I would like to first say that just like anything involving humans (or any living creature), nothing is exact. In reality there is never a sharp threshold when it comes to O2 toxicity or for that matter decompression obligation. (IMHO) I always roll my eyes
when I hear the term “undeserved decompression hit”, but that is a subject for another discussion.
How susceptible we are to O2 varies per individual and it even varies by the time exposure and some other factors. But it is never an exact threshold limit.
From most of my reading, the 1.4 atm seems to be a fairly acceptable limit (both for the technical community and recreational divers), but you may notice the three calculations on the left of my page.
I normally use 1.4, but for short periods I have momentarily even exceeded the 1.6 atm threshold. At the same time for most of my actual diving we rarely get even close to O2 exposure of 1.2 atm.
Notice that 1.6 is often referred to as the contingency threshold. It is the often the allowable limit in shallower water during decompression when a diver may be using higher content O2 (even pure O2 at 20 feet max) to accelerate decompression.
The first calculation is for D-max1.2, the second is for a D-max using 1.4, and the third equation is for what we call D-cont using 1.6 (that is for maximum contingency depth).
So with the most common Nitrox 32, I like to stay shallower than 100 feet, but I am not too concern about 110 ft (for a short excursion). Most of the dives are probably shallower than 90 feet, which keep my PPO2 exposure to 1.19 atm or less.
Both of my computers (the Perdix and my older one) are both set to a max PPO2 to 1.4, but I also have the alarms on silence. I do keep an eye on it.
BTW, I do remember when I started diving it was considered acceptable a PP of O2 up to 2.0 atm. That is why air at the time was considered OK down to 298 feet of salt water. Never mind the narcosis level, but this was based on the believed acceptable limits for CNS (central nervous system) O2 toxicity.
A pure O2 rebreather was considers OK down to 33 feet of salt water.
And the Navy used to test their divers for how O2 susceptibility.
In a chamber or with a full-face mask the Navy and commercial divers still pushes O2 exposure during decompression. With a full-face mask you are not as concerned about drowning if you have a seizure from CNS high O2 exposure. You are not going to drop the regulator out of your mouth.