A certificate displayed in the changing room. Most people pass by without a glance, but I always stop. The "Onsen Analysis Report"—this is the resume that tells the true story of the hot spring.

Source name, spring temperature, pH level, total dissolved substances. At first glance, it seems like a jumble of complex numbers and technical terms. However, once you learn to decipher this report, the world of hot springs expands dramatically. Where does the hot spring come from? What components does it contain? How concentrated is it? All of this is clearly indicated by the numbers.

The first time I seriously read an onsen analysis report, my understanding of hot springs changed completely. What I had vaguely felt as "pleasant" became a scientifically backed natural blessing. In this article, I will explain how to read an onsen analysis report in a way that even beginners can understand.

What is an Onsen Analysis Report: A Legally Mandated Certificate

An onsen analysis report is an official document issued based on the Hot Springs Act. A registered agency designated by the prefectural governor scientifically analyzes the hot spring and issues the results as a certificate.

All hot spring facilities are required to display this report in a visible location. On the wall of the changing room, at the entrance to the bath, sometimes even inside the bath. Framed and carefully displayed.

This report is not just a formal document. It reveals whether the spring is truly a "hot spring," what components it contains, and how it is used. It is the only source of information that hot spring facilities cannot falsify.

The date of analysis is also important. The components of a hot spring can change over time. Ideally, the analysis report should be within 3 to 5 years. An outdated report may not accurately reflect the current state of the hot spring.

Deciphering the Spring Quality Name: The First Impression of a Hot Spring

The most prominent part of an onsen analysis report is the "spring quality name." "Sodium-Chloride Spring," "Calcium-Sulfate Spring," "Simple Hot Spring." These are names used to classify hot springs.

The spring quality name is expressed in the format "Cation-Anion-Type of Hot Spring." It may seem difficult, but understanding this allows you to grasp the character of the hot spring instantly.

A "Sodium-Chloride Spring" has components similar to seawater and tastes salty. It has a high heat retention effect and is known as "heat water." After bathing, your body stays warm for a long time.

A "Calcium-Sulfate Spring" is effective for healing wounds. Known as "healing water," it has been visited by people with injuries for therapeutic purposes.

A "Simple Hot Spring" does not have specific components reaching a defined amount but has a temperature of 25°C or higher. It is gentle on the skin with little stimulation, making it safe for everyone from babies to the elderly.

By just looking at the spring quality name, you can imagine the texture, scent, and effects of the hot spring. This is the beginning of the enjoyment of reading an onsen analysis report.

Total Dissolved Substances: A Number Indicating the "Concentration" of the Hot Spring

The "total dissolved substances" is what I pay the most attention to in an onsen analysis report. This number indicates how much mineral content is dissolved in 1 kg of hot spring water.

Less than 1,000 mg/kg is "hypotonic," 1,000 to 10,000 mg/kg is "isotonic," and over 10,000 mg/kg is "hypertonic," indicating very high concentration.

I once bathed in a hot spring with over 20,000 mg/kg of total dissolved substances. The moment I entered the water, I was amazed by its density. The sensation of your skin tightening and the hot spring components soaking into your body. Even after bathing, my skin remained moist, and the effect lasted for hours.

A larger number is not necessarily better. If the concentration is too high, it can be harsh on the skin. However, the total dissolved substances are an important indicator of the "power" of the hot spring.

pH Level: Acidic or Alkaline?

The pH level indicates whether the hot spring is acidic or alkaline. A pH of 7 is neutral, below 7 is acidic, and above 7 is alkaline.

A "strongly acidic spring" with a pH below 3 has a tingling sensation. It is effective for skin diseases but can be too strong for those with sensitive skin. After bathing, it is essential to rinse off with a shower.

An "alkaline spring" with a pH of 8.5 or higher is known as "beauty water." It removes old keratin from the skin, leaving it smooth. This spring quality is very popular among women.

By looking at the pH level, you can understand the intensity of the hot spring's stimulation. Those with sensitive skin should choose a neutral hot spring for peace of mind. Conversely, if you seek strong effects, choosing a hot spring with an extreme pH level is an option.

"Usage Status" Section: The Most Important Information

The most important section in an onsen analysis report for identifying authentic hot springs is the "usage status" section.

This section describes how the source is used: "free-flowing," "circulated," "mixed use." Whether water is added or heated, and whether disinfectants are used. Everything is written here.

"Free-flowing source, no added water, no heating, no disinfectants." If this is stated, you can be sure it is a high-quality hot spring. It is the ideal hot spring where you can enjoy the source as it is, without any additions.

However, many facilities require some form of treatment. "Water added due to high temperature," "heated in winter," "combined with circulation filtration." When such descriptions are present, it is necessary to understand how much they affect the quality of the hot spring.

Honest facilities disclose everything. They do not hide but reveal the facts. This honesty is the mark of a trustworthy hot spring facility.

The Enjoyment of Reading an Onsen Analysis Report

Once you can read an onsen analysis report, the way you enjoy hot spring trips changes.

Upon arriving at a facility, first check the onsen analysis report. Spring quality, components, usage status. Absorb this information before bathing. Then, you can clearly feel the unique characteristics of each hot spring.

"This smoothness is due to high metasilicic acid," "This saltiness is because it's a Sodium-Chloride Spring," "This sulfur scent is proof of total sulfur over 2 mg/kg." While soaking in the hot spring, compare the report's contents with the actual sensations. This intellectual enjoyment adds depth to the hot spring experience.

Onsen Analysis Reports: The Gateway to Hot Springs

At first, an onsen analysis report may seem complex. Technical terms, chemical formulas, complicated numbers. However, once you learn how to read it, it becomes the best tool for deeply understanding hot springs.

When you visit a Japanese hot spring, be sure to check the onsen analysis report. It contains the story of that hot spring. Geology, history, and the mystery of nature. By deciphering the onsen analysis report, you become a true connoisseur of Japanese hot spring culture.

Having the eye to identify authentic hot springs is the first step to enjoying Japanese hot springs more deeply.