Unlike traditional antioxidants (like Vitamin C or E) which are large molecules, molecular hydrogen is the smallest molecule in existence. This allows it to penetrate cellular membranes and reach the mitochondria and nucleus, where DNA damage often occurs. Its primary benefit lies in its ability to selectively neutralize the hydroxyl radical ($cdot OH$), the most cytotoxic of all reactive oxygen species https://www.clare-coltd.jp/hydrogen-inhalation .