At the risk of sounding like a smart arse....
Not in a million years will you get bent freediving 30'!
The principle behind decompression sickness,{the bends} is that nitrogen, which makes up approx 80% of the air that we breath, becomes liquid under pressure.
When breathing compressed air at depth, tiny droplets of liquid nitrogen are absorbed into the bloodstream through the normal breathing process. The more time at depth, the more nitrogen is absorbed.
To rid the body of this excess nitrogen, one must surface slowly or stop at certain predetermined depths to allow that liquid nitrogen to turn back into a gas and escape through the lungs as one breathes.
By surfacing to fast, as the water pressure decreases these droplets will become a gas again forming bubbles in your bloodstream. These bubbles tend to aggregate in the joints, especially the shoulder, but can occur anywhere in the body including the brain.
Short term can cause excruciating pain and even death.
Longterm can lead to bone necrosis.
To run the risk of being bent at 30', you've got to spend something like 4 hours breathing compressed air at this depth and then swim rapidly to the surface.
Granted, there have been literally a FEW cases of freedivers suffering the bends, but these guys were all snorkelling repeatedly in excess of 100' for at least 5 hours.
Personally, I find these claims hard to believe.
But it COULD happen at these depths.
Cheers Lou.