A hot (73.3 °C) sodium chloride-bicarbonate spring, cooled to about 48 °C at the bath. Alkaline pH gives it a soft, faintly slippery feel, while the salty chloride backbone is rounded out by bicarbonate softness. Mineralization is moderate at ~1,437 mg/kg, with a useful 130 mg/kg of metasilicic acid layered in for skin smoothness.
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
Aches & joints
Warmth eases stiffness and raises the pain threshold. Radon and sulfate springs show the strongest evidence for joint and muscle pain.
stiff shoulders
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
- 急性疾患(特に熱のある場合)
- Active tuberculosis
- Malignant tumor
- 重い心臓病
- 呼吸不全
- Renal failure
- Bleeding disorders
- Severe anemia
- その他一般に病勢進行中の疾患
- 妊娠中(特に初期と末期)
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⁺393mgP70 national94%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺12.4mgP29 national3%
- Potassiumminor; matters for drinking limitsK⁺9.6mgP45 national1%
- LithiumLi⁺1.4mgP86 national1%
- MagnesiumMg²⁺0.50mgP20 national0.2%
- Iron (II)Fe²⁺0.50mgP52 national0.1%
- StrontiumSr²⁺0.50mgP68 national0.1%
- BariumBa²⁺0.90mgP85 national0.0%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻299mgP63 national45%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻488mgP75 national43%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻58.7mgP43 national7%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻24.0mgP75 national4%
- FluorideF⁻2.2mgP59 national0.6%
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂147mgP97 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃130mgP65 national
Measured at the source
On the certificate
- Chronic digestive disorders
- Hemorrhoids
- 冷え症
- 慢性皮膚病
- Chronic gynecological conditions
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.
- Donaubauer AJ et al. Serial radon spa therapy on pain in musculoskeletal disorders. Frontiers in Immunology, 2024.
- Verhagen AP et al. Balneotherapy for rheumatoid arthritis. Cochrane Database of Systematic Reviews CD000518, 2015.
- 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.