Let's be objective with a real world example of the risks involved (non-contributing, glip & flippant, contemptuous "holier than thou" & self-righteous posts like Lamont & Bob's above aside):
If you're in warm tropical 27deg C waters, no current, good viz, on an external easy tour of a wreck (like in Truk Lagoon, where I'll be in two weeks
), you can cognitively accommodate to deep air. You have trained muscle memory to handle most contingencies albeit you may be slower to process, problem solve and react . . .
so you strategically mitigate your activities to avoid potential tactical overload in an emergency situation --i.g. don't penetrate the wreck; don't physically exert yourself to CO2 retention levels & starting the vicious dark narc cycle;
don't do any extensive computations on-the-fly to your deco schedule; go no deeper than 60m or ppO2 1.47 etc. --keep it all nice, easy and simple. . .
That above being said, in my experience . . .here's what can start to "innocuously" happen while on Deep Air:
You can compensate for the narcosis with increased concentration on the task at hand, but unfortunately that may come with the detrimental loss of overall "situational awareness": for example, you can consciously concentrate hard in tying in a clean secondary tie with your penetration reel, but you then inadvertently tangle your SPG in the process, in addition to silting-out the passage behind you (actually happened to me inside the Nippo Maru). Plus, it takes more time to problem-solve and assess contingency scenarios when you're cognitively impaired;
your margin for recoverable mistakes & errors of judgment becomes less and less; you become more susceptible to being overwhelmed by cascading adversity (i.g. tangled penetration line, followed by a primary light failure, with your buddies signaling that you're at thirds SPG Pressure-turn-around-to-egress). . .
