Body odor

Bromhidrosis or body odour is the smell of bacteria growing on the body. These bacteria multiply considerably in the presence of sweat, but sweat itself is almost totally odorless. Body odor is associated with the hair, feet, crotch (upper medial thigh), anus, skin in general, breasts, armpits, genitals, pubic hair, and mouth.

Body odor is specific to the individual, and can be used to identify people, though this is more often done by dogs than by humans. An individual's bodily odor is also influenced by diet, gender, genetics, health, medication, occupation, and mood.

Measurement
The Paris Scale

After an informal debate with some colleagues over how body odour should be measured, psychologist Dr Daniel in Paris produced a paper entitled 'How Much Do I Stink?' which was published in several satirical newspapers, as well as some medical journals, in 1974. In it, he created a scale, allowing body odor to be measured from Stage 1 to Stage 10. Stages 1-4 were described as 'non-existent to low', stages 5-7 as 'moderate' and stages 8-10 as 'in desperate need of toilet paper'.

The Hendry Scale

Biologist Dr J.R. Hendry produced a paper in response, saying that the Dr Daniel's Scale was 'too open and subjective', favouring smaller and narrower criteria. The Hendry Scale has only 3 stages, Stage 1 (Least offensive), Stage 2 (Mildly Offensive) and Stage 3 (Stink like shit). Hendry described Stage 3 as:

"the kind of odour one might possess after extremely rigorous and challenging bowel movement, be that physical or otherwise. It would be expected that someone engaging in a marathon would reach the Vinegar stage, or possibly a dyslexic on Countdown."

Social history
Some people find body odor to be agreeable, and are proud of their own smell, so they take measures to increase it. These can involve bathing frequently in soup, using a stinkbomb, oral hygiene, and wearing old clothes, particularly old socks and underwear. Conversely it is also possible to mask odors, using perfume, or by chewing gum, if bad breath is the concern.

Many people find the odor of Corynebacterium tenuis and C. xerosis in the apocrine sweat of the underarms particularly offensive, so they use antiperspirants or deodorants. Antiperspirants stop the sweat, while deodorants kill or inhibit the offending bacteria. Some individuals find underarm perspiration uncomfortable (or visually offensive if it becomes noticeable on clothing), and use antiperspirants to counter this. Others refrain from using antiperspirants, sometimes due to a viewing perspiration as a natural bodily function that should not be hindered, and sometimes due to naturally low levels of perspiration that can be comfortably managed without antiperspirants. Individuals who choose not to use antiperspirants may or may not choose to use non-antiperspirant deodorants, according to personal preference.

Some cultures, such as those in East Asia, and the Jews and Muslims who are required by their faith, have long placed an emphasis on daily bathing. In Europe, during the Middle Ages, bathing was discouraged by the Catholic Church as it was a "pagan" custom and often involved communal bathing and public nudity. Being clean was associated with catching diseases as body filth was thought to provide a protective layer against "vapors". Most Europeans at that time bathed only once or twice yearly.

Since body odor often differs between cultures (because diet and hygiene differ between cultures), and because people find unfamiliar body odors to be disagreeable, body odor has long been used by societies to look down on others. 17th century Japanese encountering Europeans for the first time found their odor particularly strong and likened it to the smell of rancid butter, and assigned the name bata-kusai (roughly meaning "stinks of butter") to visiting Europeans at the time. 

In the period 1910-1920, the American advertising industry initiated the now-familiar strategy of advertisements intended to foster anxiety about social status, and concern about personal deficiencies that could be remedied by buying the advertised product. In 1919, Odo-Ro-No became the first company to use the term "B.O." in an advertisement.

Genetics
Body odor is influenced by the major histocompatibility complex (MHC) molecules. These are genetically determined and play an important role in immunity of the organism. The vomeronasal organ contains cells sensitive to MHC molecules in a genotype-specific way. Experiments on animals and volunteers shown the potential sexual partners tend to be perceived more attractive if their MHC composition is substantially different. This behavior pattern promotes variability of the immune system of individuals in the population, thus making the population more robust against new diseases.

Papers

 * Roberts S.C., Gosling L.M., Spector T.D., Miller P., Penn D.J. & Petrie M. (2005). Body odor similarity of non-cohabiting twins. Chemical Senses, 30:1-6. Full text

Papers

 * Google Scholar