I will try to be clear, but this issue is easily misunderstood.
In all algorithms, there is a broad range of risk of scuba. Stay 15 minutes away from the NDL, and you are almost guaranteed not to get DCS. Get within a few minutes of NDL, and your chances of getting DCS increase, but it is still very unlikely that you will take a hit. Surpass NDL by a few minutes, and you are still pretty safe, because they do not make those NDLs right on the edge of a precipice of danger. Go 10 minutes into deco, you have put yourself at a much greater risk, but there is still a good chance you will be OK, depending upon the algorithm you are using. Go a half hour over NDL and you are quite likely to take a hit, and it could be a big one.
I once surfaced from a dive, and another diver, a friend of mine, surfaced at the same time. (We were not diving together). He immediately said his computer was acting funny, and when he described it, I realized it had gone into deco and was telling him how much deco he needed to do. I told him to put his regulator back in his mouth, and the two of us hung out at 20 feet for 20 minutes. His computer was already in error mode, so I had no idea how much deco he had owed. He thought 10 minutes when I asked him later. He was fine.
Similarly, there is a broad range in the seriousness of what we all DCS.
- Some tech divers call then "the niggles," the symptoms they encounter after a dive that eventually go away.
- Richard Pyle famously changed his ascent procedures based on his observations of the differences between dives after which he felt fatigued and generally crappy and dives where he felt fine. His "Pyle stops" were designed to prevent what was actually "subclinical DCS." My buddy on a Truk Lagoon liveaboard told me he was going to go to bed and skip a dive. He slept for 13 hours, after which he was fine. I am sure that was an example of subclinical DCS.
- On that same trip, another diver got skin bends. It resolved with surface oxygen. She skipped a couple of dives and then returned to diving.
- I had a buddy go through 3 days of chamber treatments after a weekend of diving with me. We did all the same dives. I was fine. His symptoms did not fully resolve for 6 months.
- After an unprecedented dive at very high altitude, a friend of mine became paralyzed on the surface, and it took three months of therapy at a famous rehabilitation hospital before he could walk again.
- After a famous incident in Cozumel a few years ago with a bounce dive to 400 feet and no decompression stops, a DM will never walk again. He did not have access to a rehabilitation hospital.
- His diving buddy, Opal Cohen, lived for several months after that incident.
- Chrissie Rouse surfaced from 230 feet without any deco stops. He made it to the chamber, but he died there.
- His father, Chris, died just after they reached the surface.
So, no, there is no bright line for what is safe for deco. There is no single level of seriousness for DCS. So what do we do? We have to decide what the bright line is for us, and for just about everyone (including me), that means following a specific algorithm. You just have to understand that big picture and make your decisions appropriately when the need arises. For example, I once had a buddy do a panicked, sprinting ascent to the surface from 85 feet because he had looked at his Suunto computer and seen that he had only two minutes to go before he reached NDL. He seriously believed that meant he had about two minutes to reach the surface before he got full blown, paralyzing and possibly fatal DCS. Don't be that guy.
