1. Why Do Clothes Worn for a Day Smell When Sweat Is Odorless?
The majority of sweat is water, and sweat from eccrine glands is considered to be almost odorless [1]. The smell is explained as arising when bacteria living on the skin surface break down small amounts of proteins in apocrine sweat [1]. The same thing happens on clothes. Research by Kao Corporation has been reported. When 23 test garments worn by adult men were examined, bacteria of the Micrococcus group were listed as related to sweat odor. Odor components included short-chain fatty acids like isobutyric acid and medium-chain fatty acids like 4-methylpentanoic acid [2]. Kao states that it is important not to increase the bacteria causing the smell while wearing clothes and to remove them from the clothing during washing [2]. By changing clothes, you can separate the garment that has absorbed sweat and become a breeding ground for bacteria from your body and wash it. However, this research was read through news reports, and the cause of the odor is not necessarily limited to just one type of bacteria. Section 4 examines whether one type of bacterium causes the odor. Before that, let us look at the numbers to see how much sweat clothes are actually absorbing.
2. How Much Sweat Gets Into Clothes? Results From a Running Experiment
There is an experiment that investigated how much sweat transfers to clothes. 8 men ran for 50 minutes in a wind tunnel, and the total sweat produced from their whole bodies was about 1083 grams. Of this, the cotton T-shirt absorbed about 101 grams of sweat, which accounts for about 9.3% of the total [3]. In this experiment, sweat moved from the skin to the clothes, and the clothes received the sweat. It is also pointed out that when clothes get wet, it may affect the evaporation of sweat [3]. In the same experiment, the amount of sweat absorbed by polyester shirts (4 people) was about 40% of that for cotton (51.4 grams and 126 grams per square meter) [3]. The amount a garment absorbs changes depending on the material. However, because this is a small experiment with 8 men and 1 type of cotton T-shirt, it differs from everyday amounts. Even without exercise, moisture that we do not perceive as sweat comes out of the skin. When Japanese adults spend time in comfortable temperatures, this insensible perspiration is said to be about 900 grams per day [4]. With poorly ventilated clothes, the humidity inside the clothing rises, and it is said that when temperatures are low, condensation may occur and affect insulation [4]. The closer the clothing is to the skin, the more it becomes a place that receives sweat and moisture. So, does this amount change depending on the season or the person?
3. Do Children Sweat More in Summer? How Many Sweat Glands We Have
It is said that Japanese people have an average of about 2.3 million eccrine glands [1]. These are sweat glands that produce sweat to lower body temperature. There is a study that investigated whether the way sweat is produced changes by season. Researchers from Waseda University, Niigata University, and University of Tsukuba had 405 people aged 6–17 and 52 young adults participate. In the 111 people who measured both spring and summer, it was reported that for both boys and girls, sweat production in summer increased to about 1.5 times that of spring [5]. This means children’s sweat production also changes by season. However, this measurement was sweat from the forearm induced by medication. It is not the amount of everyday sweat itself [5]. Still, it serves as a clue when considering that clothes are more likely to receive larger amounts of sweat in summer. So, what kind of bacteria create odors on clothes that have received sweat? Is it only one type of bacterium?
4. Is Only One Type of Bacterium Responsible for Sweat Smell?
Is the cause of sweat odor just one type of bacterium? In 2022, German researchers examined household towels. Towels from homes with odors contained Moraxella, Staphylococcus, Corynebacterium, and Micrococcus, among others. It is said that towels from homes without odors contained more soil bacteria [6]. The research team believes that odor is not determined by a single type of bacterium but involves multiple factors [6]. Because this survey was small, involving only 48 households, we should be careful not to overgeneralize the results. Still, the fact that Micrococcus, which was cited as a cause of sweat odor in previous research, is included among the bacteria found on odorous towels connects with that earlier study. However, it does not prove causation. Also, bacteria are not necessarily villains. In a 2024 announcement by RIKEN, it was reported that skin bacteria help make the upper stratum corneum neutral (in mouse experiments). Skin bacteria are part of the body’s normal mechanisms [7]. Having bacteria on the skin itself is natural. The problem is when they increase enough on sweat-receiving clothes to create an odorous state. So, how often should we wash clothes to prevent this state? Researchers note that foul odor is multifactorial and it is unlikely that only one species causes it [6].
5. Do We Need to Wash Clothes Every Day? France's Call to Wash Less
There are costs to washing too much. The French Agency for Ecological Transition (ADEME) reportedly called on people that "clothes do not get dirty every time you wear them," stating that increasing the number of washes unnecessarily increases electricity use and water pollution, and damages clothes faster [8]. Reports also introduce guidelines from older ADEME documents: underwear should be washed after 1 wear, sportswear after 1–3 wears, cotton shirts after 4–5 wears, and jeans after 15–30 wears [8]. The items closer to the skin that receive sweat have shorter washing intervals [8]. However, this list of numbers is a citation from reports on older documents and is not established as fixed official rules. Current guidelines use three categories—"after 1 wear," "after several wears," and "after several weeks"—and do not specify days [8]. In other words, the materials do not say that "you must change everything every day" nor that "you can wear it for any number of days." The view is that the timing for washing changes depending on the amount of sweat, the type of clothing, and whether it touches the skin. As a guideline for washing frequency, there is a call from the French Agency for Ecological Transition (ADEME) [8].
6. Try It at Home: Check Where Your Clothes Absorbed Sweat
With an adult at home, look at clothes you have worn for a day. Gently touch the areas that were in contact with your skin with your fingers to check for moisture and smell. You do not need to sniff everywhere; noticing within a comfortable range is sufficient. After touching them, wash your hands. Compare the inside of the sleeves of an undershirt and an outer garment to consider whether the part closer to the skin received sweat. You might also notice that the care label tags list washing temperatures and drying methods for each material.