The minimum source temperature to qualify as an Onsen (温泉), which literally means hot spring in Japanese, is 25°C.

The typical bathing temperature is around 41°C. Some places with a lower source temperature may artificially heat up the water (ka-on 加温), while others with a higher source temperature may mix cold water (ka-sui 加水) to achieve a comfortable bathing temperature.

Many of us would have learned in our chemistry lessons that soaps are made of alkaline substances. So it should be of no surprise that alkaline baths feel slippery. They are also often described as having a soft texture.

An alkaline bath is effective in removing sebum and keratin from our skin, leaving it soft and smooth; and hence, it is often called a "beauty's bath" (bijin-no-yu 美人の湯). One should, however, be cautious of excessive soaking in strong alkaline baths, which may removes too much oil and moisture from our skin, resulting in rough skin instead.

Alkaline baths are thought to mitigate illnesses brought about by acidic blood, which includes kidney stones, rheumatism, etc.

Acidic baths are often describe as feeling firmer and harder. They are bactericidal and are beneficial to people with skin conditions. Acidic baths are effective in removing dead skin and stimulating skin growth, and are thus, good for chronic skin conditions such as eczema, athlete's foot, atopic dermatitis, etc.

As acidic baths may be over-stimulating for people with a weak body constitution, thorough rinsing of the body should be considered. It is not advisable to drink water from acidic onsen to prevent damage to the gastric.

The osmotic pressure is measured by the amount of dissolved substances per kg of water. The higher the osmotic pressure, the easier it is for our body to absorb the spring water's minerals.

Since a high osmotic pressure corresponds to a high level dissolved substance, baths of such nature are usually cloudy or opaque, and are called Nigori-yu (濁り湯).