HHO is safe and non-combustible at concentrations of 4% or less hydrogen in the breathing mix, so I am told. Part of the safety modifications for doing this involve throttling the gas production to a safe level.

You can find most of this information in material published by George Wiseman and others, starting with the links in my articles.

And yes, while breathing pure HHO would be possible, you wouldn't want to be anywhere near a spark or a flame... the result would not be pretty! :O