“And, if it is a mixture, what is the exact percentage of H₂ in the gas produced?”
PureONE is a mixed-gas machine commonly called an HHO or hydroxy generator. It produces a consistent mixture: approximately 66% molecular hydrogen and 33% oxygen.
Adjusting the percentage control on the front of the machine changes the gas production rate, not the gas composition. Lowering the setting reduces the amount of gas delivered; if you add the gas to your breath, it mixes with ambient air and the effective concentration decreases.