A sodium chloride spring rising at seaside Akazawa in Ito. With total dissolved solids of 19.90 g/kg it is a hypersaline strong-salt source exceeding ocean salinity, leaving a mineral film on the skin and retaining warmth well after the bath. It is weakly alkaline and soft on the skin. As the source emerges at 35.5 °C, the water is reheated for bathing.
Good for
Warming
Salt forms a film on the skin that slows heat loss, so the warmth lingers long after the bath (the classic heat-keeping water, nettō 熱の湯).
cold sensitivity
Smooth skin
Bicarbonate and alkalinity gently lift dead keratin while silica hydrates, leaving skin smooth. This is the basis of the beautifying-water (bijin-no-yu 美人の湯) reputation.
dry, rough skin
Cuts
Mineral-rich chloride, sulfate and sulfur waters have a long-recorded soothing effect on minor cuts and slow-healing skin.
minor wounds
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
- acute phase of illness (especially with fever)
- active tuberculosis
- advanced malignant tumors
- severe anemia
- serious heart disease
- serious kidney disease
Full composition
The bar is each ion's share of charge (mval%) within its group; over 20% names the spring. Gases, silica and trace metals are measured by mass only.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺5,177mgP97 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺1,206mgP96 national
- Magnesiummild; a laxative if you drink itMg²⁺635mgP99 national
- Potassiumminor; matters for drinking limitsK⁺138mgP94 national
- Manganesetrace; companion to iron springsMn²⁺2.1mgP91 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺0.80mgP59 national
- Aluminumshows up in acidic volcanic waterAl³⁺0.10mgP54 national
- CopperCu²⁺< —mg
- Iron (III)Fe³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻10,850mgP97 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,825mgP99 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻67.2mgP34 national
- Bromidetrace, sea-originBr⁻13.1mgP90 national
- Iodideyellows in air; helps blood lipidsI⁻0.08mgP30 national
- HydroxideOH⁻0.0070mgP24 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃29.6mgP12 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.7mgP54 national
- Metaarsenious acidHAsO₂0.04mgP38 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂6.5mgP32 national
Measured at the source
On the certificate
- chronic pain or stiffness of muscles or joints
- mild paralysis
- peripheral circulatory disorders
Evidence
- Maeda M. 温泉の医学的効果とその科学的根拠. J. Hot Spring Science 70:197–207, 2021.
- Kario K et al. Hemodynamic Effects of Hot Spring Bathing. Hypertension Research 46(3):711–720, 2023.
- Naito Y et al. A Hot-Spring Water Improves Inflammatory Conditions in an Atopic Dermatitis Model. Biomedicines 13(11):2707, 2025.
- Li H et al. Bicarbonate Ionized Water Bathing Enhances Natural Killer Cell Activity. Scientific Reports 14:51851, 2024.
- Takeda M et al. Hot spring bathing practices have a positive effect on mental health in Japan. Heliyon 9(9):e19631, 2023.