A few of you brought up the important point (IMHO) that NDL is a fuzzy line... more like a broad expanse...
I think that the issue of terminology did something to add confusion to the debate.
An NDL is determined by the mathematics of a given, specific, decompression algorithm. It's not a grey area. It's not fuzzy. It's a very clear number on a table or computer screen.
Different algorithms provide different NDL for otherwise identical dive parameters. In addition, most algorithms can be made more conservative via manual settings - reducing the NDL.
However, DCS presentation itself IS a grey area. There are a myriad of known factors that can vary the potential onset and severity ...and probably quite a few unknown factors also.
It's important to recognise that diving
within a given algorithm NDL is not a guaranteed assurance that you
won't get DCS.
Pertinent to some stances in this debate, it's also correct to recognize that
exceeding an NDL is also not a guaranteed assurance that you
will get DCS.
Divers increasingly have the freedom to select an NDL for their diving. Gone are the days when the only choice for most divers was between a PADI table NDL and a US Navy table derived NDL.
There are now dozens of dive computers on the market, running many different algorithms... each calculating more conservative premises aggressive diving limits.
Very few recreational divers select a computer based on algorithm concerns. The algorithm is barely mentioned in most dive computer advertising. When it is mentioned, most divers aren't knowledgeable enough to place it into a personal context or risk consideration.
Personally, I think that the moment you strap on compressed air (of any mixture) and submerge yourself into water, you incur some form of decompression malady, however slight.
I'd agree that any dive has physiological effects on the body; including effects that persist post-dive. This includes the formation of bubbles.
However, those physiological effects may not be significant enough to create diagnostic signs or symptoms according to the current criteria of DCS. That does not mean that these effects are non-damaging in ways, as yet, not understood by hyperbaric medicine.
I have a dive buddy that has been bent twice well below NDL... , he's been bent twice from rec depths. Why?
There's undoubtedly physiological factors that make some individuals highly prone to DCS. Vice-versa, there's individuals that can do extraordinarily aggressive diving and seem near-immune to DCS.
This is why the general advice is always to dive conservatively. Historically, that's been through staying comfortably within an NDL. In the age of diving computers and varied algorithms, it also means choosing a conservative algorithm and/or setting for an algorithm.
Obviously, it takes significant diving experience to determine your own personal risk factors with DCS. Each dive adds to your individual sample... your personal statistics.
I'd suggest that it takes many hundreds, if not thousands, of dives to confidently determine your general susceptibility to DCS.
Even then, your susceptibility will vary depending on many factors, not least your age, general health and fitness. A prudent diver should expect increasing susceptibility as they get older and/or if their diving habits change.
Paying attention to your post-dive vitality may help a quicker understanding of your susceptibility, without the need to push your 'sampling' to the level where you need hyperbaric medical treatment.
Obviously, to achieve this requires both experience and astute observation.
No-deco, Lite-deco... there is always a risk that is personal and can't be guaranteed by computers or tables... no matter who created them. Fortunately for most of us, they all seem to work okay.
But that still sucks for the few it doesn't work for.
It pays to stay very conservative until you have a high degree of confidence in understanding your own DCS susceptiblity. Many divers erring towards aggressive diving seem to underestimate the time and parameters needed to have that real confidence.
The issue with 'lite deco' isn't so much about physiological factors. Undertaking effective staged decompression should
still dictate a reasonably low diffused inert gas volume on surfacing.
The DCS risk with 'lite' deco stems from whether or not the diver is competently able and properly equipped to
guarantee that they won't reach the surface with a harmful volume of inert gas in their body.
Creating that guarantee is what polarizes 'lite' deco into a potentially very safe or a very risky diving approach.
