While the immediate risks are obvious and well-documented, I'm curious to know whether there are any long-term risks associated with lengthy breath-holding such as that undertaken by free divers, even when it doesn't result in a black out.

My gut feeling says yes, but I have absolutely no evidence to back up that and it's pure speculation. Something worthy of further study, perhaps, although it would only affect a minority of the population.

An impressive feat but one I wouldn't personally be undertaking.