From: http://news.bbc.co.uk/2/hi/entertainment/4946804.stm I was reading this through with interest from the Scuba angle and was rather puzzled when I got to the bottom: Mr Harris thinks Blaine will have an unfair advantage when he attempts the world record. "He will be breathing compressed air under water so he will have a much higher concentration of oxygen molecules," he says. "In competitive freediving, we are positively banned from breathing pure oxygen before the event." Can anyone explain exactly how the % of oxygen in compressed air is different to the percentage in uncompressed air? I hope he's been misquoted, either that or he needs to resit his diving exam. The partial pressure of O2 at depth is of course higher, but the concentration is unchanged. Unless of course he's breathing Nitrox or pure O2, but the max depth on O2 at 1.6 bar ppO2 is only 6 metres and has serious CNS issues if used for days at the surface, let alone at any depth. As for the O2 ban, isn't that to do with why you shouldn't hyperventilate before freediving? It reduces the level of CO2 to below that needed to form the stimulus to breathe at the end of a breath hold whilst the level of O2 drops below the level needed for consciousness resulting in drowning. Corn.