An iron(II)-bearing sodium-magnesium bicarbonate spring that oxidizes reddish-brown on contact with air. The 48.4 °C source is delivered to the bath cooled to about 42 °C; the bicarbonate character also leaves skin feeling smooth. Iron measures 20.2 mg/kg — at the iron-spring threshold — with a generous 989 mg/kg of free CO₂.
Good for
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
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
From a certificate analyzed in 2007.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were removed from the official list of contraindications then, or include a condition that was. If you have a health condition, ask a doctor before bathing.
- acute illness (especially when accompanied by fever)
- active tuberculosis
- malignant tumors
- severe heart disease
- respiratory failure
- renal failure
- hemorrhagic disease
- severe anemia
- other generally progressing illnesses
- pregnancy (especially early and late stages) (dropped from the official list in 2014)
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⁺702mgP82 national56%
- Magnesiummild; a laxative if you drink itMg²⁺145mgP95 national22%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺138mgP80 national13%
- Potassiumminor; matters for drinking limitsK⁺94.1mgP92 national4%
- AmmoniumNH₄⁺25.3mgP99 national3%
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺20.2mgP96 national1%
- LithiumLi⁺1.1mgP76 national0.3%
- ZincZn²⁺2.5mgP98 national0.1%
- BariumBa²⁺1.8mgP93 national0.1%
- StrontiumSr²⁺1.1mgP80 national0.1%
- AluminumAl³⁺0.30mgP67 national0.1%
- ManganeseMn²⁺0.40mgP61 national0.0%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻3,260mgP99 national98%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻31.6mgP21 national2%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻30.6mgP36 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃222mgP90 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂13.0mgP63 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂989mgP99 national
Measured at the source
On the certificate
From a certificate analyzed in 2007.
This list uses the wording the Ministry of the Environment replaced in 2014, so it may not match a current certificate. Items marked † were dropped from the official list then.
- neuralgia
- muscle ache
- joint pain
- frozen shoulder
- motor paralysis
- stiffness of joints
- bruises
- sprains
- chronic digestive disorders
- hemorrhoids
- sensitivity to cold
- cuts
- burns (dropped from the official list in 2014)
- chronic skin disease (dropped from the official list in 2014)
- menstrual disorders (dropped from the official list in 2014)
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.
- 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.