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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. Therefore, more smell is created with factors causing more sweating, such as wearing shoes and/or socks with inadequate air ventilation for many hours. 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 materials used to make socks, but provide less ventilation than cotton or wool do when used for the same purpose. Wearing polyester or nylon socks may increase perspiration and therefore may intensify foot odor. Because socks absorb varying amounts of perspiration from feet, wearing shoes without socks may increase the amount of perspiration contacting feet and thereby increase bacterial activities that cause 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 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.
Methods of extinguishment may be used even before onset of the odor as prevention. However, a very effective and cheap way to prevent foot odor is with sodium bicarbonate (a mildly basic white salt also known as baking soda, bread soda, cooking soda, bicarbonate of soda, sodium bicarb, bicarb soda, or simply bicarb). Sodium bicarbonate will create a hostile environment unsuitable for the bacteria responsible for the bad smell. Four pinches of it on each foot everyday are usually enough (two inside the sock and two on the insole of the shoe). Sometimes it might take one or two days before the shoes completely lose their old smell. Washing your feet and applying the sodium bicarbonate daily are also potentially useful solutions.
While there are a number of other remedies, sodium bicarbonate, if bought in a supermarket, costs approximately 20 times less than common odor-eaters or odor-killer powders.
Swabbing feet twice daily with isopropyl alcohol, found at your local drug store, for two weeks is a cheap and highly effective cure. One can also periodically remove their footwear, to reduce foot moisture and thereby reduce bacterial spawn.
Some types of powders and activated charcoal insoles, such as odor eaters, have been developed to prevent foot odor by keeping the feet dry. Special cedarsoles can be recommended for this purpose because of their antibacterial characteristics. Hygiene is considered important in avoiding odor, as is avoidance of synthetic shoes/socks, and rotation of the pairs of shoes worn.
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