You've been an instructor for 40 years and probably diving even longer than that, so let me ask you this.
In all that time have you ever seen, read, experienced, or heard of a diver on open circuit scuba having an accident that was directly related to improper use of Nitrox?
That's a really good question, and the answer is a little complicated.
First of all, improper use of nitrox is a serious problem in technical diving. It usually involves using the wrong mix at depth, and the most common reason is a mistake in what was in the tank being breathed. There have been a number of such incidents, and the details of those incidents are instructive in terms of a complete reply to the question, as I will explain later.
The one recreational diving incident I do know of is actually the reverse of your question--a diver intended to dive nitrox, made a last minute switch to air, forgot to changes his computer, and got bent because he dived air to nitrox limits. But that is not really what you are talking about, is it? You are asking about oxygen toxicity.
CNS Oxygen toxicity is a consequence of breathing too high a partial pressure of Oxygen
for too long. People tend to forget there is a time element involved--you don't dip below your MOD for a few seconds and go into convulsions. The problem is we can't accurately predict what PPO2 and what length of time are going to cause a problem. In the tech diving incidents I know, the time was longer than one would expect. In one case, a diver took EANx 50 to about 200 feet and was diving it for a surprisingly long time before toxing. In another case, a diver thought he had air in his doubles and dived at about 160 feet for about 20 minutes before trouble hit--he had EANx 36 instead of air. In another case, a diver insisted his stage bottle had air and dived it at about 100 feet for a while before the pure oxygen it actually contained did him in.
So let's say we had a recreational diver with an AL 80 diving with 36%, which has a standard MOD of 90 feet (1.4 standard) and a contingency depth of 110 feet (1.6 standard). According to PADI nitrox tables, his NDL for 90 feet is 40 minutes, but with an AL 80, he certainly won't be there that long. If he violates his MOD and goes to 100 feet instead, the longest he could stay there with a very good SAC rate is 30 minutes. Will that be enough to put him over the edge? Let's say he thinks he is diving air and goes to 120 feet. His air NDL is 13 minutes there, but he will probably be ascending before that because of gas supply limits.
In short, for a recreational diver with an AL 80, the gas supply will greatly limit the chances of oxygen toxicity, assuming that the fill is within the normal range for recreational nitrox. If you think you are diving air and accidentally have nitrox, the air NDLs will also limit the risk. It is still possible though, so no one should intentionally violate the standard limits. Increase the size of the tanks or get fills beyond the recreational limits, and the potential changes.
