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The main cause is foot sweat (also see focal hyperhidrosis). Sweat itself is odorless, but it creates a beneficial environment for certain bacteria to grow and produce bad-smelling substances. These bacteria are naturally present on our skin as part of the human flora. The front part of the foot is where it produces the most sweat. Therefore, more smell is created with factors causing more sweating, such as wearing shoes with inadequate air ventilation for many hours. Socks generally do not cause foot odor on their own, but when worn along with shoes, socks can help to trap the hair on the feet, especially on the toes, may contribute to the odor's intensity by adding increased surface area in which the bacteria can thrive.
Given that socks directly contact the feet, their composition can have an impact on foot odor. Polyester and nylon are common synthetic materials used to make socks, but generally provide less ventilation than cotton or wool do when used for the same purpose. Wearing polyester or nylon socks may increase the amount of perspiration because they do not let the feet breathe as well as cotton. This may intensify foot odor, although polyester and nylon can help reduce blisters by keeping the perspiration away from the foot. Cotton's natural antibacterial properties can reduce odors better than synthetics although many synthetic socks are treated with chemicals to help reduce odor.  Since the insides of shoes are typically not washed (unlike socks), and since socks also help to absorb perspiration from feet, wearing closed-toe shoes without socks allows sweat to accumulate excessively inside shoes, enabling bacteria to grow over time. Bacterial growth is facilitated in this case because closed-toe shoes do not receive adequate air ventilation (especially in the toe area, where the feet produce the greatest amount of sweat), so the sweat cannot evaporate while the shoe is being worn, in addition to there being no socks to absorb the sweat. Dead skin cells, dirt and oils can also accumulate in the shoe from the foot over time which can increase odor inside the shoe. Mold and fungus are also more likely to form if socks are not worn which can increase odors and other foot issues. If closed-toe shoes (such as ballet flats or heels) must be worn without socks, the best prevention of foot odor is to slip the shoes off momentarily, at least part-way, whenever the feet start to feel "hot" or sweaty, to allow sweat to evaporate from the feet and shoes. Foot powders are also available to manage moisture and reduce odor.
The quality of foot odor is often reported as a thick, cheese-like smell. Some describe the smell like that of malt vinegar. However, it can also be ammonia-like. Brevibacteria are considered a major cause of foot odor because they ingest dead skin on the feet and, in the process, convert amino acid methionine into methanethiol, which has a sulfuric aroma. The dead skin that fuels this process is especially common on the soles and between the toes. The brevibacteria is also what gives cheeses such as Limburger, Bel Paese, Port du Salut, Pálpusztai and Munster their characteristic pungency.
Propionic acid (propanoic acid) is also present in many foot sweat samples. This acid is a breakdown product of amino acids by Propionibacteria, which thrive in the ducts of adolescent and adult sebaceous glands. The similarity in chemical structures between propionic acid and acetic acid, which share many physical characteristics such as odor, may account for foot odors identified as being vinegar-like. Isovaleric acid (3-methyl butanoic acid) is the other source of foot odor and is a result of actions of the bacteria Staphylococcus epidermidis which is also present in several strong cheese types.
Bart Knols, of Wageningen Agricultural University, the Netherlands, received an "IG Nobel" prize in 2006 for showing that the female malaria mosquito Anopheles gambiae "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 uses a blend of eight chemicals, which is four times more effective than an actual human.
Once foot odor has begun, it can be extinguished, or at least alleviated, by either aromatic deodorants that neutralise the odor by their own smell, or by absorbers of the odor itself.
Among the earliest foot deodorants were aromatic herbs such as allspice, which nineteenth-century Russian soldiers would put in their boots.
Odor absorbers include activated charcoal foot insert wafers.
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. Avoid putting on shoes with dirty feet and dirty socks. When walking barefoot, feet should be cleaned with soap and water before putting shoes on.
Always wear clean, dry socks when wearing shoes. Socks should be changed daily or more frequently if feet sweat excessively. Avoid socks that are 100% synthetic. Cotton/synthetic blends are generally best.
Use sodium carbonate as a home-remedy against foot odor and for the prevention of foot odor. While there are a number of other remedies, sodium bicarbonate is the least expensive. However, the extent of the antimicrobial effect on the odor-causing bacteria is unclear.
Swabbing feet twice daily with isopropyl alcohol - found at your local drug store - for two weeks is an inexpensive and highly effective cure.
Remove footwear periodically to allow shoes to air out and to allow perspiration to evaporate.
Some powders and activated charcoal insoles, such as odor eaters, have been developed to prevent foot odor by keeping the feet dry. Special cedarsoles and socks with silver fibers can be recommended for this purpose because of their antibacterial characteristics. Using boric acid powder is an effective way for both treatment and prevention of the odor.