An iodine-bearing sodium chloride strong-salt spring drawn from a drilled, pumped well in Tokyo's Ariake district. Dissolved solids reach 29.8 g/kg — approaching ocean salinity — and at the wellhead the water is pale brown and clear, strongly salty on the lips, with a faint mineral-oil note. Weakly alkaline and richly mineralized, it leaves the salt film that keeps bathers warm long after soaking; the certificate's annex documents it for cuts, peripheral circulation disorders, cold sensitivity, and dry skin. Metasilicic acid is generous too, leaving a thin moisturizing silica film. Per the Article 18 posting, the bath uses source water only — no dilution — heated to bathing temperature, recirculated, and chlorine-disinfected.
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, netto 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
Recovery
Warmth and buoyancy lower stress markers and improve sleep; habitual bathing is linked to lower rates of depression.
fatigue, stress
Take care
- Active stage of any illness (especially with fever)
- Active tuberculosis
- Advanced malignant tumor or severe anemia with marked physical weakness
- Severe heart or lung disease causing breathlessness with slight exertion
- Severe kidney disease with edema
- Gastrointestinal bleeding
- Visible bleeding
- Acute exacerbation of a chronic illness
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⁺10,310mgP99 national90%
- Magnesiummild; a laxative if you drink itMg²⁺218mgP97 national4%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺303mgP89 national3%
- AmmoniumNH₄⁺149mgP100 national2%
- Potassiumminor; matters for drinking limitsK⁺319mgP99 national2%
- Iron (II)Fe²⁺11.3mgP89 national0.1%
- ZincZn²⁺0.60mgP93 national0.0%
- ManganeseMn²⁺0.30mgP53 national0.0%
- Aluminumshows up in acidic volcanic waterAl³⁺< —mg
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻17,530mgP99 national98%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻633mgP81 national2%
- BromideBr⁻92.8mgP100 national0.2%
- IodideI⁻43.1mgP100 national0.1%
- SulfateSO₄²⁻17.4mgP29 national0.1%
- CarbonateCO₃²⁻0.90mgP17 national0.0%
- Fluoridetrace; drinking limits applyF⁻< —mg
- Hydrogen sulfidesulfur character — antibacterialHS⁻< —mg
- Thiosulfateoxidized sulfurS₂O₃²⁻< —mg
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃152mgP69 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂17.1mgP71 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂38.3mgP64 national
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- ArsenicAs< —mg
- CadmiumCd< —mg
- CopperCu< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg< —mg
- LeadPb< —mg
Measured at the source
On the certificate
- Chronic muscle or joint pain and stiffness (rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains — chronic phase)
- Muscle stiffness associated with motor paralysis
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Stress-related symptoms (sleep disorders)
- Cuts
- Peripheral circulation 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.