I haven't looked into Kangen water yet. I don't know how much hydrogen "sticks" in the water when Brown's gas bubbles through it; I just know that we need to drink it straight away after bubbling, because hydrogen is such sneaky stuff... it can escape from just about anything!

The percentage is straight math, based on lung capacity and breathing rate. George works out the details pretty clearly in his manual for the AC50 machine. You can easily find a copy of that manual on the AC50 page, linked frequently from my articles. ;)

I also did the math on reaching that level in the volume of a typical sized room... It would take days and days, and that would only be if the room were sealed... So, really, the main caution is keeping sparks and flames away from the machine outlet (and from any gas bags surrounding body parts).