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1. Molecular hydrogen (H₂) is the smallest and lightest molecule in nature.
   Because of its tiny size and neutral charge, H₂ can diffuse rapidly through cell membranes and enter places many larger molecules cannot, including mitochondria and the cell nucleus.

2. H₂ acts as a selective antioxidant, not a blanket one.
   It reacts with highly damaging radicals—especially the hydroxyl radical (•OH)—but does not neutralize useful signaling molecules like hydrogen peroxide (H₂O₂), which cells need for normal function.

3. Hydrogen gas is nonpolar and electrically neutral.
   This allows it to pass through lipid membranes without needing transport proteins or receptors, unlike vitamins or enzymes.

4. When H₂ neutralizes a hydroxyl radical, the end product is water.
   The reaction converts a highly destructive free radical into harmless H₂O, making hydrogen unusual among antioxidants because it leaves no toxic by-products.

5. The human body already produces small amounts of hydrogen naturally.
   Gut bacteria generate hydrogen gas during fermentation of certain foods, but most of it is exhaled or lost, which is why external sources (like hydrogen-rich water) are studied for potential benefits.