I would hazard that the "old" USN standard of 1.2 was ats guage? seems excessively , and uncharacteristically cautious, and I know that USN divers routinely dove to 60+ fsw (feet sea water) on 100% O2for quite some time. a PPO2 of 2.0 is what DCIEM has deemed potentially acutely toxic for an unacceptable percentage of the population. When I was in the Navy ( 2002) we used 1.6 ATA as the in water cutoff, or approx 25 fsw for 100% O2. For in water decompression, Canada uses a 30' O2 stop(1.8+) and in the chamber, oxygen is used @ 40' for decompression, and @ 60' for treatment. in each case, exposure is moderated by using a 25 minute on, 5 minute off routine, and there is a chamber attendant inside with the person. As you are aware, there are 2 types of o2 toxicity, pulmonary toxicity, and CNS. The NOAA #s, don't jive with what I remember and would possibly be a misprint? Where did you get the reference, I am surprised by those #'s. In summary, 1.6 ATA is quite moderate as opposed to historical norms, and very safe. Just to put these #'s into context for Navy seals to dive on oxygen rebreathers @ even 1.5 ATA ( 2 hour max according to NOAA), they would have to swim @ 16', & @ 1.3, they would be skimming the surface @ around 9'