Unfortunately there just isn't any way to definitively test your sensitivity to oxygen. Most of current "wisdom" is based on anecdotal evidence as has already been mentioned. I do recall however going back through most of the "toxicity" accidents when I was going through my cave as well as rebreather training and did not find any events (even counting multi-day dive profiles) that were reported with pO2's below the 1.2 threshold, however nearly 100% of seizures underwater will be fatal. (Someone feel free to correct me if they find evidence to the contrary as I may find myself diving a 1.0 as Ken does.) With the stakes that high it seems to be prudent to lower your pO2's for the working portion of the dive to 1.2 as increasing even to 1.4 offers marginal benefit.

As far as nitrogen being "anesthetic" and or "narcotic"...

We have all experienced the affect of nitrogen "narcosis". There does appear to be some type of affect with regards to impairment of motor function, motivation , and judgement. How this actually occurs is largely unknown. Hell we still don't even know for sure how many modern anesthetics exert their effects. General anesthetic gases as well as IV anesthetics will prevent as well as "break" a seizure very rapidly. I would expect nitrogen to play a minimal role in the concentrations our bodies see to prevent or attenuate a seizure.
With Hal Watts I would say that on certain days when he felt the narcosis more so than others is probably based on descent rate. The faster he descends, the more rapid the increase in the "dose" of nitrogen he received. For instance, if I have an entire vial of Diprivan which is an IV anesthetic (Michael Jackson Juice), I can give that entire vial to a patient and they will remain "awake". They will be impaired but observers would describe them as awake. Liken that to a slow descent. Now I take that same vial of Diprivan and I give the whole thing in one push then unconsciousness will ensue. That would be more of the rapid descent type. Rapidly increasing the dose of an "anesthetic" or "narcotic" drug will increase its onset and side effects. Basic pharmocology.

I expect the answer to oxygen sensitivity lies in consideration of many different mechanisms but a large part of it in my opinion is based on acute (single dive very high pO2) and chronic exposure (multi day diving). Not to mention oxygen free radical scavenging which likely plays a role as well. With so much individual variation from a physiologic standpoint as well as from a dive practice standpoint we may not ever get to the bottom of it. Way too much physiology to discuss on this forum but again it seems prudent to me to lower working pO2's because no one wants to find out how sensitive they are to oxygen the old fashioned way.

Sean Costabile, MD
Cardiovascular Anesthesiologist