Japan sorts its therapeutic springs into ten categories by what is dissolved in the water. The category tells you what the bath will smell like, feel like and look like.
Mild and low in dissolved minerals. Usually clear and gentle on the skin, which is why these baths are often recommended for first-timers.
Towns with simple spring water
Salty water. The salt leaves a thin film on the skin, so you stay warm well after leaving the bath.
Towns with chloride spring water
Smooth, slightly slippery water. Often nicknamed bihada-no-yu, the water for beautiful skin.
Towns with bicarbonate spring water
Clear water with a faintly bitter taste. Known for holding heat.
Towns with sulfate spring water
Fine bubbles cling to your skin. Rare in Japan, and usually cooler than other springs.
Towns with carbon dioxide spring water
Turns reddish brown as the iron meets air. The water is clear at the source.
Strongly acidic, sometimes below pH 2. It stings on cuts and is not for long soaks.
Towns with acidic spring water
Contains iodine and often turns yellowish over time. Uncommon.
Towns with iodine spring water
The classic rotten-egg smell and milky white water. It can tarnish silver jewellery, so take it off.
Towns with sulfur spring water
Contains trace radon that leaves the water quickly. Also called radium or radon springs.