 60 fsw for 61 minutes and then ascend without a stop longer than one minute, you would violate the navy tables as well as every computer currently sold in the U.S., yet there is less than a five percent chance (probably less than a two percent chance) you would get DCS. Indeed, until you extend the no stop limits at 60 fsw to 80 minutes, the incidence of decompression sickness is less than five percent. Current statistics seem to indicate that the incidence of DCS in scuba divers is somewhere between 1:5,000 on the high side and 1:200,000 on the low. Thus, the chance of getting even one case of DCS in a lifetime of diving is relatively remote. This risk is always present, however, and, although there are ways to reduce the risk, there is no way to eliminate it entirely.
The next ground rule is that this discussion is restricted to only to stop diving, as it is the policy of all the major teaching organizations in the United States that dives requiring decompression stops are not in the realm of recreational diving. It is not clear whether dives that require staged decompression are always more risky than no decompression dives. If we adhere to the concept that the greater the depth-time profile (DTP), the greater the risk for DCS, we should find more DCS in staged decompression dives. The safety stop at 10 to 15 feet is not considered a decompression stop since it is an elective stop when used after a no decompression dive and is not described in standard air decompression tables.
The final ground rule has to be that I will only discuss depth-time profiles as a cause of decompression sickness. There is a great deal of lore concerning factors that predispose an individual to decompression sickness, however, almost all are no more than old wives’ tales. There are no hard data to suggest that divers who are older, less fit, fatter, female (menstruating or not), etc. are more prone to DCS. The only factor that has ever been shown to play a role in the occurrence of decompression sickness in humans is the DTP.
In order to understand the relationship between the DTP and DCS, it is important to realize the limitations of our understanding of this malady and the limits of our ability to describe safe dive profiles. For some types of DTPs we have a relatively large amount of information available, whereas for other types we have much less. We know the most about single no stop dives. Currently available data seem to suggest that by reducing the U.S. Navy no stop time limits by approximately 10 percent, asymptomatic bubbling can be prevented. Thus, if you are only going to make one dive, following your computer (all of them have algorithms that effectively reduce the navy limits by about 10 percent for single dives) or shortening the navy limits by about 10 percent (the Huggins or Bassett recommendations) should produce dives that are, for practical purposes, as safe as is reasonably possible.
This recommendation, however, ignores the thorny issue of safety stops and slower ascent rates. Currently there is no evidence to suggest that on a single dive, as described above, a safety stop or a slow ascent rate is required. On the other hand, there is a suggestion that rapid uncontrolled ascents are possibly related to a small percentage of cases of DCS. Whether this is mediated through an unnoticed arterial gas embolism, a patent foramen ovale or some other factor, no one knows, however, a safety stop demands good buoyancy control and makes uncontrolled ascents unlikely.
Since most sport divers don’t make just one dive, the preceding advice is insufficient for the repetitive diver. Currently we have a very limited data base for repetitive diving. In fact, we do not know for sure whether repetitive diving confers an additional risk of DCS over and above the additive risk of the individual dives. Certainly each repetitive dive creates additional gas loading and the residual nitrogen must be taken into account with each successive dive. But, each additional dive also creates physiologic changes associated with gas unloading. Thus, if dive #1 and dive #2 in a series were both close to the no stop limits, a diver might be exposed to a fair amount of decompression stress, even though he/she was technically within the limits of the tables or the computer being used. A similar third or fourth dive might then additively produce enough decompression stress that DCS would occur even though the diver technically followed the tables or the computer. Thus, it is felt by many authorities that the greater the number of dives one makes in a series (especially if they repetitively bring the diver close to the limits of the computer or tables being used), the greater the risk of developing DCS, even if one follows procedures correctly and properly accounts for residual nitrogen.
We are still talking about a relatively low incidence of DCS, even though the risk may be higher than the risk of a single dive. Unfortunately there is no way we can currently quantify this problem and therefore we have no way of reliably incorporating this concept into either tables or computers. Thus, in the setting of repetitive dives, the concept of a safety stop makes sense from a strictly decompression point of view. Similarly, a slow ascent rate will both provide for additional linear decompression time as well as lowering the risk of an uncontrolled ascent and whatever problems may be associated with it. Finally, backing off the limits slightly for repetitive dives also makes sense under these circumstances. (For the interested diver there are now computers available on the market that alter their calculations and progressively become more conservative with each repetitive dive.)
Unfortunately, all of the above is somewhat ambiguous and it is therefore difficult to make concrete suggestions. One cannot scientifically justify firm rules. On the other hand, there are certain things a diver can do to keep the risk of decompression sickness reasonably low:
1) Always try to surface with air to spare in your tank. There is nothing like an empty tank to create anxiety, discomfort and uncontrolled ascents. Even if these do not lead to DCS they certainly do lead to arterial gas embolism.
2) Follow your tables or your computer. Most of the cases of serious decompression sickness I see are still caused by DTPs unrestrained by the use of either dive tables or computers.
3) If you are making repetitive dives consider either a safety stop (the greater the number of dives in a series the longer the safety stop should be) or reducing your limits further.
There is no right or wrong way to accomplish these goals and I have intentionally not been specific because there are many different ways people dive. In the Caribbean, a safety stop can often be accomplished by spending the last portion of the dive on the top of the reef looking for critters at 20 to 25 feet. Whatever method you use, the underlying concept is relatively straightforward: If you push the tables or your computer you may increase the risk of DCS. If you are diving a lot, take it easy or back off a little. Remember the old saw: “There are old divers and there are bold divers but there aren’t any old, bold divers.”
Last but not least, if you do get symptoms, go to a recompression chamber as soon as possible. In general, the longer you wait for treatment, the greater the likelihood of permanent disability. DCS happens and I would rather deal with a minor problem (too) early than a major problem (too) late.