Why, when everyone has any discussion about issues that are about physiology issues, decompression, O2 tolerance, etc, do people expect a black and white answer.
The human body is not a machine, not identical individual to individual. Even developing data on a specific individual,the result is not repeatable day to day.
It is not possible to state if you do x profile you will be guaranteed dci free. Add 1m or 1 minute and you are guaranteed to be bent.
Dive with a PO2 of 1.6 you are 100% guaranteed safe, a PO2 of 1.61 you are guaranteed to have a hit.
There are far to many variables. Most theory / best practice is based on a mix of theory, analysis, and practical experience. It is heavily weighted to minimise risk.
I don't think any one would be happy if you said its 50:50 that you will be Ok, or 90%, or 99% etc.
Point 1.
Physical fitness is a variable in the amateur diving community.
Age is a variable in the amateur diving community.
Environment is a variable.
How our physiology behaves day to day a variable.
Data is limited because of the ethics of human experimentation.
Despite this, the Navy's of the world have carried out tests.
One of the repeated results is the variability of the results, even with the same individual.
In the case of O2 tolerance, when the US Navy was carrying out tests. they could put the same super fit individual through the same tests, on occasions the individual could withstand very high PO2's BUT, on other occasions they would fit on PO2's below 2.0. There was no particular 'trigger' that they could determine for this variability.
There where triggers they could introduce that would increase the likelihood of a fit (CO2, Heavy work, Stress, Fatigue, Cold, Heat etc).
If they kept the PO2 below 1.6 (1.4), then it appeared statistically unlike that the diver would fit.
Along time ago I was taught a PO2 of 1.6 is the maximum..... In IDEAL conditions.
For any adverse, remove 0.1. - work, cold, fatigue, stress, etc. So a PO2 for 1.4 for the 'dive phase', 1.6 for the rest (decompression) phase.
Now, my maximum PO2 is 1.3, but then I'm on CCR. In an emergency, I allow a higher PO2 for off loop bailout, because the PO2 will crash as I ascend. For that brief instance, of high PO2, the risk of a CNS hit is very low.
Yes, you can do a dive with PO2 of 2.0 and have a safe dive, today, tomorrow, but eventually your luck will run out. Thats your problem, except of course, it becomes your buddies problem, and the rest of the dive team. Potentially, my problem when someone decided there are to many accidents.