A sodium-calcium chloride spring drawn at a mixing tank from a very hot 76 °C source. Weakly alkaline and soft on the skin, it is nearly colorless and transparent with a faintly salty finish on the lips. It is one of the hotter natural sources, and carries around 131 mg/kg of metasilicic acid that helps leave skin feeling smoother.
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
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.
- Magnesiummild; a laxative if you drink itMg²⁺1.1mgP26 national2%
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺344mgP65 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺17.4mgP36 national
- Iron (II)rusts brown in air; iron for anemia when drinkableFe²⁺< 0.10mg
- Aluminumshows up in acidic volcanic waterAl³⁺< 0.05mg
- CopperCu²⁺< 0.05mg
- Iron (III)Fe³⁺< 0.05mg
- Manganesetrace; companion to iron springsMn²⁺—mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻1,744mgP85 national81%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻476mgP86 national16%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻12.9mgP8 national3%
- Bromidetrace, sea-originBr⁻1.7mgP69 national
- Iodideyellows in air; helps blood lipidsI⁻0.30mgP62 national
- Hydrogen phosphateHPO₄²⁻< 0.10mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< 0.10mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< 0.10mg
- HydroxideOH⁻0.02mgP28 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃131mgP65 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂9.7mgP54 national
- Metaarsenious acidHAsO₂0.40mgP81 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂8.8mgP38 national
Measured at the source
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.