Article: Daily Limits for CNS Oxygen Toxicity

[redacted]

Thanks!

OK, a little housekeeping. I suggest ignoring the 90-minute half-time because I have never found any credible scientific reference to it in relation to CNS toxicity. (There is reference in some studies on Pulmonary Toxicity which is irrelevant in this discussion, IMHO). Unfortunately, I only had one conversation with Bill Hamilton and wish I'd taken better notes; however, if memory served he used a rather choice expletive to describe the notion that CNS exposures decayed over time. "If that were the case," he said. "We would not have had to develop a 24-hour CNS table."

Do the maths and it becomes virtually impossible to approach the daily CNS limits if one subscribes to a half-time for CNS toxicity.

Also, worth thinking about, is that there are documented episodes of CNS "accidents" to divers whose oxygen exposure was well within NOAA depth and time limits (for example 1.3 bar for 95 minutes). This has lead to a general community speculation that some folks are more prone to CNS that others. THIS IS ABSOLUTELY PLAUSIBLE, however, it's my strong belief that the strongest factor in these cases is that the diver ignored or was unaware of the daily CNS limits. The dives on which incidents occur are invariably the last of a series of dives... all within single dive limits but taken as a group, beyond the limits of the recommended daily dose.


Therefore, I plan (and teach) dives based on daily limits without any reference to a half-time. CNS is one of the few things I fear... actually, one of MANY (LOL) but it seems reasonably easy to manage.

If you are going to assume that the 24 hour period to clean is good, but don't trust the scientific basis for the 90 minute half-time; why not try a 4 hour halftime. That would be the most conservative estimate, assuming the exponential decay and that clean means a period of 6 half times as is used for nitrogen off-gassing. That also establishes a bracket to work within which, considering time required for exposure and surface intervals, could easily lead one to accept something in the 2 to 3 hour haft-time range that should be quite workable if you do the math.

 

[Doppler]

If you are going to assume that the 24 hour period to clean is good, but don't trust the scientific basis for the 90 minute half-time; why not try a 4 hour halftime. That would be the most conservative estimate, assuming the exponential decay and that clean mean a period of 6 half times as is used for nitrogen off-gassing.

Well... I guess first question is, is there any Scientific basis for the 90-minute half-time? I have not found one. Does CNS loading follow a similar path to nitrogen when it decays? I don't know. Admittedly, the half-time concept is pretty well established with a lot of stuff outside of and beyond scuba but does it apply to CNS? I don't know. I see your logic with a four-hour half-time but feel that the calculations behind the original work at NOAA (keep exposures within a 24-hour time frame inside the daily limits) easier to administer.

Probably not a good time to bring up the issue of the CNS Clock... another construct that Bill Hamilton took exception to... yet almost every textbook from the major training agencies promotes its use.

 

[exposure]

Really great article, thanks for sharing it. Always interesting to remind essential knowledge with some more detail than the one provided during a basic certification. By the way, Steve's book rock! I am so impatient to read the other one.

 

[donhealy]

This was a great blog. I have been looking for a thorough understanding of CNS and OTU's since I went to Nitrox and bought my Cobalt computer. Got real worried when I saw some of the CNS data when diving live aboards. Doing 5 dives a day for 5 days requires some special attention.

 

[ragbager]

What is the science for the difference in single dive max O2 times and multiple dive max O2 times on the NOAA CNS tables?