I am not terribly concerned about what it says in the NOAA manual. I am a lot more concerned about real world experience and the various factors that affect me on a dive.
The US Navy did studies in the 1950's on O2 enriched air mixtures and found a large degree of variability to O2 toxicity. One of the factors was that the test subects were former or current helmet divers who had developed a tolerance to high CO2 levels due to the poorly ventilated helmets of the day. This high CO2 tolerance and the lower respiration/SAC rates these experienced divers exhibited made them more suceptible to an O2 hit. In the end the US Navy was not able to determine a totally safe yet still operationally beneficial PPO2 level and abandoned the whole concept. In short the US Navy found that for working divers (with low SAC rates/high CO2 tolerance) a safe PPO2 level is low enough that you gain relatively little benefit over a 21% percent 02 mix, so why bother.
Recreationally speaking nearly all rec training agencies intitially used a 1.6 PPO2 and MOD's based on that number. However it is also true that nearly all recreational agencies now use a PPO2 of 1.4 and recommend MOD's based on that lower number. I suspect the move to a lower PPO2 had a lot to do with agencies being unable to control the type of diving that the nitrox divers they mint will ultimately do. And I suspect it had a lot more to do with an over abundance of lawyers, some of whom are willing to sue on a contingency basis on behalf of the surviving relatives of moronic and deceased divers who failed to apply common sense to their nitrox diving.
Tech training agencies seem to use an PPO2 of 1.4 for the working portion of the dive and use a PPO2 of 1.6 for the deco portions of the dive for the improved offgassing that occurs during the less active deco portion of the dive.
Personally, I think a decent education on the topic and a lot of common sense will serve you a lot better than rigorous adhearance to agency dogma.
My thoughts on the subject:
1. If you are one of those divers obsessed with having the lowest SAC on the boat or in your group of dive buddies, either stop trying to lower your consumption rates by extreme or artifical means or don't dive nitrox or do deep air dives (below 130'). If you persist in doing both, a PP02 of 1.0 to 1.2 may be something you want to consider as a maximum.
2. Some people are just more suceptible to an O2 hit than other people and your personal suceptibility varies from day to day. It is important for any nitrox diver to be aware of and alert for the symptoms of an impending PPO2 hit.
3. More exertion means more CO2 retention and greater risk of an O2 hit at a given PPO2. So if you are diving in a current, working under water or other wise exerting yourself more than normal - or are planning a dive where these situations could conceivably occur - plan to use a lower PPO2 and it's shallower MOD.
4. A quality regulator with low inhalation and exhalation efforts is beneficial interms of reducing CO2 retention. A cheap regulator does not mix well wit either deep diving or relatively high PPO2 nitrox diving.
5. A full face mask and a good dive buddy will go a long way toward making an O2 hit surviveable if one occurs since it is the drowning after you spit out your regulator that kills you, not the convulsion itself.
6. Caution is a good thing, but there is also a point of diminishing returns with reducing PPO2 levels. As you continue to lower the PPO2 and MOD, you gain less benefit in terms of reduced deco obligations and in turn reduce either the bottom time or safety margin added to an air decompression table's NDL's (depending on why you dive nitrox).
7. Think about what you do and why you do it and don't base what you do soley on agency dogma (particularly if you don't thoroughly understand the course content or if the course content was lacking). And don't base what you do on what you read on the internet.
