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Foot odor (or foot odour) is a type of body odor that affects the feet of humans. It is sometimes considered to be an unpleasant smell, but can also be the target of foot fetishism, more specifically as a form of olfactophilia.
The main cause is foot sweat (also see focal hyperhidrosis). Sweat itself is odorless; however, it creates a beneficial environment for certain bacteria to grow, producing odorous substances. These bacteria are naturally present on our skin as part of the human flora. The front part of the foot produces the most sweat.
The smell is exacerbated by factors that increase sweating, such as wearing closed-toe shoes. Socks can trap foot hair, especially on the toes, and may contribute to odor intensity by increasing surface area on which bacteria can thrive.
Given that socks directly contact the feet, their composition can affect foot odor. Synthetic materials like polyester and nylon afford less ventilation to the foot than do cotton or wool, leading to increased perspiration and odor, although they can also reduce incidence of blisters by wicking away perspiration. Many synthetic socks are treated with chemicals to help reduce odor.
Wearing closed-toe shoes (e.g., ballet flats or pumps) without socks leads to accumulation of sweat, dead skin cells, dirt, and oils, further contributing to bacterial growth. Momentarily slipping off shoes whenever feet start to feel "hot" or sweaty can help prevent odor.
Brevibacteria are considered a major cause of foot odor because they ingest dead skin on the feet and, in the process, convert the amino acid methionine into methanethiol, a colorless gas with a distinctive sulfuric aroma. The dead skin that fuels this process is especially common on the sole and between the toes. Brevibacteria also give such cheeses as Limburger, Bel Paese, Port Salut, Pálpusztai and Munster their characteristic pungency.
Propionic acid (propanoic acid), a breakdown product of Propionibacteria amino acid metabolism in adolescent and adult sebaceous gland ducts, is also present in many foot sweat samples. The similarity between the chemical structures of propionic acid and acetic acid may account for the tendency of foot odors to be identified as smelling like vinegar.
Isovaleric acid (3-methyl butanoic acid), another other source of foot odor, is produced by Staphylococcus epidermidis, a bacterial species normally resident on human skin and present in several strong-smelling varieties of cheese.
Bart Knols of Wageningen Agricultural University in the Netherlands received a 2006 "Ig Nobel Prize" for demonstrating that the female Anopheles gambiae mosquito, known for transmitting malaria, is "attracted equally to the smell of Limburger cheese and to the smell of human feet". Fredros Okumu, of the Ifakara Health Institute in Tanzania, received grants in 2009 and 2011 to develop mosquito attractants and traps to combat malaria. He used a blend of eight chemicals four times more effective than actual human secretions.
Maintaining good foot hygiene is the best way to prevent foot odor as it eliminates odor causing bacteria and removes dead skin cells as well as sebum. A foot file or pumice stone can be used to remove dead skin cells. Dirty feet and dirty socks, instead of clean, dry socks changed frequently can contribute to odor. Socks that are made of synthetic materials are more prone to becoming smelly than natural or blended fabrics, such as a blend of cotton and polyester.
Sodium bicarbonate is used as an inexpensive home remedy for the prevention of foot odor. However, the extent of the antimicrobial effect on the odor-causing bacteria is unclear. A solution of 10% household bleach in 90% water kills the odor-causing bacteria. A brush can be used to clean between the toes. Swabbing feet twice daily with isopropyl alcohol is another home remedy.
Some powders and activated charcoal insoles, such as odor eaters, have been developed with the purpose of preventing foot odor by keeping the feet dry. Special cedarsoles and socks with silver fibers with antibacterial characteristics are available.
Bamboo fabric socks are naturally anti-microbial, anti bacterial and moisture wicking, cutting odor and abating athlete's foot.
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