Inhaled molecular hydrogen behaves as a selective therapeutic antioxidant — a distinction that matters. Unlike broad-spectrum scavengers that strip harmful and beneficial radicals alike, dissolved and gaseous H2 preferentially reduces the two most destructive species in human pathology: the hydroxyl radical (•OH) and peroxynitrite (ONOO−). Both are neutralized into plain water. Essential immune-signaling reactive oxygen species — superoxide, hydrogen peroxide, nitric oxide — are left untouched, preserving the oxidative signaling the immune system actually depends on.
The selectivity is matched by reach. Molecular hydrogen is non-polar, charge-neutral, and roughly 2 daltons in mass — the smallest molecule in nature. That physical profile lets it diffuse straight through lipid biomembranes, bypass the blood-brain barrier, and penetrate the mitochondria and cell nuclei where oxidative damage concentrates. Landmark peer-reviewed work (Ohsawa et al., Nature Medicine, 2007) documented precisely this selective neutralization, and hundreds of subsequent studies have mapped the effect across organ systems. The pharmacological shorthand: H2 goes where conventional antioxidants cannot, reacts only with the radicals that do the worst damage, and leaves behind nothing but water.