What Is pH in Hot Springs?
pH measures how acidic or alkaline water is on a scale of 0 to 14. Most hot springs fall between 6.0 and 9.5, though some volcanic springs can dip below 3.0 while certain travertine-depositing springs exceed 10.0. The pH of a hot spring is determined by the minerals dissolved in the water and the gases rising from deep geological formations.
How pH Affects the Soaking Experience
Slightly alkaline springs (pH 7.5–9.0) tend to feel silky on the skin and are often described as having a softening effect. Mildly acidic springs (pH 5.0–6.5) are traditionally associated with skin-tightening and exfoliating properties. Japanese onsen culture distinguishes between these types, prizing both for different therapeutic purposes.
The pH also determines which minerals stay dissolved in the water. Alkaline conditions favor silica and sodium bicarbonate, while acidic conditions tend to carry more iron, aluminum, and sulfate. This means pH indirectly shapes the full mineral profile of each spring.
Traditional & Emerging Research
Balneotherapy research suggests that alkaline bicarbonate springs may support skin barrier function and hydration. Acidic sulfate springs have a long history of use for chronic skin conditions in European and Japanese spa medicine. A 2018 review in the International Journal of Biometeorology noted positive outcomes for atopic dermatitis patients bathing in acidic sulfur springs.
Disclaimer: The information on this page is for educational purposes only and does not constitute medical advice. Always consult a healthcare provider before using hot springs for therapeutic purposes.