I got 1.4/1.6 from PADI/NAUI/NOAA and 1.2 from GUE.
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According to an article by Bitterman in the Undersea and Hyperbaric Medical Society Journal in 2004 (found here), CNS toxicity is not determined solely by the partial pressure of the inspired oxygen. Other factors, such as level of exertion, carbon dioxide build-up, the amount of light (ie, darker water = higher risk), age, gender, medications, and even Circadian rhythms may play a role in the risk of CNS oxygen toxicity.
With so many other potential factors in play, it is easier to understand why identical dive profiles might be safe one day, yet result in disaster on another day. It is also easier to see why it makes sense to dive conservatively.
"Conservative" may be defined differently depending upon those other factors mentioned. So, diving in colder, darker water might mean using 1.2 or 1.3 as the cutoff, whereas drift diving in the sunny, warm Caribbean might mean that 1.4 or even 1.5 could be okay for young, healthy individuals.
As you lower risk O2 toxicity risk, you increase your DCS risk, and vice versa. It's all a bit of a crapshoot with enough science and experience thrown in to make ourselves feel better about pretending to be fish!
Its not quite that simple. Risk isnt linear.
Reducing from 2.0 to 1.9 for example has a far higher reduction in incidents than dropping from say 1.4 to 1.3.
As CNS tox (all we're worried about here) is a factor of partial pressure and exposure time you can look at the dive limit figures to get a feel for that.
The risks at 1.4 are very very small. Ive only heard 2nd hand about 2 possible incidents and on those as far as im aware it hasn't been proven it was that causing it.
At 1.3 i haven't heard of any incident.
At these low levels other factors such as increased CO2, medication and other factors begin to have more of an effect.
These 1.3 to 1.2 issues are mainly going to be of interest to long duration CCR divers who use a fixed setpoint for long periods of time.
R
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Of course, I was just trying to illustrate my question.
I was really looking for quantification. I do understand the have been CNS at 1.4, 1.5, and 1.6. I accept that 1.4 entails less risk than 1.6 (In the same sense that I would accept that driving at 55 entails less risk than driving at 60).
I ask because I still use 1.6 as my limit. I really don't have cause to go over 1.4 very often but when a deeper easy (no work, short [10 min] exposure) dive presents itself, I don't hesitate to push into the 1.4 to 1.6 range. My level of work on these dives is little different than what I imagine a deco stop (rest stop) to be. I do this based on my perception that 1.6 is also a very very small risk in spite of my age (65). A somewhat greater risk, but still very very small and undefined risk.
Im not aware of age being an increased risk for CNS tox at all.
I do know of tox incidents at 1.6. Although rare there are some. MOST of them could potentially have other causes such as CO2 retention from working and so on.
Non-deco diving i use 1.4 as a bottom gas but will drop to 1.6 if theres something work taking photos of and its not a hard work dive.
Deco diving its 1.4 bottom to avoid running against the limits later in the dive when on deco which is 1.6.
O2 tolerance and susceptibility is massively variable. I think its the GUE tech book but it summarises a study that shows just how much it varies person to person and the same person day to day. The data was all over the place with no clear trends.
I do know of tox incidents at 1.6. Although rare there are some. MOST of them could potentially have other causes such as CO2 retention from working and so on.
Non-deco diving i use 1.4 as a bottom gas but will drop to 1.6 if theres something work taking photos of and its not a hard work dive.
Deco diving its 1.4 bottom to avoid running against the limits later in the dive when on deco which is 1.6.
O2 tolerance and susceptibility is massively variable. I think its the GUE tech book but it summarises a study that shows just how much it varies person to person and the same person day to day. The data was all over the place with no clear trends.
It is interesting to see the reasonings that folks have for their personal limits. For them, they are valid selections.
I know of no CNS tox cases at 1.2 (some @ 1.3), so I use 1.2 where a tox would be a life ending event while overhead-restricted.
I'm willing to take a slim chance of a tox on a NDL dive, where I can be taken to the surface, so I'll use 1.4 for recreational dives.
Do I expect these numbers to change? Of course - they seem to trend lower as our experience as a community increases. For example, in the late 70's, we routinely deco'd at 2.0 and used 1.6 as a normal exposure.
All the best, James
I know of no CNS tox cases at 1.2 (some @ 1.3), so I use 1.2 where a tox would be a life ending event while overhead-restricted.
I'm willing to take a slim chance of a tox on a NDL dive, where I can be taken to the surface, so I'll use 1.4 for recreational dives.
Do I expect these numbers to change? Of course - they seem to trend lower as our experience as a community increases. For example, in the late 70's, we routinely deco'd at 2.0 and used 1.6 as a normal exposure.
All the best, James
Answers to some of the questions and comments posed above are found in the article I referenced earlier.
Figure 2 shows a graph from a Lambertson study. There was no CNS toxicity below ppO2 of 2.0. However, the study was performed in a chamber, and there is evidence that wet immersion presents greater risk than seen in a hyperbaric oxygen chamber. The take-home point from the figure, though, is to look at the CNS Toxicity curve, and note the sharp, exponential rise at the elbow of the curve once the toxicity threshold is reached.
The big question is, where is that elbow when diving?
The answer to that is: Who knows? Look at Figure 4 where Bitterman references a study by Donald. Donald took the same diver and had him do the same hyperbaric oxygen profile 20 times over 3 months. Each time he "dove" until neurological symptoms occurred, and each time the symptoms occurred after a different length of exposure ranging between 5 minutes and 2.5 hours! Heck, that might as well be random!
Bitterman concludes: "As can be seen, there are large day to day variations in time duration of symptoms, suggesting that there is no fixed, personal, predetermined threshold of tolerance to oxygen toxicity."
String: One of the risk factors mentioned is age. When Bitterman talks about possible mechanisms for how CNS toxicity develops, he discusses vascular modulation and enhanced antioxidant states. These are things that can be affected by age very much, so it seems reasonable that age might be a factor. (As a diver who is a little above the average age, it certainly makes me want to be a little less cavalier than I have been.)
Figure 2 shows a graph from a Lambertson study. There was no CNS toxicity below ppO2 of 2.0. However, the study was performed in a chamber, and there is evidence that wet immersion presents greater risk than seen in a hyperbaric oxygen chamber. The take-home point from the figure, though, is to look at the CNS Toxicity curve, and note the sharp, exponential rise at the elbow of the curve once the toxicity threshold is reached.
The big question is, where is that elbow when diving?
The answer to that is: Who knows? Look at Figure 4 where Bitterman references a study by Donald. Donald took the same diver and had him do the same hyperbaric oxygen profile 20 times over 3 months. Each time he "dove" until neurological symptoms occurred, and each time the symptoms occurred after a different length of exposure ranging between 5 minutes and 2.5 hours! Heck, that might as well be random!
Bitterman concludes: "As can be seen, there are large day to day variations in time duration of symptoms, suggesting that there is no fixed, personal, predetermined threshold of tolerance to oxygen toxicity."
String: One of the risk factors mentioned is age. When Bitterman talks about possible mechanisms for how CNS toxicity develops, he discusses vascular modulation and enhanced antioxidant states. These are things that can be affected by age very much, so it seems reasonable that age might be a factor. (As a diver who is a little above the average age, it certainly makes me want to be a little less cavalier than I have been.)
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