[Expez]

The article says it will "diffuse away" which implies they just let it escape the device. It'll then move through the tissue / fluids, reach the blood and eventually be breathed out. I did a quick google search to see if we already had some level of hydrogen gas in our body.

It seems that bacteria in the human gut already produce some hydrogen gas and that breath contains measurable levels of it[0]. This was news to me, but means that dumping a tiny amount of H2 into the body is probably fine.

[0] https://link.springer.com/article/10.1007/s00216-013-7606-6