As Charlie has mentioned, the mechanism for CNS toxicity is not well understood, but, like narcosis, symptoms diminish quickly as PO2 falls below 1.4. In the case of narcosis, what's odd is that the narcotic affects of nitrogen seem to be almost entirely directly related to PN2 and have practically nothing to do with total absorbed nitrogen - that is, if you dive deep enough to get narced, you'll get narced right away and it won't get measurably worse over time if you don't go any deeper, and it will subside almost immediately when you ascend. At least with oxygen, there's a reason it doesn't accumulate like nitrogen, as oxygen is both metabolized in all the tissues, and scavenged by hemoglobin from the blood.biz_nate:I'm still curious. Several people have stated that as you head for the surface, tox symptoms would improve. I understand that there's a threshold at which this would be true, but until this threshold is reached, wouldn't you still be accumulating exposure? It seems a basic integration problem...
Therefore...
In the cited case, where the nausea persisted all the way to the surface and for awhile afterwards, I'd rule out both CNS oxtox and narcosis, and point the finger at food, drink, a bug or CO2 as the most likely culprits - and a possibility of CO/bad gas... but not oxtox or narcosis.
Rick