Deodorants (Deodourants) are substances applied to the body, most frequently the underarms, to reduce the body odor caused by the bacterial breakdown of perspiration. A subgroup of deodorants are "antiperspirants", which prevent odor and reduce sweat produced by parts of the body. Antiperspirants are typically applied to the underarms, while deodorants can also be used on feet and other areas in the form of body sprays.
Human sweat itself is largely odorless until it is fermented by bacteria that thrive in hot, humid environments such as the human underarm. The armpits are among the consistently warmest areas on the surface of the human body, and sweat glands provide moisture. Underarm hair adds to the odor by providing increased surface area on which this bacteria thrive. Body odor is controlled by reducing moisture, killing bacteria or over powering the bacteria's smell with perfume.
Deodorants — classified and regulated as over-the-counter (OTC) cosmetics by the U.S. Food and Drug Administration (FDA)  — work to inhibit the growth of bacteria which cause odors. The first commercial deodorant, Mum, was introduced in the late nineteenth century. Deodorants are usually alcohol-based, which kills bacteria effectively. Deodorants can be formulated with other, more persistent antimicrobials such as triclosan, or with metal chelant compounds that slow bacterial growth. Deodorants also often contain perfume fragrances intended to mask the odor of perspiration.
Deodorants may be combined with antiperspirants — classified as drugs by the FDA — which attempt to stop or significantly reduce perspiration and thus reduce the moist climate in which bacteria thrive. Aluminum chloride, aluminum chlorohydrate, and aluminum-zirconium compounds, most notably Aluminum zirconium tetrachlorohydrex gly and Aluminum zirconium trichlorohydrex gly, are the most widely used antiperspirants. Aluminum-based complexes react with the electrolytes in the sweat to form a gel plug in the duct of the sweat gland. The plugs prevent the gland from excreting liquid and are removed over time by the natural sloughing of the skin. The blockage of a large number of sweat glands reduces the amount of sweat produced in the underarms, though this may vary from person to person.
A popular alternative to modern commercial deodorants is Ammonium alum, which is a common type of Alum sold in crystal form. It has been used as a deodorant throughout history in Thailand, the Far East, Mexico and other countries.
Deodorants and antiperspirants come in many forms. What is commonly used varies in different countries. In Europe, aerosol sprays are popular, as are cream and roll-on forms which are prevalent in less affluent parts of the world. In the United States, solid or gel forms are dominant.
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Email rumors surfaced on the Internet in the early 1990s that antiperspirants have a link in causing breast cancer; these are now widely considered to be an urban myth. According to the National Cancer Institute (NCI) and the American Cancer Society (ACS), these rumors are largely unsubstantiated by scientific research.
The rumors suggested that antiperspirants keep a person from sweating out toxins and that this would help the spread of cancer-causing toxins via the lymph nodes. While lymph nodes do help to clear toxins, they are not connected to the sweat glands. Furthermore, sweat is not a significant route for eliminating toxins from the body.
NCI discusses two studies that address the breast cancer rumor: A 2002 study of over 800 patients at Fred Hutchinson Cancer Research Institute that found no link between breast cancer and the use of antiperspirant/deodorant; and a study of 437 cancer patients, published in 2003 by the European Journal of Cancer Prevention, which found a correlation between earlier diagnosis of breast cancer and antiperspirant/deodorant use. The NCI's analysis of the second study said that it "does not demonstrate a conclusive link between these underarm hygiene habits and breast cancer. Additional research is needed to investigate this relationship and other factors that may be involved."
One school of thought, advanced by the studies of researcher Phillipa Darbre, PhD, hypothesizes that particular substances in deodorants, such as preservatives called parabens, or aluminum salts such as aluminum chloride used in antiperspirants, get into the bloodstream or accumulate in breast tissue, where they enhance or emulate the effects of estrogen, which stimulates the growth of cancerous breast cells. The ACS and other scientists consider these studies to be early and inconclusive, but merit further research; Darbre also stated that her findings did not show causality. The main reservations have to do with the source and significance of the parabens or other toxic substances. Michael Thun, MD, of the ACS argued that even if some of the substances in antiperspirants do promote tumor growth, the risk from cosmetic use appears minuscule compared with other known tumor promoters — from 500 to 10,000 times less potent than taking oral estrogen or being obese.
Aluminum has been established as a neurotoxin. Aluminum chloride, an aluminum salt that is commonly used in antiperspirants, is also commonly used in studies on aluminum-induced neurotoxicity. Aluminum itself adversely affects the blood-brain barrier, is capable of causing DNA damage, and has adverse epigenetic effects. Research has shown that the aluminum salts used in antiperspirants have detrimental effects to a number of species such as non-human primates, mice, dogs and others. An increased amount of aluminum is also present in the brains of many Alzheimer's patients, although this link does not currently seem to be causal.Template:Disputed-inline
An experiment with mice found that applying an aqueous solution of aluminum chloride to the skin resulted in "a significant increase in urine, serum, and whole brain aluminum." Other experiments on pregnant mice showed transplacental passage of aluminum chloride.
Commercially-manufactured deodorants may also target areas of the body other than the armpits, such as the genitals, and particularly the female genitals. Such products are sometimes the target of sexually graphic humor.
Aluminum zirconium tetrachlorohydrex gly, a common antiperspirant, is a cause of "armpit stains" on clothing, reacting with our bodies to create yellow stains. 
- How Products Are Made: Antiperspirant/Deodorant Stick
- About: Urban Legends and Folklore – Antiperspirants Cause Breast Cancer
- Rados, Carol (July–August 2005). "Antiperspirant Awareness: It's Mostly No Sweat". FDA.gov. Retrieved 2007-03-22.
- Antiperspirants/Deodorants and Breast Cancer: Questions and Answers