1. Natural Water, Natural Mineral Water, Mineral Water, and Bottled Water: Four Names on Bottles
When comparing product names on bottles, you will see similar names lined up: "Natural Water," "Natural Mineral Water," "Mineral Water," and "Bottled Water." The Kanagawa Prefectural Institute of Public Health explains that this is a classification of four types based on the source of the raw water and the differences in processing during manufacturing [2].
The basis for this division is the guidelines from the Ministry of Agriculture, Forestry and Fisheries. These rules were established as a notification from the Director-General of Food Distribution in March 1990 and revised in 1995 [1]. They are not laws with penalties but serve as a manual for labeling.
If the name is determined by "what was done" to the water, the first thing to consider is the water that has undergone the least processing.
2. What Processing Is Not Done to Water Labeled "Natural"?
Natural Water is defined as groundwater taken from a specific source that has not undergone physical or chemical processing other than sedimentation, filtration, and heat sterilization [1]. This means the water is settled to remove debris, filtered, and heated to reduce bacteria, but ingredients are not added or removed.
Natural Mineral Water is a subset of this. It uses groundwater that has dissolved inorganic salts from underground layers while moving or accumulating underground. Groundwater that bubbles due to natural carbon dioxide is also included [1].
Although the name includes "mineral," the guidelines do not set a standard value for the amount of minerals [1]. The criterion is not whether there are many or few minerals, but whether the water is "water that has not been processed beyond the permitted range" after the underground dissolution.
So, what are waters that have been processed called?
3. How Is Processed Mineral Water Made, and What Does Its Label Say?
Mineral Water is made from Natural Mineral Water as the raw material, but it undergoes adjustments to stabilize quality. These processes include adjusting mineral content, aeration (blowing air into the water), or mixing Natural Mineral Water from multiple sources [1].
Bottled Water (drinking water) covers everything else. The raw water can be tap water or other sources [1].
Here, the wording on the label becomes clear. Words like "natural" or "nature," and similar terms, cannot be displayed for anything other than Natural Water and Natural Mineral Water. Words that suggest or imply effects like pharmaceuticals are also prohibited [1].
Additionally, the ingredient list on the label must state the type of raw water in parentheses after "water." Examples include shallow well water, deep well water, spring water, mineral spring water, hot spring water, and underground river water. If the water has been adjusted or mixed, the processing method must also be written [1].
Reading these labels reveals much about the water's origin. So, what are the numbers on the edge of the label?
4. What Is Water Hardness, and Why Is the "Soft Water" Line Not the Same Everywhere?
Hardness is a value that converts the amount of calcium ions and magnesium ions dissolved in water into the equivalent amount of calcium carbonate (mg/L) [4]. The calculation is approximately "Calcium (mg/L) × 2.5 + Magnesium (mg/L) × 4.1" [5]. For example, if calcium is 10 mg/L and magnesium is 2 mg/L, the calculation is 10 × 2.5 + 2 × 4.1, resulting in a hardness of approximately 33 (this number is just a calculation example).
This hardness value is used to classify water. A document from Fukuoka City divides 0–60 as soft water, 60–120 as moderately soft water, 120–180 as hard water, and 180 and above as very hard water. The US Geological Survey (USGS) uses the same lines for 60 and 120, but calls 60–120 "moderately hard water" [5]. However, in examples from manufacturer materials, a chart shows 100 and below as soft water, 100–300 as moderately hard water, and 300 and above as hard water [6].
Because of these differences, the same water might be called "soft water" by one standard and "moderately hard water" by another. Japan's water quality standards for tap water include a hardness figure of 300 mg/L, but this is based on factors like soap lathering. The target for "delicious water" is set at a hardness of 10–100 mg/L [4].
You might wonder if hard water is good or bad for health. According to a 1996 assessment by the World Health Organization (WHO), there is no clear evidence that hardness has negative health effects, and no guideline values for health impacts have been proposed [4]. Therefore, good or bad cannot be determined by hardness alone.
If the numbers differ by country or organization, is the water itself also very different between countries?
5. Why Do Some European Bottled Waters Have Extremely High Hardness?
In examples from manufacturer materials, European waters are listed with hardness values of approximately 307, 401, 674, and 1551 mg/L. Examples of Japanese water show values like approximately 50 mg/L [6]. Since the specific brands corresponding to these values cannot be identified from this material, no brand names are mentioned.
The reason given is that Japan has many steep mountains, so rainwater flows quickly into the sea and has a short residence time in underground layers. In contrast, Europe has vast plains where water remains in underground layers for a long time [6]. The material cites this as the reason for the difference in mineral content between Japanese and European water.
Some imported waters do not undergo sterilization or disinfection. Under the Food Sanitation Act, mineral water types generally require sterilization or disinfection, and most Japanese products fall under this rule. Products that do not undergo sterilization are subject to stricter standards regarding raw water quality and environmental protection around the water source. These products are mainly imported from Europe [3]. Not sterilizing is not considered "better"; it simply means the standards are structured differently.
In terms of volume, domestic production of mineral water types in 2025 was approximately 4.86 million kL, while imports were approximately 119,000 kL. Imports account for about 2.4% of the total, while domestic production accounts for about 97.6% [7] (calculated from actual figures, not upper limits). Although imported water is common in stores, most water by volume is domestic.
6. How to Read a Bottled Water Label: Product Name, Ingredients, and Hardness
Choose one bottle of water from your home and read the label in this order. First, check the product name: which of the four types is it? Next, look at the ingredients: what kind of raw water is listed in the parentheses? Finally, if hardness is listed in the nutritional information, what is the number?
If hardness is listed, compare that number to the lines for "soft water" at 60 and 100 that we discussed. Some bottles do not list hardness, so in that case, you can rely on the product name and ingredients. When comparing the taste of bottled water, please do so with an adult.
If you want to read more, you can look up the Ministry of Agriculture, Forestry and Fisheries guidelines [1] or the Kanagawa Institute of Public Health Newsletter [2].