Sayamago Onsen is an alkaline simple thermal spring at pH 8.8, drawn from a 34.4 °C well. The water shows a faint yellow tint, a touch of salt on the lips, and a light hydrogen sulfide note at the source. Mineralization is light (0.419 g/kg), led by sodium and bicarbonate, and the alkalinity gives the water a smooth, slippery feel — it sits squarely in the Japanese 'beauty-water' family.
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 2021.
- Active stage of any illness (especially with fever)
- Active tuberculosis, advanced malignant tumor, severe anemia or other marked physical weakness
- Severe heart or lung disease causing shortness of breath 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⁺116mgP33 national92%
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺7.0mgP21 national6%
- MagnesiumMg²⁺0.50mgP20 national0.5%
- PotassiumK⁺0.50mgP5 national0.2%
- AmmoniumNH₄⁺0.10mgP23 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻202mgP58 national62%
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻41.2mgP24 national22%
- Carbonatestrongly alkaline; slippery feelCO₃²⁻24.0mgP72 national15%
- Hydrogen sulfideHS⁻0.10mgP25 national0.2%
- SulfateSO₄²⁻0.30mgP4 national0.2%
- FluorideF⁻0.20mgP16 national0.2%
- Hydrogen phosphateHPO₄²⁻0.30mgP58 national0.1%
- Bromidetrace, sea-originBr⁻0.10mgP14 national
- NitrateNO₃⁻0.10mgP26 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃25.0mgP9 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂1.2mgP13 national
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂< —mg
- Free hydrogen sulfide"egg" smell — opens vessels, antibacterialH₂S< —mg
- Mercurytoxic trace — part of the drinking-safety checkHg0.0001mgP24 national
- CadmiumCd< —mg
- ChromiumCr< —mg
- CopperCu< —mg
- LeadPb< —mg
Measured at the source
On the certificate
From a certificate analyzed in 2021.
- Chronic muscle or joint pain or stiffness
- Rheumatoid arthritis, osteoarthritis, lower back pain, neuralgia, frozen shoulder, bruises, sprains and the like (chronic phase)
- Muscle stiffness associated with motor paralysis
- Cold sensitivity, peripheral circulation disorders
- Reduced gastrointestinal function, mild hypertension
- Impaired glucose tolerance, mild hypercholesterolemia
- Mild asthma or emphysema
- Hemorrhoid pain
- Autonomic nervous system instability, stress-related symptoms
- Convalescence, fatigue recovery, general health improvement
- Autonomic nervous system instability, insomnia, 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.
- 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.