A blended source: five wells — Nos. 2 and 4 plus the Yamada, Oshita and Tohoku springs — are combined at Asama Onsen's central distribution tank, which is where the sample for this certificate was drawn. The water is an alkaline simple thermal spring at pH 9.0, soft on the skin with the slippery feel that places it in Japan's beauty-water family. Mineralization is light at 440 mg/kg, but 54 mg/kg of metasilicic acid leaves the kind of thin silica film that makes skin feel noticeably smoother afterwards. At the wellhead the water carries a faintly sulfurous taste and a light rotten-egg note of H₂S; the certificate's appendix lists it as documented for autonomic nervous system instability, depressed mood and insomnia.
Good for
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
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, severe anemia, or other states of marked physical debilitation
- Severe heart or lung disease causing breathlessness on 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⁺90.5mgP26 national71%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺31.5mgP47 national28%
- PotassiumK⁺1.6mgP19 national0.7%
- AmmoniumNH₄⁺0.10mgP23 national0.1%
- LithiumLi⁺0.04mgP15 national0.1%
- StrontiumSr²⁺0.20mgP42 national0.1%
- MagnesiumMg²⁺0.03mgP4 national0.0%
- AluminumAl³⁺0.01mgP18 national0.0%
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻200mgP65 national73%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻33.1mgP22 national16%
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻22.0mgP14 national6%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻4.8mgP31 national3%
- Fluoridetrace; drinking limits applyF⁻1.3mgP52 national1%
- HydroxideOH⁻0.20mgP67 national0.2%
- Hydrogen sulfideHS⁻0.20mgP36 national0.1%
- BromideBr⁻0.10mgP14 national0.0%
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃54.0mgP32 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂0.80mgP11 national
- ArsenicAsNDmg
- CadmiumCdNDmg
- ChromiumCrNDmg
- Mercurytoxic trace — part of the drinking-safety checkHgNDmg
- LeadPbNDmg
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)
- Cold sensitivity
- Peripheral circulation disorders
- Reduced gastrointestinal function (heaviness, bloating)
- Mild hypertension
- Impaired glucose tolerance (diabetes)
- Mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Stress-related symptoms (sleep disorders, depressed mood)
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.
- 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.