熱海353号泉(混源)Atami Spring No. 353 (Blended)
Atami Spring No. 353 is a blended sodium-calcium chloride source emerging at 55.2 °C as a hot, hypertonic water. With 19.74 g/kg of dissolved solids it sits squarely in the richly mineralized band — body and heat-retention are noticeable, and a faint mineral film tends to linger on the skin afterwards. Weakly alkaline, so the feel on the skin stays soft despite the strong mineralization.
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
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.
- Sodiumsalt's cation — pairs with chloride to make the water saltyNa⁺3,680mgP96 national
- Calciumsettling and calming; can leave a white mineral bloomCa²⁺3,208mgP100 national
- Magnesiummild; a laxative if you drink itMg²⁺194mgP96 national
- Potassiumminor; matters for drinking limitsK⁺73.2mgP90 national
- Chloridetable-salt anion — coats the skin and holds heat inCl⁻11,030mgP98 national
- Sulfate"gypsum / Glauber's salt" — vessels and wound healingSO₄²⁻1,419mgP99 national
- Bicarbonate"soda" — softens skin, leaves it smoothHCO₃⁻36.1mgP22 national
- Bromidetrace, sea-originBr⁻11.5mgP90 national
- Metasilicic acidmetasilicic acid — a natural moisturizer for silky skinH₂SiO₃51.9mgP30 national
- Metaboric acidmetaboric acid — mildly antibacterialHBO₂11.7mgP60 national
- Metaarsenious acidHAsO₂< —mg
- Free carbon dioxidefizz — widens blood vessels (strong above 1000 mg/kg)CO₂17.8mgP49 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.