Learn how acidic hot springs are defined under Japan’s Ministry of the Environment standards, plus famous baths like Kusatsu, Tamagawa, and Zao, and key cautions for skin, eyes, and wounds.
Published: Dec 24, 2025
Learn how acidic hot springs are defined under Japan’s Ministry of the Environment standards, plus famous baths like Kusatsu, Tamagawa, and Zao, and key cautions for skin, eyes, and wounds.
Published: Dec 24, 2025
Acidic springs are one of the 10 types of therapeutic hot springs, characterized by spring water that contains hydrogen ions above a certain level. They are common in volcanic areas and are known in Japan as highly distinctive baths that often feel sharp when you enter.
In short, whether a spring is classified as acidic is determined not by how “sour” or “tingly” it feels, but by the amount of dissolved components in the water. Under the Ministry of the Environment’s standards, a hot spring that contains hydrogen ions above a certain level is defined as an acidic spring. This article neutrally explains that definition, representative famous baths such as Kusatsu, Tamagawa, and Zao, and the cautions you should keep in mind for skin, eyes, and wounds when soaking in strongly acidic water. For the sensory aspect of acidic versus alkaline water, that is, pH, see The Guide to Onsen pH Values, and for the overall picture of the 10 spring types, see The Guide to Hot Spring Types. Here, we focus only on acidic springs as a spring category.
This article provides general information and does not guarantee any specific therapeutic or health benefits. If you have sensitive skin, wounds, or a medical condition, do not push yourself. Try especially strong waters only for short periods at first, and follow the facility’s guidance.
Whether a bath is classified as an acidic spring depends on the amount of hydrogen ions dissolved in the water. Under the Ministry of the Environment’s classification based on the "Guidelines for Mineral Spring Analysis Methods," an acidic spring with a therapeutic hot spring designation is defined as a hot spring that contains at least 1 mg of hydrogen ions per 1 kg of hot spring water. If a bathhouse displays "acidic spring" or "acidic - XX spring," it means the water meets this standard.
What is important here is that the spring category "acidic spring" and the pH display are based on different standards. pH is a number that indicates whether the water is acidic or alkaline, and water below about pH 3 is generally considered acidic. Since water containing at least 1 mg/kg of hydrogen ions is roughly below pH 3, acidic springs often overlap with low-pH waters. However, as a spring category, acidic springs are judged by component quantity, not by the pH number itself. For the one-dimensional sensory contrast of pH, namely the sharp sensation of acidic water versus the slippery feel of alkaline water, see The Guide to Onsen pH Values.
Also, acidic springs do not always appear alone. They are often combined with other components. Some are classified as "acidic and sulfur-containing springs," while others contain iron, aluminum, chloride, or sulfate. The exact label shown can vary by region. Even within the same acidic spring, color, smell, and bathing feel can differ depending on the components combined with it.
Acidic springs are found in many volcanic regions. Some of the best-known examples, each with its own character, are listed below. Even within the same "acidic spring" category, the acidity and combined components can differ greatly.
| Hot Spring Area | Location | Approx. pH | Features |
|---|---|---|---|
| Kusatsu Onsen | Gunma Prefecture | Yubatake approx. 2.1 / Bandaiko approx. 1.7 | One of Japan’s Three Famous Hot Springs. Many source waters are acidic and also sulfur-containing, and acidity varies by source, such as Yubatake |
| Tamagawa Onsen | Akita Prefecture | Approx. 1.1–1.2 | One of Japan’s strongest acidic springs, with hydrochloric acid as the main component. The source spring "Ohbuki" is known for the largest flow rate from a single spring outlet in Japan |
| Zao Onsen | Yamagata Prefecture | Approx. 1.9 | A strongly acidic, sulfur-containing bath. Also known for its cloudy water |
The pH values and combined components above are only examples, and the numbers differ from one source to another even within the same hot spring area. For example, in Kusatsu there is a difference in acidity between Bandaiko and Yubatake, and in Tamagawa the recorded pH can vary depending on when it is measured. The most reliable way to confirm what type of water a bath has is to check the posted spring classification or the official hot spring analysis report. Beyond these examples, there are also famous acidic springs in other regions, such as Sugawa Onsen in Iwate Prefecture.
Acidic springs, especially strongly acidic waters with a pH in the 1 to 2 range, can put more strain on the body because they are so stimulating. The basic rule is to avoid long soaks and start by observing your reaction over a short time. Even people who normally have no problem may feel the stimulation more strongly than usual if they are tired or sleep-deprived while traveling.
The three main points to watch are skin, eyes, and wounds. If you have cuts or irritated skin, acidic water can sting. Because it will also sting if it gets into your eyes, avoid putting your face into the bath or washing your face with the water. If your skin feels tight or dry after bathing, rinsing lightly with plain hot water can help reduce the irritation. People with sensitive or dry skin should keep their bathing time especially short, and if the water does not suit them, they should not force themselves.
Some hot spring areas, such as Kusatsu Onsen, have traditional bathing methods that help the body adjust to the water before entering, such as timed bathing. Some facilities provide instructions on how to bathe and how to rinse afterward, so it is safest to follow them. Acidic springs are sometimes described as having a sterilizing effect or as being good for skin troubles, but such effects vary depending on the conditions and should only be taken as a general impression. This article does not claim any specific benefits. If you have a medical condition or want to understand therapeutic effects more broadly, also see Things to Know Before Entering a Hot Spring.
Acidic springs often contain sulfur as well. As seen in Kusatsu and Zao, which are known as "acidic and sulfur-containing springs," regions connected to volcanic activity often show high acidity and sulfur content at the same time. In such cases, the water may smell like rotten eggs or appear milky white.
The smell, cloudiness, and bath sediments are characteristics of sulfur springs, and the mechanisms behind them, including tarnishing of silver accessories and precautions in poorly ventilated areas, are covered in The Smell and Color of Sulfur Springs. Acidic springs and sulfur springs are classified by different standards, but in actual baths they often overlap. There are also strongly acidic springs discussed in the context of radioactive springs, such as Tamagawa Onsen in Akita, and The Standards and Safety of Radioactive Springs (Radon Springs) may be helpful here. If you want to find a facility with acidic spring water, you can search from the facility list.
In acidic springs, metal accessories and watches can be damaged. This is because acid acts on the surface of metals, so rings, necklaces, earrings, and similar items are best removed before bathing. In waters with sulfur content, silver products may also turn black.
Strongly acidic water can also affect bath tubs, piping, and the area around the spout. Users do not need to take special measures, but the fact that acidic springs are powerful enough to affect facilities is a clue to their strong stimulation.
It is one of the 10 types of therapeutic hot springs and refers to a hot spring that contains at least 1 mg of hydrogen ions per 1 kg of hot spring water. They are common in volcanic regions and are characterized by a sharp, tingling sensation. If the posted information or hot spring analysis report says "acidic spring" or "acidic - XX spring," the water meets this standard.
No, the standards are different. An acidic spring is a spring category judged by components, specifically the amount of hydrogen ions, while pH is a number that shows whether the water is acidic or alkaline. Because water containing at least 1 mg/kg of hydrogen ions is roughly below pH 3, the two often overlap, but they are separate criteria. The sensory side of pH is covered in the pH Values Guide.
Avoid long soaking and try it for only a short time at first. If you have wounds, it may sting, so be careful about your skin condition, and do not put your face into the water because it will sting if it gets into your eyes. People with sensitive skin or those feeling unwell should especially avoid overdoing it, and if your skin feels tight after bathing, a light rinse with plain hot water can help.
Acidic springs are sometimes described as having a sterilizing effect or as being good for skin problems, but their effects vary depending on the conditions and cannot be stated definitively. The indications for therapeutic hot springs are based on repeated use over a certain period, and individual differences are large, so it is best not to expect too much.
In areas related to volcanic activity, high acidity and sulfur content often appear at the same time. In waters known as "acidic and sulfur-containing springs," such as those in Kusatsu and Zao, smell and cloudiness may be present. For more on how the smell and color are produced, see The Smell and Color of Sulfur Springs.
Acidic springs are therapeutic hot springs that contain at least 1 mg of hydrogen ions per 1 kg of hot spring water, and they are highly distinctive waters often found in volcanic regions. Whether a spring is acidic is determined by component quantity, not by the tingling sensation, and the standard is separate from pH. Across Japan, famous baths such as Kusatsu, Tamagawa, and Zao differ in acidity and in the components they contain, and many also include sulfur.
As a traveler, you can enjoy strongly acidic waters safely by avoiding long soaks, paying attention to skin, eyes, and wounds, and removing metal products. Without making definitive claims about benefits, and without being overly cautious, the appeal of acidic springs lies in their clear individuality, unlike any other spring type. For the overall picture of each spring type, see The Guide to Hot Spring Types, and for how acidic to alkaline waters feel on the skin, see The Guide to Onsen pH Values.
Acidic springs are one of the 10 types of therapeutic hot springs, characterized by spring water that contains hydrogen ions above a certain level. They are common in volcanic areas and are known in Japan as highly distinctive baths that often feel sharp when you enter.
In short, whether a spring is classified as acidic is determined not by how “sour” or “tingly” it feels, but by the amount of dissolved components in the water. Under the Ministry of the Environment’s standards, a hot spring that contains hydrogen ions above a certain level is defined as an acidic spring. This article neutrally explains that definition, representative famous baths such as Kusatsu, Tamagawa, and Zao, and the cautions you should keep in mind for skin, eyes, and wounds when soaking in strongly acidic water. For the sensory aspect of acidic versus alkaline water, that is, pH, see The Guide to Onsen pH Values, and for the overall picture of the 10 spring types, see The Guide to Hot Spring Types. Here, we focus only on acidic springs as a spring category.
This article provides general information and does not guarantee any specific therapeutic or health benefits. If you have sensitive skin, wounds, or a medical condition, do not push yourself. Try especially strong waters only for short periods at first, and follow the facility’s guidance.
Whether a bath is classified as an acidic spring depends on the amount of hydrogen ions dissolved in the water. Under the Ministry of the Environment’s classification based on the "Guidelines for Mineral Spring Analysis Methods," an acidic spring with a therapeutic hot spring designation is defined as a hot spring that contains at least 1 mg of hydrogen ions per 1 kg of hot spring water. If a bathhouse displays "acidic spring" or "acidic - XX spring," it means the water meets this standard.
What is important here is that the spring category "acidic spring" and the pH display are based on different standards. pH is a number that indicates whether the water is acidic or alkaline, and water below about pH 3 is generally considered acidic. Since water containing at least 1 mg/kg of hydrogen ions is roughly below pH 3, acidic springs often overlap with low-pH waters. However, as a spring category, acidic springs are judged by component quantity, not by the pH number itself. For the one-dimensional sensory contrast of pH, namely the sharp sensation of acidic water versus the slippery feel of alkaline water, see The Guide to Onsen pH Values.
Also, acidic springs do not always appear alone. They are often combined with other components. Some are classified as "acidic and sulfur-containing springs," while others contain iron, aluminum, chloride, or sulfate. The exact label shown can vary by region. Even within the same acidic spring, color, smell, and bathing feel can differ depending on the components combined with it.
Acidic springs are found in many volcanic regions. Some of the best-known examples, each with its own character, are listed below. Even within the same "acidic spring" category, the acidity and combined components can differ greatly.
| Hot Spring Area | Location | Approx. pH | Features |
|---|---|---|---|
| Kusatsu Onsen | Gunma Prefecture | Yubatake approx. 2.1 / Bandaiko approx. 1.7 | One of Japan’s Three Famous Hot Springs. Many source waters are acidic and also sulfur-containing, and acidity varies by source, such as Yubatake |
| Tamagawa Onsen | Akita Prefecture | Approx. 1.1–1.2 | One of Japan’s strongest acidic springs, with hydrochloric acid as the main component. The source spring "Ohbuki" is known for the largest flow rate from a single spring outlet in Japan |
| Zao Onsen | Yamagata Prefecture | Approx. 1.9 | A strongly acidic, sulfur-containing bath. Also known for its cloudy water |
The pH values and combined components above are only examples, and the numbers differ from one source to another even within the same hot spring area. For example, in Kusatsu there is a difference in acidity between Bandaiko and Yubatake, and in Tamagawa the recorded pH can vary depending on when it is measured. The most reliable way to confirm what type of water a bath has is to check the posted spring classification or the official hot spring analysis report. Beyond these examples, there are also famous acidic springs in other regions, such as Sugawa Onsen in Iwate Prefecture.
Acidic springs, especially strongly acidic waters with a pH in the 1 to 2 range, can put more strain on the body because they are so stimulating. The basic rule is to avoid long soaks and start by observing your reaction over a short time. Even people who normally have no problem may feel the stimulation more strongly than usual if they are tired or sleep-deprived while traveling.
The three main points to watch are skin, eyes, and wounds. If you have cuts or irritated skin, acidic water can sting. Because it will also sting if it gets into your eyes, avoid putting your face into the bath or washing your face with the water. If your skin feels tight or dry after bathing, rinsing lightly with plain hot water can help reduce the irritation. People with sensitive or dry skin should keep their bathing time especially short, and if the water does not suit them, they should not force themselves.
Some hot spring areas, such as Kusatsu Onsen, have traditional bathing methods that help the body adjust to the water before entering, such as timed bathing. Some facilities provide instructions on how to bathe and how to rinse afterward, so it is safest to follow them. Acidic springs are sometimes described as having a sterilizing effect or as being good for skin troubles, but such effects vary depending on the conditions and should only be taken as a general impression. This article does not claim any specific benefits. If you have a medical condition or want to understand therapeutic effects more broadly, also see Things to Know Before Entering a Hot Spring.
Acidic springs often contain sulfur as well. As seen in Kusatsu and Zao, which are known as "acidic and sulfur-containing springs," regions connected to volcanic activity often show high acidity and sulfur content at the same time. In such cases, the water may smell like rotten eggs or appear milky white.
The smell, cloudiness, and bath sediments are characteristics of sulfur springs, and the mechanisms behind them, including tarnishing of silver accessories and precautions in poorly ventilated areas, are covered in The Smell and Color of Sulfur Springs. Acidic springs and sulfur springs are classified by different standards, but in actual baths they often overlap. There are also strongly acidic springs discussed in the context of radioactive springs, such as Tamagawa Onsen in Akita, and The Standards and Safety of Radioactive Springs (Radon Springs) may be helpful here. If you want to find a facility with acidic spring water, you can search from the facility list.
In acidic springs, metal accessories and watches can be damaged. This is because acid acts on the surface of metals, so rings, necklaces, earrings, and similar items are best removed before bathing. In waters with sulfur content, silver products may also turn black.
Strongly acidic water can also affect bath tubs, piping, and the area around the spout. Users do not need to take special measures, but the fact that acidic springs are powerful enough to affect facilities is a clue to their strong stimulation.
It is one of the 10 types of therapeutic hot springs and refers to a hot spring that contains at least 1 mg of hydrogen ions per 1 kg of hot spring water. They are common in volcanic regions and are characterized by a sharp, tingling sensation. If the posted information or hot spring analysis report says "acidic spring" or "acidic - XX spring," the water meets this standard.
No, the standards are different. An acidic spring is a spring category judged by components, specifically the amount of hydrogen ions, while pH is a number that shows whether the water is acidic or alkaline. Because water containing at least 1 mg/kg of hydrogen ions is roughly below pH 3, the two often overlap, but they are separate criteria. The sensory side of pH is covered in the pH Values Guide.
Avoid long soaking and try it for only a short time at first. If you have wounds, it may sting, so be careful about your skin condition, and do not put your face into the water because it will sting if it gets into your eyes. People with sensitive skin or those feeling unwell should especially avoid overdoing it, and if your skin feels tight after bathing, a light rinse with plain hot water can help.
Acidic springs are sometimes described as having a sterilizing effect or as being good for skin problems, but their effects vary depending on the conditions and cannot be stated definitively. The indications for therapeutic hot springs are based on repeated use over a certain period, and individual differences are large, so it is best not to expect too much.
In areas related to volcanic activity, high acidity and sulfur content often appear at the same time. In waters known as "acidic and sulfur-containing springs," such as those in Kusatsu and Zao, smell and cloudiness may be present. For more on how the smell and color are produced, see The Smell and Color of Sulfur Springs.
Acidic springs are therapeutic hot springs that contain at least 1 mg of hydrogen ions per 1 kg of hot spring water, and they are highly distinctive waters often found in volcanic regions. Whether a spring is acidic is determined by component quantity, not by the tingling sensation, and the standard is separate from pH. Across Japan, famous baths such as Kusatsu, Tamagawa, and Zao differ in acidity and in the components they contain, and many also include sulfur.
As a traveler, you can enjoy strongly acidic waters safely by avoiding long soaks, paying attention to skin, eyes, and wounds, and removing metal products. Without making definitive claims about benefits, and without being overly cautious, the appeal of acidic springs lies in their clear individuality, unlike any other spring type. For the overall picture of each spring type, see The Guide to Hot Spring Types, and for how acidic to alkaline waters feel on the skin, see The Guide to Onsen pH Values.