Ok, i love free diving, have all my life. But currently i'm starting te reach my max bottom time. I was wondering if i could use pure O2 at the surface to boost the oxigen level in my blood/lungs in order to extend my bottom time. The thing i'm worried about before attempting is the fact that oxygen becomes toxic at depth. At times i do constant weight diving (sit on a sled with eights on it and decent along a steel cable) Having 9 liters of pure O2 in my lungs, might give problems when i go down on a sled. I reach 40 meter in quite short time, (when going down with a sled) and my body won't have used much of the O2. Does the O2 get toxic or is my body able to use enough of it to dilude it enough with nitrogen to keep alive?