There is a recommended maximum for hydrogen inhalation for therapeutic purposes, and it's defined by the FDA.
Here's how that works:
We normally inhale between 5 and 8 litres per minute, but when exercising this can rise to 12.
Let's look at a Hydroxy (Hydrogen and oxygen mixed) generator that makes 600 ml of gas per minute. That's 400 ml hydrogen, and 200 ml of oxygen per minute.
Now let's see how that relates to our breathing.
In a worst case scenario, we can assume that half of the time we are exhaling. (It's actually more like 1/3 inhalation and 2/3 exhalation.
While we exhale we are not taking in any of our Hydroxy mix.
Input gas volumes, therefore, are halved to 200 ml hydrogen and 100 ml oxygen. Here's the maths.
(0.2 / 5 )* 100 = 4 % hydrogen concentration
(0.2 / 8 )* 100 = 2.5 % hydrogen concentration
(0.2 / 12 )* 100 = 1.6 % hydrogen concentration
So to remain within FDA's guidelines, (for adults) and to stay below the 4% is to use any hydrogen generating device that outputs at less than 600 ml of hydrogen gas per minute.