Molecular hydrogen is the smallest and lightest molecule in existence, consisting of two hydrogen atoms. Its tiny size allows it to penetrate cell membranes easily, reaching deep into tissues and even crossing the blood-brain barrier. Research suggests that H₂ acts as a selective antioxidant, neutralizing harmful free radicals while leaving beneficial reactive oxygen species (ROS) untouched. This unique property makes it a powerful tool in combating oxidative stress, a key contributor to aging and chronic diseases.