Closed-circuit rebreathers use high concentrations of oxygen, like 100%, to avoid nitrogen narcosis at depth.

“So, uh a closed circuit rebreather for example, like a drager rebreather like the original ones, uh those early rebreathers and even now it's high concentration. It's like a essentially 100% oxygen, right? You're breathing 100% oxygen. So there's no nitrogen. There's 80% nitrogen air we're breathing right now, right? Uh there's no nitrogen. So you avert the potential for nitrogen narcosis.”