Diver0001:
Diver0001 is correct but there is a little more to it and discussing the maximum PPO2 that is considered acceptable is normally a good way to start an argument.
PPO2's far lower than 1.6 or 1.4 are still toxic given enough time and PPO2's well above 1.6 can be tolerated for short times. The specific times are where it gets interesting as they can vary from diver to diver, from day to day, with workload and with other confounding variables such as elevated CO2 levels and the amount of nitrogen in the mix.
A PPO2 of 1.6 is considered the acceptable max for sport diving and many people feel the more conservative limit of 1.4 is better for portions of the dive where you are exerting yourself. But as a rule, the higher the PPO2, the shorter the period of time you can be exposed to it without potential problems.
Short term problems with high PPO2 include central nervous sysytem problems up to and including convulsions where the diver is likely to lose his reg and then drown. Longer term exposures to high 02 levels can cause pulminary problems due to oxygen at high partial pressures damaging lung tissue. (This can even occur with 100% O2 at 1 ATA given enough time.) Really long term problems can also occur such as bone necrosis but this is not an issue in sport diving.
