Not wanting to give you an entire course online, but ppO2 is the partial pressure of oxygen. 1.2, 1.4 and 1.6 are accepted limits for certain types of diving by certain agencies. Take a nitrox course to get the full info. The increased level of oxygen used in a standard recreational nitrox mix will have a higher partial pressure as you go deeper, within the recommended maximum depth of 130ft and using air, there is no problem, but as you start enriching the air or go deeper the partial pressure will go up and the risk of oxygen toxicity (both pulmonary and CNS) will increase and need to be monitored - its one of the minuses for using nitrox, the principal plus being longer bottom times. Its a great course and the use of nitrox is beneficial too