Sort of a general comment made earlier and worth repeating for late comers to this thread....
The models, rules, algorithms - whatever you want to call them - that we use for oxygen toxicity (both CNS and pulmonary) are just our "best effort" to give guidance about what is likely safe versus what is likely not safe, or at least what is entering into a realm of risk that most people would rather avoid. What these "rules" are certainly
not is a precise measure of anything.
Is the half-time credit for CNS really a half-time? If so, is the time 90 minutes or 120 minutes (does God like to work with round numbers?). How variable is that half time, if it is, for different people? We know, as
@doctormike explained that time/exposure are variables in CNS risk, though the relationship is not linear. What is that relationship, exactly? Don't know. We can approximate it based on empirical data, but this isn't like the situation where we can measure it how long it takes to metabolize alcohol (even that is variable, though at least it can be directly measured).
How is the risk of a CNS or pulmonary hit affected by other factors like whether you vape, are a CO2 retainer, have a lot of inert gas loading? What about how hard you are working or whether you are cold or tired or hungover or on meds? Some people can tolerate 1.3 PO2 day after day without it hurting their eyes, others are sensitive to hyperoxic myopia and cannot. Lots of mysteries.
I can tell you that I would be far more comfortable hitting 125% on the CNS clock because I did an hour or two of deco at a high PO2 than I would be hitting 95% if I were swimming into a current at 100'. We know exertion plays a factor. We also know that our model does not account for it.
These formulas - the CNS clock and OTUs - are just approximations work "enough" to serve as the basis for responsible dive planning. They are intentionally conservative -- precisely because we don't know everything. There is no guarantee that following these guidelines will keep you 100% safe, though it is believed that the risk would be quite small. It's a spectrum of risk. At the limits, what works one day may not work the next.
That is a very long disclaimer and response to some of the posts scattered among the last several pages that seem to imply a certitude about these guidelines that is probably misplaced. It doesn't make a lot of sense to split hairs about oxtox because we don't have a defined hair to split.
