Onsen uses natural hot spring water heated beneath the ground and must be at least 25C, although some onsens get as hot as 100C. Balneotherapy – medical treatments through the use of bathing are widely practiced in Japan. They believe the mineral-rich water (magnesium and sulphur) from an onsen helps to promote skin health, whereas the heat from the water helps to reduce inflammation, muscle pain, and boost your immune system.
Onsens are often looked upon as a treat due to its cost, whereas sentos are everyday bathhouses that are more affordable. Sentos are communal indoor public bathhouses that simply use regular hot water just like you would if you ran a hot bath at home.