http://www.mindspring.com/~divegeek/oxyconcern.htm
For the "average" recreational diver its sure seems pretty safe...
The "Numbers"
Larry Harris Taylor, Ph.D.
I believe the relative concerns are about oxygen toxicity can be best expressed by looking at some computer-generated numbers, with the understanding that different computer programs may give slightly different absolute values for the numbers quoted. The exact numerical value is NOT as important as the relative magnitude of the number and its comparison to the accepted tolerance limits. Besides, in the clinical situation, even though numbers are important tracking devices and guidelines, therapeutic protocols (time/depth/breathing gas) are evaluated and determined by the medical staff based on individual patient behavior and needs. Lastly, while 1440 OTU's is the allowed daily dose of oxygen, under physician's care in a hyperbaric procedure, doses in excess of 1700 OTU's may be acceptable.
A diver breathing 100% Oxygen at sea level (1 ata pressure) accumulates 1 OTU per minute. An injured diver breathing from a demand inhalator mask typically consumes a DAN jumbo D cylinder in approximately 50 minutes. So, each DAN cylinder consumed represents about 50 OTU 's delivered to the patient. Thus, a diver would need to consume (1440 0TU allowed per day / 50 OTU per cylinder) 28.8 DAN jumbo D cylinders in a continuous 24-hour session to reach the allowed whole body daily dose. Most divers do NOT carry this quantity of oxygen with them AND, more importantly, transfer of the patient to the emergency medical chain of response occurs long before oxygen toxicity becomes a critical factor.
Now, let's address accumulation of OTU's during recreational diving on typical recreational diving breathing gases.
Table One examines the % CNS toxicity (as a measure of CNS toxicity risk) and accumulated OTU's (as a measure of whole body or pulmonary toxicity risk) for dives on air to the no decompression limits of the US Navy tables. These numbers are well below the daily-allowed dose of 1440 OTU.
Table 1: Oxygen Toxicity Values While Breathing Air
Depth (fsw) Time (min) % CNS OTU's
40 200 0 0.00
50 100 12 9.10
60 60 8 14.67
70 50 8 19.04
80 40 7 20.40
90 30 7 19.11
100 25 6 19.07
110 20 6 17.85
120 15 5 15.51
130 10 4 12.10
140 10 5 13.50
There has been some discussion in the recreational diving literature, especially in the early 1990's, about withholding oxygen to divers who have made dives using a breathing mix of oxygen-enriched air. So, let's look at "the numbers" for oxygen-enriched air. Table Two is a compilation of oxygen toxicity values for dives to the no decompression limits while breathing NOAA Mix 1 (32 % oxygen). Again, these values are far below thresholds of concern.
Table 2: Oxygen Toxicity Values While Breathing NOAA Mix 1 (32 % Oxygen)
Depth (fsw) Time (min) % CNS OTU's
40 310 55 149.19
50 200 45 133.17
60 100 29 84.16
70 60 20 60.80
80 50 22 59.42
90 40 20 54.38
100 30 18 46.32
110 25 18 43.55
120 25 22 47.71
130 20 36 42.15
In addition, Table Three is a compilation of oxygen toxicity values for dives to the no decompression limits while breathing NOAA Mix 2 (36 % oxygen). As with Mix 1, these values are below thresholds of concern.
Table 3: Oxygen Toxicity Values While Breathing NOAA Mix 2 (36 % Oxygen)
Depth (fsw) Time (min) % CNS OTU's
40 310 68 200.48
50 200 56 166.46
60 100 35 103.36
70 60 26 73.35
80 60 31 85.01
90 50 31 80.43
100 40 31 72.39
110 30 42 60.36
To help put this is a bit more of a proper perspective, consider (only for the purposes of examining the "numbers" for oxygen toxicity: this is certainly not a suggested mission profile. The "numbers" here are taken to absurdity only for purposes of illustration) making three consecutive dives, with no surface interval, to 60 fsw for 60 minutes. The accumulated % CNS and OTU's for this series of decompression dives is shown below in Table Four.
Table 4: Oxygen Toxicity Values For Three Consecutive Dives of 60 fsw for 60 minutes
Mix % CNS OTU's
Air 25 43.90
Mix1 (32 % O2) 54 152.40
Mix 2 (36 % O2) 64 188.49
It should be obvious that even after three consecutive 60 fsw dives for 60 minutes, a diver would be well below the allowed OTU accumulation of 1440. I also suggest that, especially in cold waters, thermal considerations have more control of the dive durations than whole body oxygen toxicity while diving conventional mixes to traditional recreational diving depths.