[ethbr1]

Indeed, if you're doing extreme free diving, it would make more sense to breathe a more depth-compatible (nitrogen-replaced) mix at the surface before diving, no?

Then your lungs (and blood) would be full of a safer gas mix at depth.

I'm not sure if any of the deeper gas mixes are safe to breathe at surface level though? I think so?

[nradov]

Yes, some "technical" freedivers do pre-breathe nitrox or pure oxygen at the surface before descending.

https://www.deeperblue.com/technical-freediving-are-breathho...