What types of onsen have you tried?

1. Simple spring

Many famous hot springs are of this type.

Refers to an onsen in which none of the included components reached a defined amount even when the water is over 25°C. This type of onsen does not stimulate your body, and is widely used for various beneficial effects.
Effective for: Recovery from fatigue, nerve pain, insomnia, hardening of the arteries, high blood pressure, etc.

2. Simple carbon dioxide spring (simple carbonic acid spring)

Very warming even at low temperatures.

Your blood vessels expand when bathing in this type of onsen, which lowers blood pressure. There aren't many simple carbon dioxide springs in Japan.
Effective for: Paralysis, muscle/joint pain, contusions, high blood pressure, hardening of the arteries, cuts, sensitivity to cold, menopausal disorders, infertility, etc.

3. Carbonated spring (bicarbonate earth spring, sodium bicarbonate spring)

You'll be refreshed after bathing in this so-called "cold spring."

There are two types: bicarbonate earth springs, which have alkaline properties, and sodium bicarbonate springs.
Effective for: muscle/joint pain, contusions, cuts, chronic skin diseases, etc.

4. Chloride spring (salt spring)

Ideal for the elderly or people recovering from illness.

This type of spring contains salt and resembles the properties of seawater. After bathing, the salt on your skin prevents the evaporation of sweat, keeping you nice and warm.
Effective for: muscle/joint pain, contusions, sprains, sensitivity to cold, chronic women's diseases, infertility, etc.

5. Sulphate spring

Prevents hardening of the arteries and also calms and relaxes you.

There are three types: Calcium sulphate, sodium sulphate, and magnesium sulphate springs.
Effective for:
Calcium sulphate spring: Rheumatism, bruises, cuts, burns, etc.
Sodium sulphate spring: High blood pressure, hardening of the arteries, external wounds, etc.
Magnesium sulphate spring: Same as calcium sulphate spring and sodium sulphate spring

6. Ferruginous spring (iron spring)

Non-oxidized ferruginous springs that are close to the source are especially good!

There are two types of ferruginous springs: those that contain iron carbonate, and those that contain melanterite. When the spring water first gushes forth from the source it is colorless and transparent, but it turns a dark reddish-brown color when exposed to oxygen, and its effects also grow weaker.
Effective for: Anemia, rheumatism, menopausal disorders, hypoplastic uterus, chronic eczema, etc.

7. Sulphur spring (sulphur spring, hydrogen sulphide spring)

This type of spring is very stimulating, so watch out for overheating and overexposure.

This type of onsen is known for its characteristic odor of hydrogen sulphide, and causes the blood vessels to expand. However, poor ventilation can be dangerous and should be avoided.
Effective for: High blood pressure, hardening of the arteries, chronic skin diseases, joint pain, etc.

8. Acidic spring (alunite spring)

To prevent stress on your body from overexposure to hot water, make sure to pour fresh water on your body after bathing.

This type of spring is extremely stimulating, even to the extent that it can permeate your skin. People with sensitive skin should watch out for overexposure. Acidic springs are used to treat chronic skin diseases and other ailments.
Effective for: chronic skin diseases, chronic women's diseases, diabetes, athlete's foot, etc.

9. Radioactive spring

Inhale for the most beneficial effects.

Radioactive springs contain miniscule amounts of radioactivity, and are also referred to as radium springs or radon springs. Radiation is absorbed by the skin and respiratory organs. However, there's no need to worry since it leaves your body immediately after bathing.
Effective for: High blood pressure, hardening of the arteries, nerve pain, rheumatism, calming stress, gout, etc.