I feel that I perhaps shouldn't be asking this as it's bound to be pretty nasty - but... What the hell happened for you to spend a day in an hyperbaric chamber Russ?!
I believe the balanced flue to our boiler was partially blocked, so there wasn't enough oxygen for the gas to burn completely. Carbon monoxide was drawn from the flue into my bedroom through an open window, where I had been happily breathing it in all night, for several weeks.
The easiest way to remove carbon monoxide from the blood, where it is absorbed more readily than oxygen, is to force it out with high pressure oxygen. I spent eight hours in a Perspex tube at 5atm, nude, with a very bored nurse watching me.