[sheepeck]

I would say this:
Think about professional top freedivers - they go over 100m deep. Limit depth for diving with air as breathing gas is somewhere around 65m. So - will you also recommend to those freedivers not to go below that depth? In their case the PPO2 also goes up.

But I think that you won't insist on it. IMHO the amount of air in lung is so small that it's irrelevant.

Still, would be nice to have some entry about this matter from more experience diver.