I can only say my experience, and granted that's not much, made my first 2 dives to 100' or so today using air(always used nitrox before when going to this depth). I can say this much, somewhere around 85-90 feet, I noticed it. Did notice myself taking more time than usual thinking over what I was doing before doing it. I was down in a cool lil cavern(Paradise Springs in florida) and had decided before we went down that we'd poke our heads back into the cave portion(5-10 feet at the most). I can easily see though if I hadn't noticed it, it could have turned into something dangerous.