The human body produces a variety of odors in the form of volatile chemical substances that stimulate the sense of smell. While most of these body odors are considered socially unpleasant, there are a few that are considered benignand some that are believed to serve as attractants. Those odors considered to be most noxious are produced in the intestinal tract, the mouth, on the feet, and under the arms.
Intestinal tract gas, known medically as "flatus," is a normal product of digestion. Flatus is generated when bacteria that reside in the gut process thecarbohydrates in food that cannot be broken down by digestive enzymes, or from swallowing gas-containing substances such as carbonated beverages or excessive amounts of air. The nature of its odor depends on the types of bacteria that colonize the human intestinal tract, as well as the types of food that are consumed. While usually expelled through the anus, excessive gas may pass through the mouth, also known as "eructation," or "belching" (burping).
Bad breath, or "halitosis," may be caused by excess bacteria growing on or between teeth that may lead to tooth decay, and by gum or gingival disease. In80 to 90 percent of cases, something in the mouth, such as plaque (an almostinvisible film of bacteria), and cavities cause halitosis. A dry mouth also causes an offensive smell. While saliva's natural antibacterial action cleansthe mouth and washes away food particles, it is its reduced flow at night that causes "morning breath".
Bad breath may also be caused by something in the lungs or gastrointestinal tract. When certain food substances such as onions, garlic, and alcohol are consumed, their digestive byproducts are carried through the bloodstream and exhaled by the lungs, producing a characteristic odor on the breath that can linger for many hours. Pharmaceutical drugs and the use of tobacco products canalso cause offensive mouth odor. Various disease conditions are associated with bad breath, including certain liver disorders that produces an ammonia-like odor, respiratory tract cancers and infections, and the odor of acetone caused by high blood sugar among diabetics. Health conditions such as sinus infections may also contribute to halitosis. In order to prevent mouth odors, meticulous oral hygiene should be practiced, including: brushing and flossing regularly; replacing toothbrushes frequently; brushing the tongue to remove bacteria; encouraging the production of saliva to wash away food particles; andusing a fluoride-containing or plaque-fighting mouthwash.
A rare genetic disorder called trimethylaminuria, or "fish odor syndrome," causes individuals to produce a fish-like odor, not only on their breath, but also in their sweat and urine. A condition that was not formally recognized bythe medical community until the 1970s, there are literary references to thesyndrome dating back thousands of years--such as Hindu tales that aptly describe the disorder. Information about the syndrome remains limited causing those with trimethylaminuria to be labeled with inaccurate diagnoses such as poorhygiene or psychiatric problems, while some patients may never be correctlydiagnosed.
While its cause remains unknown, recent studies have shown that trimethylaminuria results from defects in an enzyme that breaks down trimethyl-amine, a byproduct of protein digestion released by bacteria existing in the gut. This byproduct is the small molecule responsible for the foul or garbage-like smellat low concentrations and a fishy odor in greater amounts. With the discovery of the gene implicated in trimethlyaminuria in 1997, scientists are studying its correlation to a patient's diet and periods of increased stress that cause the condition to become more prominent. The syndrome appears to be more common in women than in men, with the disease worsening around puberty, just before and during menstrual periods, after taking oral contraceptives, and around menopause. The disorder may also develop in those who have liver or kidney disease. Although there is no cure for trimethlyaminuria, the condition maybe controlled with a special diet and low doses of antibiotics.
Excess sweating, a condition known as "hyperhydrosis," often results in underarm, foot, and general body odor, especially when bathing is not regular. Thehuman body possesses three major types of skin glands--sebaceous glands (most commonly around the face and forehead), eccrine (or sweat) glands, and apocrine glands. While more than two million eccrine glands over most of the bodyrelease an odorless sweat that consist of 99 percent pure water along with traces of salt and potassium, a limited amount of apocrine glands located in the armpits, the genitals, and anywhere there is hair produce a viscous substance. This sticky matter is also made on hands, cheeks, and scalps, and when it is exposed to a warm environment, the sweat encourages the growth of certain bacteria and fungi. These microorganisms digest components of sweat and release volatile chemicals that are responsible for the acrid odor associated with sweat.
Some of the common bacteria responsible for body odor include micrococci, staphylococci, aerobic and anaerobic corneforms, and pityrosporum species. Apocrine glands enlarge and become active during puberty, and stress can also cause the glands to constrict, developing more sweat from the skin. A woman's body temperature will increase a full degree higher before she perspires, causing men to sweat more than women. Those people with excess hair also sweat moredue to their numerous hair follicles where aprocrine glands originate. Caucasians and blacks have more aprocrine glands than Asians, who, as a result sweat less. "Bromidrosis" is the medical term used to describe the condition ofunpleasant body odors produced by excess sweating. This condition is so rareamong Japanese men, that historically, a strong body odor could disqualify them from military service.
Body odors that are considered benign include the scent produced by a mother's breast milk and the individually unique odor of every infant. Scientific studies have demonstrated that human babies and mothers can recognize one another solely on the basis of odor. Scent-communicating chemical compounds are also contained in human sweat, urine, breath, saliva, breast milk, skin oils, and sexual secretions. Genital and anal odors are also emitted by humans. Somepeople consider strong genital odors to be an attractant. In a famous nineteenth-century love letter, Napolean Bonaparte instructed his mistress Josephine that she must not wash, since he would soon be returning from one of his European campaigns and wanted to enjoy her sexual scent. During Victorian times, nice-smelling young English women would sell handkerchiefs scented with their body odor, and during the last century, perfumeries tried to mass market the smell of sexual attraction by bottling the gentle musk deer's aroma--hoping to duplicate this animal's biochemical bouquet and its presumed effect on the opposite sex.
The act of perceiving an odor, whether it be pleasing or noxious, is called olfaction. At the top of the nasal cavity in humans is a small patch of skin called the olfactory epithelium. Only a few centimeters square, it contains some five million olfactory neurons. When these neurons sense and recognize specific odor molecules, they send signals to the olfactory bulb, which is partof the brain. The sensation of smell happens quickly, with olfactory cells responding within milliseconds. Odors are also rapidly adapted to, with olfactory receptors adapting about 50 percent within the first second, while remaining adaptation happens very slowly after this. Only a microscopic quantity ofa substance needs to be present in the air for it to be smelled (also known as minimal identifiable odor or MIO), allowing for an extremely low thresholdto be smell's major characteristic. The substance methyl mercaptan can be detected in concentrations as low as 1/25,000,000,000 mg per milliliter of air.Individual reaction to the perception of odors is highly variable, and appears to be determined at least somewhat by culture and learning. In previous centuries, when current sanitation and personal hygiene practices differed, humans were presumably more tolerant of bodily odors.
Throughout much of the twentieth century, it was assumed that humans were unable to detect pheromones (a substance that provides chemical means of communication by way of smell) due to the presumed lack of a vomeronasal organ (VNO)--a pair of tiny cigar-shaped sacs in the nasal cavity. Many animal species rely on the VNO as an excessory olfactory system to detect the presence of pheromones, or a type of bodily odor that may advertise the sexual readiness ofa potential animal mate. It was long believed that body odor played no role in human sexual attraction. Recent studies have proven the presence of a human's VNO, and its importance along with the brain's olfactory bulb continue tobe studied. The nerve cells in this organ, like those in the olfactory epithelium, appear able to sense and recognize specific odor molecules. Scientistsspeculate that human pheromones are involved in sexual attraction, and that the sensory perception of pheromones may occur unconsciously.
One way that pheromones may be exchanged is through universal human greetingssuch as a handshake or a kiss. The Eskimo kiss of rubbing noses is a mutualsniffing of body smells, and only in the Western world has this been modifiedto a kiss. The hands and face are the two most accessible concentrations ofscent glands on the human body other than the ears. Humans have a denser skinconcentration of scent glands than almost any other mammal, and researcherscontinue to study the significance of the human body's odor on interpersonalinteractions.