Probability in Relation to “The Bends” – Changing the Way We Think About Dive Safety

One of the features I’ve been emphasizing about the SAUL algorithm (besides its greater accuracy), is that it allows the diver to choose a level of risk that he/she feels is appropriate.   Since this is quite different from the way we initially learned to think about dive safety, now is a good time to review the differences and to go over some basic ideas and facts about probability.

When you were first learning to dive, you would have been told that, in diving, there is always some small probability of decompression sickness (DCS), but that you can minimize your risk by diving safely – all of which is true.   But, once you were introduced to dive tables and/or dive computers, there was no further discussion of probability in relation to DCS.  Instead, you went on to look at dive tables to work out no-decompression limits (NDL’s), and then proceeded to dive computers which, in effect, automated the working out of  NDL’s.

The problem with NDL’s  (and with current dive computers) is that they are deterministic.   That is, they have an absolute dividing line: below it you’re okay, above it you’re in trouble.  As with speed limits on highways, there may be a tendency to continually push the limit – until you get caught or “bent”.  (Although, to be fair, PADI dive tables, do have a disclaimer of sorts on them advising that you shouldn’t actually plan to dive to the limits, that the limits are something of an absolute maximum.  But, then, you’re left wondering – “How close to the limits is too close?”)

While some current dive computers offer riskier or less risky options, how can anyone make a sensible choice without a clear idea of how much risk each option involves?

So let’s look at the probability of DCS in diving.  In the higher ranges, if you were to dive each of the listed profiles on the PADI recreational diving tables, right at their no-decompression limits (on air, including a 3 minute safety stop at 15 fsw), your risk of DCS would average out to .00264, which is 264 chances in 100 thousand or, roughly, just over a 1 in 400 chance of getting bent.  (I averaged over all the listed profiles because the risks at the limits of the different profiles are not equal.)  To see the comparative risks of PADI’s NDL’s for different profiles, check out my post on How Saul Relates to the PADI Dive Tables.

On the other hand, most recreational diving typically involves much lower risks.  Project Dive Exploration, which solicits and collects actual  dive data from large numbers of divers found (in one subset of their data, that roughly corresponds to typical recreational diving) a DCS rate in the neighbourhood of .0007, which is about 70 chances in 100 thousand.  Or, to put it another way, only about 1 chance in 1,400 of getting bent.

What factors should you consider when choosing an acceptable level of risk?  Obviously, less risk is preferable to greater risk.  No one wants to get bent.  (On the other hand, the least risk of all is to stay out of the water.  Not a serious option.)   But less risk generally does mean less bottom time.  So what do you do?

Regardless of the level of risk you choose, there is some possibility (even if very small) of getting DCS on any single dive.   As you increase the number of dives, you’re more likely to experience the consequences of a higher level of risk.  Using the (not recommended) 1 in 400 level of risk mentioned above, if you were to dive 400 times at that level of risk you’re quite likely to get bent.

It’s probably unnecessary to remind you (but I’ll do it anyhow) that a 1 in 400  risk doesn’t mean exactly 1 incident of DCS in every 400 dives, but that, over a large enough number of dives, or a large enough number of divers, the number of cases of DCS will average out to 1 in 400.   Meaning that, while you might manage those 400 dives without getting bent at all, you’re about twice as likely to get bent one or more times.  And, if you were to continue at that same level of risk through 1000 dives….  Well, just don’t. (To see the probability calculations, click here.)

If you dive a lot, you should be looking at a more conservative level of risk.  (If you don’t dive a lot, best to avoid high risk dives, even if your air consumption is slow enough to manage them.)   That doesn’t have to mean you can never consider a higher level of risk.  Perhaps, for that “once-in-a-lifetime” dive you may never get a chance at again, (i.e., a single exploration of  a particular wreck, where the depth and length of time involved would be a little too risky for regular diving), raising the risk level may make sense.  Just as long as every particularly interesting dive doesn’t get a similar classification.

Are there personal factors that affect your probability of getting DCS?   There are but, at the present time, only one has been confirmed with any certainty.   That one is, of course, the presence of a PFO.  If you’ve been diagnosed with one and are still permitted to dive, you’ve no doubt been advised to dive more conservatively.  Other factors that have sometimes been believed to increase your probability of  DCS are: old age, obesity, and being female.  Recent research suggests that old age may well increase the risk of DCS, but that obesity or being female (excluding pregnancy situations) probably do not.  Temporary factors that may increase your probability of getting DCS are dehydration and diving in cold water.   If you have some concerns that one or more of these factors (or any other issue not mentioned here) could potentially increase your risk of DCS, you could compensate by lowering the level of risk you choose to dive with.

 


Comments

Probability in Relation to “The Bends” – Changing the Way We Think About Dive Safety — 2 Comments

  1. I do not agree that if the chance of a DCI is 1 in 400, and I do not suffer a bend in 400 dives, it does not mean I am twice as likely to get a DCI in the next 400 dives. That is like saying that if I do not roll a 6 on the dice in 50 rolls I am more likely to roll a 6 in future rolls. If anything, the fact that no DCI has occurred may indicate that the probability estimate of 1 in 400 is too high.

    • You appear to have misread my post. Your criticism would be justified had I actually spouted the nonsense you seem to think I did. (i.e., that, if the chance of DCI is 1 in 400, and you don’t get bent in the first 400 dives, then you’re twice as likely to get bent in the next 400. I said nothing of the sort. My reference to “twice as likely” was a simple restatement of the probability calculations (which can be viewed by clicking on the link to them in the post). The probability of completing 400 dives at that risk level without getting bent on any dive is roughly 33.3%. On the other hand, if you sum up over all the other probabilities – the probability of getting bent exactly once in 400 dives; the probability of getting bent exactly twice in 400 dives; the probability of getting bent exactly three times in 400 dives…etc., you will come up with a total of roughly 66.7%. In other words, while there is about a 1 in 3 chance of completing 400 dives at that risk level without getting bent, your chances of getting bent at least once are about 2 out of 3 – double your chances of completing them unbent. With a 1 in 3 chance (where risk of DCI is 1 in 400) of completing a string of 400 dives without getting bent, doing this would give you no real reason to doubt the accuracy of the stated risk assessment. Of course, should you somehow stretch your string of no-bends dives (at that same risk level) to 1,000 dives, you might legitimately question whether the risk was genuinely 1 in 400. (While it’s not impossible, the likelihood of a string of 1,000 dives without getting bent, where the risk level is 1 in 400, is just 8 chances in 100 – so, yeah, at that point I would be suspicious too.)

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