1. Why Rinsing Once Does Not Cut Water Use in Half
In 2009, Kao Corporation shared a calculation. For 4 kg of clothes with a water setting of 47 liters, using two rinses used 130 liters of water in total. Using one rinse used 102 liters. [1]
This is a reduction of 28 liters, or about 22%. That is roughly three buckets of water saved, but it is far from half. Electricity use also drops by a similar percentage. [1]
Water use does not drop by half because water is used for more than just rinsing. The water used during the washing cycle remains even if you change the number of rinses. [1]
On the other hand, the Japan Soap and Detergent Association showed a different example in 2011. If two rinses used 120 liters, a detergent designed for "one rinse" could reduce the rinse water to 60 liters, which is half. It also expects electricity use to drop by 10 to 30% compared to traditional two-rinse methods. The association notes that results vary depending on the washing machine and conditions. [2]
So, "half" refers only to the rinse water. For the entire laundry process, Kao’s example shows a reduction of just over 20%. It is important to remember that these are examples from specific companies or associations. [1]
2. Why Strong Cleaning Power Makes Detergent Harder to Rinse Off
The main ingredient in detergent is a surfactant. According to Kao’s technical explanation, surfactants have a difficult trade-off: the properties that make them good at cleaning are the opposite of those that make them easy to rinse out. [1]
To make rinsing easier, you want a surfactant that mixes well with water (high hydrophilicity) and does not stick to fabric fibers. To make cleaning stronger, you want a surfactant that mixes well with oil stains (high lipophilicity). [1]
This is like tape: the stickier the tape, the harder it is to peel off. However, in reality, this is about the properties of very small molecules, not tape. [1]
Kao explains that they balanced these two traits by combining two types of surfactants. When combined, the molecules move more easily and quickly attack dirt. The small particles that wrap around the dirt (micelles) have high hydrophilicity, so they separate easily from the cloth. Kao’s tests confirmed that cleaning and rinsing performance is no different from traditional detergents. [1]
3. How Making Detergent More Concentrated Reduces the Amount Used
Another trick is making the detergent more concentrated. As water is removed from liquid detergent, the surfactant molecules stack up in layers and become hard to dissolve in water. Kao solved this by making the water-loving part of the molecule larger. This allowed them to concentrate 1 kg of liquid detergent down to 400 g for the Attack Neo product released in 2009. [1]
The amount of detergent used also changes. In a Life Cycle Assessment case study by the Japan Chemical Industry Association, washing 4 kg of clothes required 16.8 g of concentrated liquid detergent for one rinse. Traditional detergent for two rinses required 41.8 g. [3]
16.8 divided by 41.8 is about 0.4, so the amount used is about 40%. [3]
This is an example comparing two specific products and does not apply to all detergents. However, if you use less detergent, there is less residue to wash away during rinsing. [3]
4. How Zero-Rinse Detergent Uses Half as Much Surfactant
Lion’s "zero-rinse" detergent (Acron) chose a different path. According to the researcher who developed it, they reduced the surfactant to about half the amount in traditional detergents. They made up for the lost cleaning power with other ingredients, such as polymers that prevent dirt from returning to the fabric. [4]
When comparing skin oil (sebum) stains, zero-rinse washing removed about as much dirt as traditional detergent with one rinse. The amount of ingredients left on the clothes was also about the same as traditional detergents. [4]
Pay attention to the wording. It says "about the same as traditional," not "leaves nothing behind." Also, when using the zero-rinse setting, you must use only this detergent. It cannot be used with fabric softeners or bleach. [4]
5. Why Rinsing with Bath Water Can Leave Dirt on Clothes
Some families use leftover bath water for laundry. The Japan Soap and Detergent Association warns about this. With "one-rinse" detergents, there are almost no cleaning ingredients left in the rinse water. If you use bath water for the rinse, dirt from the bath water can stick to the laundry. [2]
Therefore, they recommend using bath water only for the washing step, and using clean tap water for the rinse. They do not say you cannot use bath water for washing at all. Some washing machines may not have a one-rinse setting. [2]
The association explains that one-rinse detergents leave almost no cleaning ingredients in the rinse water. This saves water, but you must be careful about the water you use. This shows the other side of one-rinse washing. [2]
6. When "One Rinse Is Enough" Is True for a Detergent
"One rinse is enough" is not true for every detergent. Kao’s frequently asked questions state that if a detergent does not have a "rinse once" label, two rinses are recommended because one might not be enough. Products that support one rinse are formulated so ingredients do not stay on the clothes. The number of rinses shown refers to the method where water is filled and drained. [5]
The amount used is also a condition. For liquid New Beads, Kao explains that if you use the recommended amount based on the load, the ingredients are less likely to stay on the fibers, so one fill-and-drain rinse is sufficient. [6]
Hitachi’s Q&A lists reasons why detergent or softener stays on clothes: using too much, putting detergent directly in the drum, undissolved detergent in cold water, or insufficient rinsing. If residue remains even when using the correct amount, they suggest increasing rinses to two or changing to a spray rinse. [7]
7. Why One-Rinse Detergent Is "Same as Traditional," Not "Leaves Nothing"
All the explanations above use phrases like "no different from traditional" or "about the same as traditional." They do not say "leaves nothing behind." Industry groups assume that some detergent ingredients remain on clothes. [8]
In February 2025, the Japan Soap and Detergent Association presented materials to the Ministry of Health, Labour and Welfare. One exposure scenario for laundry detergents includes "skin contact with ingredients remaining on clothes." This does not mean residue is safe. It means there is a framework to evaluate safety, including cases where ingredients remain. [8]
However, most claims of "same level" are based on the manufacturers' own tests. No new measurements by universities or public institutions comparing residue by rinse count, or studies linking detergent residue to skin problems, were found in this research. The labels and usage guidelines on detergent bottles are currently the best clues for readers to check. [8]
Safety assessments assume some residue remains. This does not mean residue is safe; it means there is a framework to evaluate skin impact assuming residue exists. Therefore, do not declare "one rinse is safe" or "it is dangerous." The realistic approach is to check the label and conditions for each detergent. [8]
8. How to Check Your Own Detergent for Rinse Instructions
Let’s look at your detergent. Check the back of your detergent bottle or bag. Look for a "rinse once" label and see if it lists usage amounts based on the weight of clothes or water volume. [1]
If you have detergents with and without the "rinse once" label, compare how the instructions are written. Check how the recommended amount changes with the amount of laundry. Also, check if your washing machine’s rinse setting matches the detergent’s label. [1]
For those who want to read more, here are two recommendations. The Chemical Society of Japan’s explanation page describes detergent molecule design with diagrams. [1] The Japan Soap and Detergent Association article covers one-rinse detergents, water and energy savings, and warnings about bath water. [2]