Posted by Kendall Raine on June 04, 2002 at 10:41:37:
In Reply to: Re: Question for you. posted by Captain Tim on June 04, 2002 at 09:20:36:
There I go again. Sorry. If the question is "will breathing 32% at 140 kill you?" the answer is "maybe."
If the question is "is Nitrox safer than air?" the answer is "maybe."
There is no magic demarcation line overwhich a diver will succumb to CNS oxygen toxicity per unit of pressure/time. Agencies teach that PO2's should be kept to no more than 1.6 ata. Because the chemical process which is believed to drive the grand mal seizures symptomatic of CNS oxygen toxity is complex, 1.6 ata is cited by the agencies as a conservative practical limit for wet dives. Please note Treatment Table 6 puts you at 2.82 ata for an extended period. Most patients don't tox. Chamber tests have shown peolple can breathe 100% O2 for hours at partial pressures well above 3 ata without toxing. The same tests have shown high variability amongst people and within people to CNS toxicity. The threshold for CNS toxicity is believed to be much lower for immersion exposures. This is believed to be related to both mechanical (work of breathing-CO2) and chemical (adreneline) factors. That is why work while breathing elevated PO2's can significantly lessen the CNS toxicity threshold. As a result, we use a much reduced max PO2 of 1.2 to 1.3 for our dives, deep or shallow, and only go to 1.6 during the resting (deco) phase of a dive. It is also a reason to use helium in breathing media to improve regulator performance, lower work of breathing and lower CO2 levels.
Would I use 32% at 140? Never. That is because, with enough time at depth, it could cause a seizure. A seizure underwater is likely to lead to drowning. Will it cause a seizure? Maybe and maybe not. Why find out? Even if it doesn't cause a seizure, prolonged high PO2's cause cummulative lung damage which can offset the decompression benefits. Again, why screw around?
Now, is Nitrox safer than air? Having talked about some of the hazards of O2, let's turn to the benefits. Nitrox lowers the fraction of N2. Since N2 is inert, or at least not used in metabolism, it is waste which must be disposed of properly to avoid bends. One way to dispose of it is not to breathe it in the first place. Nitrox tries to do that. Since Nitrox has some N2, however, you still need to dispose of the stuff through proper decompression (ascent rates, stops, etc.) The less N2 in the mix, the longer your available bottom time per unit of required deco (NDL). It's a balancing act between reducing deco obligation and maintaining O2 exposure within limits that don't cause damage.
If you dive 32% on air tables or an air computer, you have a built in safety cushion where N2 is concerned. Provided you keep the PO2 at 1.3 ata or less during the working phase of the dive, you have a good cushion on CNS toxicity for all practical exposures involving open circuit. Put another way. You are better off on a no-D dive to 100 fsw on 32% than you are on air. Will Nitrox save you from bends, stupidity or the Hand of God? No. It's an effective hedge if you use it right. Nothing more.