What is an Acidic Spring?
An acidic spring contains hydrogen ions (H⁺) at 1 mg per kilogram of spring water, creating strongly acidic water at roughly pH 2–3 — similar to lemon juice or vinegar. Most are found near volcanic areas; Kusatsu Onsen (Gunma) and Tamagawa Onsen (Akita) are the most famous examples.
The water has very strong antibacterial power; bathing may cause a tingling sensation on the skin, and the water tastes sour. They have traditionally been used as balneotherapy destinations for skin conditions and athlete's foot.
Indications for Bathing
- Atopic dermatitisThe spring's strong bactericidal effect, with a pH of 2–3, may help suppress pathogens like Staphylococcus aureus that can worsen symptoms. It also has a keratolytic effect that promotes skin turnover. Bathing should be avoided during acute flare-ups and started with short durations during remission, always after consulting a dermatologist.
- Plaque psoriasisAcidic water's powerful disinfectant properties are thought to help by strongly suppressing the growth of bacteria on the skin's surface. This action may support the improvement of the chronic inflammation associated with this condition.
- Epidermoid suppurationThe highly acidic nature of the water provides a strong bactericidal effect. This may help manage conditions involving skin surface bacteria by suppressing their growth and supporting the improvement of chronic inflammation.
- Impaired glucose tolerance (diabetes)This is one of the four official indications for bathing in acidic springs, as designated by Japan's Mineral Spring Analysis Method Guidelines. The strong acidity is believed to provide various therapeutic benefits.
How to Bathe in an Acidic Spring
- First-timers: start with 3–5 minutes and monitor your skin before extending the soak.Acidic springs are highly stimulating and can cause a stinging sensation due to their low pH. Their strong sterilizing action, while beneficial, can be intense for first-timers or those with sensitive skin. Starting with a brief soak allows your skin to acclimate and helps you monitor for excessive irritation.
- After bathing, rinse lightly with fresh water — residual acid can be overly irritating.While the spring's acidity offers potent sterilizing effects during your bath, leaving it on your skin can lead to irritation or dryness. A light rinse with fresh water helps neutralize any residual acid, protecting your skin from over-stimulation and ensuring a comfortable post-bath experience.
- Shampoo and face wash in a separate bathroom — they react with the acidic water.The onsen's water is strongly acidic, while soaps and shampoos are typically alkaline. Mixing them causes a chemical reaction that neutralizes the soap, preventing it from lathering and reducing its cleansing ability. For effective washing, always use the separate, fresh-water showers provided.
Related Onsen Facilities
References
Famous Onsen Areas with Acidic Spring
Check access and tourist information on each onsen area column
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