Hot spring pH is one number that shows whether the water is acidic or alkaline. It helps you tell, in advance, whether the bath may feel sharp and tingly or smooth and slippery on your skin.

In short, pH is only one axis: acidic to alkaline. With pH 7 as neutral, lower numbers are more acidic and higher numbers are more alkaline. The closer it gets to either end, the more distinctive the skin feel becomes. This article focuses on that single axis and neutrally explains the Ministry of the Environment’s liquid classification, the sensations of acidic and alkaline springs, and cautions for very strong acidic springs.

Note that pH does not determine a spring’s water type. The 10-category classification based on ingredients, such as sulfur springs and chloride springs, is a separate matter. For the full picture, see Guide to Hot Spring Water Types; here, we will dig deeper into the pH axis alone.

This article is for general information. If you have sensitive skin, wounds, or a medical condition, do not overdo it, and start with a short soak, especially in highly stimulating water.

What is pH?

pH is a numerical scale from 0 to 14 that indicates how acidic or alkaline an aqueous solution is. pH 7 in the middle is neutral; the smaller the number, the stronger the acidity, and the larger the number, the stronger the alkalinity. Because pH is a logarithmic scale, a difference of 1 means the hydrogen-ion concentration changes by about 10 times. For example, pH 2 and pH 3 differ greatly in acidity.

In hot springs, pH gives you a rough idea of the water’s character. Strongly acidic water is more likely to feel sharp and tingly, while highly alkaline water is often experienced as smooth or slippery. Water near neutral tends to feel milder, with less sting and less slipperiness.

Quick guide to hot spring pH categories

The Ministry of the Environment’s “Guidelines for Mineral Spring Analysis” generally classifies spring pH, or liquid property, into the following five levels. Use this as a quick reference for the tendency toward sting or slipperiness.

The numbers are based on the Ministry of the Environment’s “Guidelines for Mineral Spring Analysis (Revised in 2014).” pH is measured at the source spring, and the value in the bath may change due to dilution or the passage of time. Also, pH differs from one source to another even within the same hot spring area, so the examples above are only representative.

This classification indicates liquid property, and it is separate from the definition of an acidic spring as a therapeutic spring type. Springs called acidic are defined by hydrogen-ion content, not by pH alone. For classifications by water type, see Water Type Guide.

Acidic springs: sharp stimulation and their so-called disinfecting qualities

When you enter acidic water, your skin may feel slightly tightened, and some people notice a sharp sting. In hot spring areas known for strong acidity, that distinctive bathing sensation itself becomes part of the appeal.

Typical examples are Tamagawa Onsen in Akita and Kusatsu Onsen in Gunma. Tamagawa Onsen is said to have a pH of around 1.1, making it one of Japan’s strongest acidic springs, while Kusatsu Onsen is also known for its strong acidity at around pH 2. These waters have long been said to help with skin troubles and are sometimes described as having disinfecting qualities, but such effects vary depending on conditions. It is best to take such claims as general impressions, not as proven benefits.

Because strongly acidic water is so stimulating, it can be hard on people with sensitive skin or on days when they do not feel well. The basic rule is to avoid long soaks and start with a short test bath. If you have a wound, it may sting, and it will also sting if it gets into your eyes, so avoid putting your face into the water. In some hot spring areas, such as Kusatsu Onsen, there are traditional bathing methods that help the body adjust to the water before bathing. If your skin feels tight after soaking, rinsing lightly with plain water can help reduce the irritation.

Acidic springs often overlap with sulfur-rich water. The relationship with sulfur springs, which can have a smell and milky color, is covered in The Smell and Color of Sulfur Springs. For waters discussed in the context of radioactive springs, such as Tamagawa Onsen, see Radioactive Springs and Radon.

Alkaline springs: why they feel slippery and are called beauty waters

Alkaline water often feels smooth and silky, and people frequently describe it as gentle on the skin. Some hot springs promoted as “beauty baths” or “skin-beautifying waters” also have this alkaline tendency.

This slippery feel can be explained neutrally as a skin-side phenomenon. When alkaline water touches the skin, sebum and old dead skin cells on the surface soften and partly react, creating a smooth sensation. It is similar to how skin feels after washing with soap. In other words, the water itself is not necessarily slippery; the feeling comes from changes in the surface of the skin.

However, it is not correct to say that alkaline water always improves the skin. This is only an impression of texture, not a guarantee of cosmetic benefits. In fact, very alkaline water can remove too much oil, leaving some people feeling dry or tight after bathing. After enjoying the smooth feel, it is a good idea to follow up with moisturizing care. Even when the water feels gentle, long soaks can still cause overheating, so also consider temperature and bathing time together; Hot Spring Temperature Categories may help.

Do not choose a hot spring by pH alone

pH is a useful clue to a spring’s character, but it does not decide whether a hot spring is good or bad. If you want something stimulating, a more acidic spring may suit you; if you prefer a soft skin feel, weakly alkaline to alkaline water may be a rough guideline. In practice, though, bathing comfort changes greatly depending on ingredients, water temperature, whether the water is diluted or heated, and even how you feel that day.

This is especially true while traveling. Fatigue from moving around or lack of sleep can make even normally comfortable water feel more intense. The more extreme the pH, the more important it is not to overdo it and to start with a short soak. If you want to compare by spring type or region, narrow your search in the Facility List. Before trying strongly stimulating water, check Precautions Before Bathing in Hot Springs as well.

Frequently asked questions

Where can I check the pH of a hot spring?

It is listed as “pH (liquid property)” on the hot spring analysis sheet posted in the changing room or bathing area. The value is measured at the source spring, so it may change in the bath due to dilution or the passage of time.

Which is better for the skin, acidic or alkaline?

There is no simple answer. Acidic water tends to feel more stimulating, while alkaline water tends to feel smoother and more slippery. Which one suits you depends on your skin type and physical condition. It is more realistic to think of both as characteristics rather than as simply good or bad.

Do “beauty waters” really make your skin look better?

“Beauty waters” and “beauty baths” are not legal water-type names; they are customary labels. Alkaline water may make the skin feel smoother, but that is only a texture impression caused by a reaction with sebum and dead skin cells, not a guarantee of cosmetic benefits.

What should I be careful about in strongly acidic hot springs?

Avoid long soaks and start with a short test bath. Wounds may sting, so pay attention to the condition of your skin. It will sting if it gets into your eyes, so do not put your face in the water. If you have sensitive skin or feel unwell, it is important not to push yourself.

Is a higher or lower pH always better?

No. The strength of the pH value has nothing to do with the quality or value of the hot spring. Water near either end simply has a stronger character. The real standard is whether it feels comfortable for you.

Summary

Hot spring pH is a clue for reading skin feel along one axis: acidic to alkaline. Under the Ministry of the Environment’s classification, pH under 3 is acidic, 3 to 6 is weakly acidic, 6 to 7.5 is neutral, 7.5 to 8.5 is weakly alkaline, and 8.5 or higher is alkaline. On the acidic side, water tends to feel sharp and stimulating, while on the alkaline side it often feels slippery and silky.

That said, statements like “acidic is disinfecting” or “alkaline is good for beauty” are only common generalizations and cannot be treated as proven effects. Strong acidic springs such as Tamagawa and Kusatsu are highly stimulating and require caution about long soaks, wounds, and the eyes. If you treat pH as just one guideline and read it together with the water type, temperature, and your condition on the day, the differences between hot springs become much easier to understand.

Sources

• Ministry of the Environment, “Guidelines for Mineral Spring Analysis (Revised in 2014)”
• Ministry of the Environment, “Protection and Use of Hot Springs — Definition of Hot Springs”
• Japan Spa Association, “How to Read a Hot Spring Analysis Sheet”