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I'm coming to this thread a little late (thanks to Panix sys admin games), so please excuse me if I have missed a reply that makes mine redundant... In reply to John Crea's statement "Any breathing gas with a CO content safe to breathe at the surface is safe at depth" (paraphrased -- he said it more eloquently -- as I have misplaced the original), I would say that the statement is true, but in a deceptive way. The deception has to do withthe missing parameter: time. As others have posted, the statement is true from the perspective that O2 delivery at depth will be no more compromised than at the surface, irrespective of CO content. This is true for 2 reasons: 1) the CO-O2 partial pressures will maintain the same relationship as depth increases, causing the CO impact on O2 transport to be no worse than at the surface; and 2) the O2 transport via plasma will increase with depth, causing a net improvement in O2 transport with depth, again irrespective of CO fraction. The problem is , however, time. The statement does not mention the time factor, and that is a major ommission: a breathing gas which may not cause blackout for quite a while at the surface, may indeed cause blackout upon returning to the surface following a dive. This can be understood from the perspective that CO impairment of HbO2 (by way of HbCO) is a cumulative effect, and will not be undone by the dive (assuming no gas switching -- particularly to high PP O2 breathing mixes with reduced CO). In fact, the time to blackout from breathing a particular mix of CO/O2/etc at the surface will always be the lower limit of the time to blackout from breathing the same mix at any depth (higher ATA pressure) -- as a result of the O2 transport via plasma. However, once the surface time to blackout is exceeded, the diver will become eligable for blackout upon return to the surfac -- this being a result of the cumulative effect of the CO exposure. Another way to explain this is that the surface blackout is controlled more by the net molar quantity of CO inhaled, while depth blackout iscontrolled more by PP O2; the CO molar "clock" is always running, and the diver will be eligable to take a "hit" from it as soon as the PP O2 has receeded to a point where the molar clock is exposed. (I visualize this as the PP O2 slows the CO molar clock when PP O2 is greater than surface (hyperoxic), but the CO molar clock is accelerated an exactly corresponding ammount as PP O2 is decreased, so that the net effect of an excursion to depth on the CO molar clock is zilch.) So I would state Crea's little challenge more along the line of "For any given CO/O2/etc gas mix, it is safer (gives a longer time to blackout) to breathe it at depth than at the surface (ignoring O2 tox, etc)." Which is, of course, totally obvious, as we all know that hyperbaric treatments are theprimary treatment for CO inhalation. (The point I'm trying to make above whathas already been presented is that breathing a CO mix at depth is not "safe", but rather just buys some more time.) I haven't run any numbers, but I don't think accounting for CO2 in this equation would change any of this. -frank -- fhd@pa*.co* | Hydrogen is a light weight, colorless, ordorless, 1 212 559 5534 | tasteless gas, which, given enough time, turns into 1 917 992 2248 | people. 1 718 746 7061 | -- Unknown Astronomer-type dude
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