Hydrogen water is water infused with hydrogen gas (H2), the first element on the periodic table. Hydrogen water is different from water containing hydrogen ion (H+), the amount of which is used as a measure of acidity.

The saturation concentration of hydrogen gas in water is about 1.6 ppm, 1.6 mg/liter or 0.8 mM under normal conditions. It is nearly two million times higher than hydrogen concentration in natural water, because little hydrogen gas is present in the atmosphere.