Proportion of gases filled in cylinder of sea-divers In my book it is given that earlier cylinder were filled with 2% oxygen and 98% helium. Nowadays it is filled with 11.7% helium, 56.2% nitrogen and 32.1% oxygen.

There are few questions which come up in my mind


Earlier 2% oxygen is changed to 32%. So was 2% oxygen enough for huaman?
98% helium is reduced to 11.7% why it is so? 
I don't get reason why helium and nitrogen are filled?
Hint:
Try to use Henry's Law. According to which solubility of gases increases with increase in pressure. If we go deeper downwards, pressure increases and when we come to the surface, pressure decreases, so there might be that soem buubles are created in the blood which may be fatal. So air cylinders are made keeping this in mind.