A posting on one of the popular online scuba forums got me thinking about how we teach CNS 24-hour limits, because there was nothing but incorrect information posted. A conversation with one of the senior ITs for the agency I teach for, followed up and I realized we need to put more emphasis on this topic in the classroom… especially given a couple of recent incidents.
I dug out my teaching notes and figured posting them here was a reasonable thing to do. If you have comments or suggestions, please let me know.
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First of all, a definition of oxygen toxicity syndrome (also known as the “Paul Bert effect”)
This is severe hyperoxia caused by breathing oxygen at elevated partial pressures… usually a function of breathing something with oxygen in it at depth or breathing pure oxygen as part of a decompression strategy. The high concentration of oxygen damages cells within the diver’s body. The precise mechanism(s) of the damage is not known, but oxygen gas has a propensity to react with certain metals to form superoxides; and these may attack double bonds in many organic systems, including the unsaturated fatty acid that residues in cells. High concentrations of oxygen are known to increase the formation of free-radicals in biological systems – such as divers. The formation of these free-radicals may then begin a sort of cascade of events which may directly harm DNA and other structures. Normally, the body has many defense systems against such damage but with hyperbaric concentrations of oxygen, these systems are eventually overwhelmed over time, and the rate of damage to cell membranes exceeds the capacity of systems to control damage or repair it. Cell damage and cell death then results.
If any anyone feels that tracking oxygen exposure is a waste of effort, I feel this alone should convince them otherwise. I addition, there have been several recent incidents of CNS poisoning in divers where the dives were conducted within acceptable limits. This gives us pause for thought and reinforces the need for us to be conservative in our CNS oxygen toxicity tracking.
Before moving on to methodology for tracking NOAA Daily Limits – NOAA seems to be the most accepted scale or system – let’s recap.
The oxygen exposure time for a single dive is compared to the Single Dive Exposure Limits on the NOAA table (1.6 for 45 mins, 1.5 for 120 mins 1.4 for 150 mins and so on).
The suggested working limit for this type of exposure is 80 percent of the maximum shown in the NOAA table. (e.g. 1.4 for 120 mins, 1.5 for 96 mins or 1.6 for 36 mins). This 80 percent limit has been almost universally adopted by technical diving communities around the world. In ALL further documentation unless otherwise stated, this is what is meant when the oxygen limit is mentioned.
When tracking with a single gas (bottom mix) the exposure at depth is all that needs to be considered since the oxygen pressure during normal ascent and at the depth of a standard safety stop, must by definition be less than 0.5 bar. For all practical purposes this amount of oxygen is too low for consideration in CNS calculations for recreational diving (sport or technical).With multiple gases (the use of a decompression gas) the oxygen pressure for ALL PHASES of the dive MUST be calculated and added together to find the total single dive oxygen exposure.
If a diver reaches the limits of the Single Exposure Time on a single dive then he must take at least a two-hour interval on the surface, breathing normal air. This surface interval is thought to reduce the CNS loading by about half. Current thinking is that CNS loading is subject to a 90-minute half-time. This means that a diver who gets out of the water with a CNS “clock” at 40 percent on surfacing, will have that loading reduced to 20 percent, 90 minutes later. (This CNS 90-minute half-time is under scrutiny and may be adjusted at some point in the very near future… so stay tuned.)
If two dives are conducted with less than a two-hour surface interval, treat them as a single dive for the purposes of CNS tracking. In other words, the in water times are added together and compared against the Single Exposure Time. If one dive is at a greater oxygen partial pressure than the other, that pressure is the one used with the combined in-water times of the two dives, to calculate total CNS loading.
If two or more dives are conducted within a single 24 hour period with more than two hours at the surface between each dive, then the total in water times are added and compared against the Daily Limit to arrive at the diver’s CNS loading. We will cover this in a moment.
In more complex decompression diving, the total CNS loading for bottom time and each staged decompression stop is taken into account – including jumps in oxygen pressure when gases are switched. The total times in minutes for each oxygen pressure for the dive, the whole dive, are added together and expressed as a percentage of the allowable total single dive limit.
If a series of dives in a 24 hour period reaches the Daily Limits, then a 24 hour surface interval breathing air is the safest option to be taken before diving again.
Daily Oxygen Limits or tracking CNS on multiple dives
Daily limit tracking is essential when multiple dives are planned and is particularly important for divers doing Live-Aboard trips where the first dive of day two can easily be less than 12 hours after the last dive of day one!
I have heard it said that NOAA daily limits are a proxy for pulmonary toxicity management. They are not. This is complete nonsense. Pulmonary toxicity has nothing to do with these calculations or the need to be vigilant keeping tabs on CNS toxicity!
Examples to illustrate the efficacy and value of Daily Oxygen Pressure Time Limits
This topic is a required as part of the curriculum for both TDI Advanced Nitrox and Decompression Procedures courses. The examples with most relevance for students will be slightly different from one course to the other. For instance in a stand-alone Advanced Nitrox course, we can use the example of a photographer on open circuit scuba making several shallow nitrox dives using a mix that delivers a partial pressure of 1.4 bar at depth. Since the reef is shallow, he can pull bottom times of an hour. Here are three dives that seem plausible.
By the way, these profiles where derived using V-Planner version 3.81 software by Ross Hemingway, and the algorithm being used is VPM – B.
DIVE PLAN #1
Surface interval = 1 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.
OTU’s this dive: 103
CNS Total: 40.7%
107.5 cu ft Nitrox 50
107.5 cu ft TOTAL
DIVE PLAN #2
Surface interval = 0 day 2 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.
OTU’s this dive: 103
CNS Total: 56.8%
107.5 cu ft Nitrox 50
107.5 cu ft TOTAL
DIVE PLAN #3
Surface interval = 0 day 2 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 60ft (1) Nitrox 50 50ft/min descent.
Level 60ft 58:48 (60) Nitrox 50 1.41 ppO2, 26ft ead
Asc to 40ft (62) Nitrox 50 -10ft/min ascent.
Surface (66) Nitrox 50 -10ft/min ascent.
OTU’s this dive: 103
CNS Total: 63.2%
107.5 cu ft Nitrox 50
107.5 cu ft TOTAL
Each dive is ‘safe’ from the point of view of CNS because none approaches the 80 percent margin, and none brings the diver close to required decompression (26 foot EAD!). HOWEVER, at the end of these three dives, the diver has about 180 minutes at a PO2 of 1.4 bar which maxes out his allowable daily dose.
According to NOAA’s table, he has to stay out of the water for 24 hours. I teach that there is no allowance made on the daily limit for the supposed 90-minute half-time decay of CNS loading… with the jury still out on what exactly happens to trigger a CNS episode, this seems the most logical and conservative practice to adopt. 
This becomes more compelling given the aging of the average diver and the widespread use of anti-nausea meds and various other pharmaceuticals and dietary supplements: none of which have been studied sufficiently to allow use to disregard their possible interactions during nitrox diving. (Note: at the finish of the examples cited above, the diver’s OTUs are at about 300 which is far less than the daily limit and consistent with levels to aim for on multi-day exposures… in other words, CNS toxicity is the issue, NOT Pulmonary)
Now, let’s look at multiple decompression dives. Many sources warn against the practice of executing more than one staged decompression dives in a day. Let’s see why that might be. It does seem odd since pulling off two or sometimes three deco dives a day is common practice in some regions, especially in warmer water.
To illustrate why this requires careful planning and CNS tracking, here are the figures for two identical decompression dives with a SIT of six hours.
DIVE PLAN #1
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen – 10ft/min ascent.
Off gassing starts at 87.3ft
OTU’s this dive: 104
CNS Total: 64.7%
148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL
DIVE PLAN #2
Surface interval = 0 day 6 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.
Off gassing starts at 87.3ft
OTU’s this dive: 104
CNS Total: 68.7%
148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL
Again, each is within the single-dive CNS limit of 80 percent or less on the clock. There is a six-hour surface interval and each dive seems to have a conservative ascent profile with the use of oxygen to optimize off-gassing. But once again, we need to consider daily CNS loading.
The total time at 1.4 bar of oxygen for these two dives is about 80 minutes… that’s equal to about 45 percent (80/180) of the NOAA limit. In addition, the total time at 1.6 is 32 minutes which is about 22 percent 32/150) of the NOAA limit. This adds up to 67 percent for the day. No worries.
But here is the issue. The NOAA daily limit is for a 24-hour period NOT a calendar day. If this diver – on a decompression course and anxious to get in the final dive before the weather turns nasty – gets an early start the next morning and – thinking all is clear because he has had a good sleep – plans a slightly deeper and longer dive, he may be pushing the limits. Here are the two dives on day one with the early morning dive on day two added.
DIVE PLAN #1
Surface interval = 2 day 0 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.
Off gassing starts at 87.3ft
OTU’s this dive: 104
CNS Total: 64.7%
148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL
DIVE PLAN #2
Surface interval = 0 day 6 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 135ft (2) Nitrox 28 50ft/min descent.
Level 135ft 37:18 (40) Nitrox 28 1.42 ppO2, 120ft ead
Asc to 60ft (42) Nitrox 28 -30ft/min ascent.
Stop at 60ft 0:30 (43) Nitrox 28 0.79 ppO2, 52ft ead
Stop at 50ft 4:00 (47) Nitrox 28 0.70 ppO2, 43ft ead
Stop at 40ft 5:00 (52) Nitrox 28 0.62 ppO2, 34ft ead
Stop at 30ft 8:00 (60) Nitrox 28 0.53 ppO2, 24ft ead
Stop at 20ft 16:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.
Off gassing starts at 87.3ft
OTU’s this dive: 104
CNS Total: 68.7%
148.6 cu ft Nitrox 28
16.0 cu ft Oxygen
164.6 cu ft TOTAL
DIVE PLAN #3
Surface interval = 0 day 10 hr 0 min.
Elevation = 0ft
Conservatism = + 3
Dec to 145ft (2) Nitrox 26 50ft/min descent.
Level 145ft 32:06 (35) Nitrox 26 1.40 ppO2, 134ft ead
Asc to 70ft (37) Nitrox 26 -30ft/min ascent.
Stop at 70ft 0:30 (38) Nitrox 26 0.81 ppO2, 63ft ead
Stop at 60ft 3:00 (41) Nitrox 26 0.73 ppO2, 54ft ead
Stop at 50ft 4:00 (45) Nitrox 26 0.65 ppO2, 45ft ead
Stop at 40ft 5:00 (50) Nitrox 26 0.57 ppO2, 35ft ead
Stop at 30ft 9:00 (59) Nitrox 26 0.50 ppO2, 26ft ead
Stop at 20ft 17:00 (76) Oxygen 1.60 ppO2, 0ft ead
Surface (78) Oxygen -10ft/min ascent.
Off gassing starts at 97.5ft
OTU’s this dive: 97
CNS Total: 61.8%
146.0 cu ft Nitrox 26
17.0 cu ft Oxygen
162.9 cu ft TOTAL
Are those examples plausible? Certainly and I’ve witnessed it or something like it many times. Are they safe? Maybe, and maybe not because the additional 32 minutes of bottom time at 1.4 bar on the third dive plus another 16 minutes at 1.6 bar to optimize deco, has brought the diver’s 24-hour CNS loading to about 95 percent of NOAA’s limits.
Accordingly, if someone did these three dives, there should be a 24 hour break before the next dive.
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