Molecular hydrogen is inert and, as mentioned above, is unlikely to react with ROS and RNS with low reactivity, which play an important role in physiological functions as signalling molecules.
Short Residence Time
Owing to its small size, molecular hydrogen can quickly enter the blood stream and diffuses into organs, tissues, cells and part of the body where most of antioxidants fail to reach. Similarly, molecular hydrogen is expelled in the breath as quickly as it enters the body. In case of the study by Shimouchi et al. (2009), hydrogen water taken orally appeared in the breath within 15 minutes and the concentration of molecular hydrogen in the breath returned to the basal level within 60 minutes (10). Unlike common drugs, the liver and the kidney are not involved in the metabolization of molecular hydrogen.
Use in Deep Diving
Hydrogen gas has safely been used in deep diving. The first report dates back to 1945 – by the Swedish engineer, Arne Zetterstrom. It was a mixture of oxygen and hydrogen gases, known as Hydrox. In late 1960’s, the United States Navy and the Compagnie Maritime d’Expertises of France also started testing the use of high-concentration hydrogen gas (up to 49%) in dives as deep as 700 m (11).
Endogenous Intestinal Gas
The health safety of molecular hydrogen has been discussed based on facts that the human gastrointestinal tract harbours trillions of microbes, namely the gut microbiota, and that molecular hydrogen is amongst the common biological gases produced by the gut microbiota – other common gasses being oxygen, nitrogen, carbon dioxide, methane and hydrogen. This is a long-accepted fact and constitutes the principle of the breath hydrogen test – a test to determine small intestinal bacterial overgrowth (12, 13, 14).