Introduction:
It has not been experimentally proven whether hydrogen gas (H2) is absorbed into the body through the skin by hydrogen-rich hot-water bathing.

Material and methods:
In this study, Hairless mini pigs, whose skin closely resembles that of humans, were bathed in hydrogen (H2)-rich hot water to assess the absorption of H2 through the skin. An H2-rich water generation line was developed to maintain a high concentration of H2 via the circulation of hot water in an 80-litre bathtub. Two hairless mini pigs (14.2 ±1.4 kg, 60 days old, 1 male and 1 female) were first placed in the H2-dissolved bath. After a washout period, one pig was bathed in an H2-dissolved bath and the other in a bath containing no H2 for 20 min. During the experiment, blood was collected from the pigs’ jugular vein, carotid artery, inferior vena cava (IVCs), and portal vein to measure the blood H2 concentration.

Results:
The H2 concentration at the IVC of the pig in the H2-dissolved bath increased from 0.733 ±0.636 ppb (w/w) to 16.9 ±4.46 ppb (w/w) after 2 min, 37.2 ±13.8 ppb (w/w) after 10 min, and 45.7 ±7.73 ppb (w/w) (H2 saturation level: 3%) after 20 min. The blood H2 concentration levels of the pig in the non-H2 bath remained below the detection limit of 0.3 ppb.

Conclusions:
Bathing in water with a high concentration of dissolved hydrogen was considered an effective means of supplying H2 to skin tissues and beyond.