1. Is the Smell of Feet Caused by Sweat or by Bacteria?
When you take off your shoes, a sharp smell often rises up. Many people assume this is simply the smell of sweat. However, a study from 2006 identified the main cause of foot odor not as sweat itself, but as the work of bacteria living on the skin [1]. Sweat contains amino acids, including one called leucine. When Staphylococcus epidermidis, a common bacterium found on human skin, breaks down this leucine, it produces an acid called isovaleric acid. This acid was identified as one of the main causes of foot odor [1]. In this process, sweat acts as the "material," and bacteria transform it into something else. However, the study did not claim that Staphylococcus epidermidis is the only cause. The same research found Bacillus subtilis on the soles of people with strong foot odor and noted a link between this bacterium and stronger smells [1]. Other bacteria might also be involved, so it is safest to think of Staphylococcus epidermidis as just one of several main contributors. So why do some parts of the same foot produce odor while others do not?
2. Why Was the Smelly Substance Found Only on the Sole of the Foot?
In 2015, a research team led by Stevens in the UK compared the sole and the top of the foot. They found that staphylococci bacteria were present almost everywhere on the foot, making up most of the bacterial population [3]. The difference lay in the quantity. There were far more bacteria on the sole than on the top of the foot. Isovaleric acid was detected only on the sole and not on the top [3]. The researchers suggest that the high number of staphylococci on the sole is likely the reason for this difference. Sweat production may also play a role. A 2021 paper cites that the number of eccrine sweat glands (the parts that produce sweat) is about 366 per square centimeter on the sole, compared to 149 on the top of the foot [4]. Since 366 divided by 149 is about 2.5, the cited numbers suggest the sole has roughly 2.5 times more sweat glands than the top. More sweat means more material for bacteria and more moisture. However, these numbers are cited from a different study, and depending on the source, values between 400 and 600 per square centimeter also exist, so it is reasonable to think of it as "hundreds of glands, much more than on the top." We now know that bacteria thrive on the sole. What happens when shoes are added to this environment?
3. How Do Different Types of Shoes Change the Temperature and Humidity Inside?
In a 2021 study by Miao and colleagues, three healthy men wore casual shoes, running shoes, and perforated (holey) shoes while the researchers measured temperature, humidity, and ventilation (air exchange) inside the footwear [4]. The casual shoes were the warmest and most humid. The temperature near the top of the foot was 30.6°C, which is about 7°C higher than the 23.4°C office environment. Humidity was measured as absolute humidity (the weight of water in one cubic meter of air). Casual shoes had 27.2 g, while perforated shoes had 18.7 g [4]. Since 27.2 divided by 18.7 is about 1.45, the casual shoes held nearly 1.5 times more water in the air than the perforated ones. Ventilation was best in the order of perforated, running, and casual shoes, which is exactly the reverse of the humidity levels. The growth of bacteria on the sole near the toes increased with higher temperature and humidity, and decreased with better ventilation [4]. The researchers stated that ventilation was the most reliable indicator [4]. However, this measurement was taken from just three people under limited conditions. These are not definitive numbers for "this shoe will always be like this." Still, it suggests a clue that airflow is linked to how bacteria grow. By the way, is isovaleric acid the only smelly component?
4. What Other Components Were Found in the Smell of Socks?
In a 2009 presentation at the Japan Society of Home Economics, a research team from Bunka Women's University examined the smell of socks. Three healthy women participated [2]. The experiment suggested that sour, pungent odors and fishy smells were related to sock odor. Octanal, nonanal, and decanal were detected in all the socks as common smelly components. The research team inferred that waste products like sweat and sebum, along with bacterial growth, are factors in unpleasant shoe odors [2]. It is important to note that detecting these substances is different from identifying the sole cause of the smell. Since this experiment examined three people over two days, we cannot say "the components of foot odor are only these three things." It is more natural to think that many substances, including isovaleric acid, overlap to create that specific smell. There are also clues for controlling odor. The 2006 study reported that fragrance components like citral, citronellal, and geraniol suppressed the production of isovaleric acid at low concentrations [1]. However, this was a result from experiments using bacteria; it does not mean sprinkling these on shoes will eliminate the smell. How can you check these findings with your own shoes?
5. How Can You Feel the Warmth and Moisture Inside Your Own Shoes?
You can try a simple observation at home. When you get home from school and take off your shoes, immediately put your hand inside to feel the warmth and moisture. On another day, do the same thing with different types of shoes (shoes with different ventilation) and write down the differences in your notebook [4]. You can also try letting air flow through them. After taking them off, open the shoe mouths wide or stand them up to let air pass through. After a while, put your hand inside again to compare how dry they feel [4]. This is just an observation that mimics the thinking of the three-person experiment; you cannot measure the number of bacteria or the specific chemical components. Still, it is an entry point to verify with your own senses the research idea that "airflow seems related to how bacteria grow." If you want to read more, Miao’s paper [4] is in English but shows temperature and humidity by shoe type in charts and tables. For Japanese readers, the abstract from the Japan Society of Home Economics [2] is short and easy to read.