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In humans, body odor serves as a means of chemosensory signal communication between members of the species. These signals are called [[pheromone]]s and they can be transmitted through a variety of mediums. The most common way that human pheromones are transmitted is through bodily fluids. Human pheromones are contained in sweat, semen, vaginal secretions, breast milk, and urine.<ref name="lund-4" /> The signals carried in these fluids serve a range of functions from reproductive signaling to infant socialization.<ref name="Damon_2021">{{cite journal | vauthors = Damon F, Mezrai N, Magnier L, Leleu A, Durand K, Schaal B | title = Olfaction in the Multisensory Processing of Faces: A Narrative Review of the Influence of Human Body Odors | journal = Frontiers in Psychology | volume = 12 | pages = 750944 | date = 2021-10-05 | pmid = 34675855 | pmc = 8523678 | doi = 10.3389/fpsyg.2021.750944 | doi-access = free }}</ref> Each person produces a unique spread of pheromones that can be identified by others.<ref name="Grammer_2005">{{cite journal | vauthors = Grammer K, Fink B, Neave N | title = Human pheromones and sexual attraction | journal = European Journal of Obstetrics, Gynecology, and Reproductive Biology | volume = 118 | issue = 2 | pages = 135–142 | date = February 2005 | pmid = 15653193 | doi = 10.1016/j.ejogrb.2004.08.010 }}</ref> This differentiation allows the formation of sexual attraction and kinship ties to occur.<ref name="Grammer_2005" /><ref name="Porter_1985">{{cite journal | vauthors = Porter RH, Cernoch JM, Balogh RD | title = Odor signatures and kin recognition | journal = Physiology & Behavior | volume = 34 | issue = 3 | pages = 445–448 | date = March 1985 | pmid = 4011726 | doi = 10.1016/0031-9384(85)90210-0 | s2cid = 42316168 }}</ref>
[[Sebaceous]] and [[apocrine gland]]s become active at [[puberty]]. This, as well as many apocrine glands being close to the sex organs, points to a role related to mating.<ref name="Oxford2007" /> Sebaceous glands line the human skin while apocrine glands are located around body hairs.<ref name="lund-4" /> Compared to other primates, humans have extensive axillary hair and have many odor producing sources, in particular many apocrine glands.<ref name="AEP" /> In humans, the apocrine glands have the ability to secrete [[pheromone]]s. These steroid compounds are produced within the peroxisomes of the apocrine glands by enzymes such as mevalonate kinases.<ref>{{cite journal | vauthors = Rothardt G, Beier K | title = Peroxisomes in the apocrine sweat glands of the human axilla and their putative role in pheromone production | journal = Cellular and Molecular Life Sciences | volume = 58 | issue = 9 | pages = 1344–1349 | date = August 2001 | pmid = 11577991 | doi = 10.1007/PL00000946 | s2cid = 28790000 | pmc = 11337405 }}</ref>
==== Sexual selection ====
Pheromones are a factor seen in the mating selection and reproduction in humans. In women, the sense of olfaction is strongest around the time of [[ovulation]], significantly stronger than during other phases of the [[menstrual cycle]] and also stronger than the sense in males.<ref>{{harvnb|Lundström|Olsson|2010|ps=:"In addition, the impact that biological factors have on our percept of body odors has recently been indirectly demonstrated by several experiments. Our percept of body odors is dependent on the sexual orientations of both the donor and the perceiver (Martins et al., 2005), and heterosexual women's percept of men's body odor varies over their menstrual cycle (Roberts et al., 2004)."}}</ref><ref>{{cite journal | vauthors = Navarrete-Palacios E, Hudson R, Reyes-Guerrero G, Guevara-Guzmán R | title = Lower olfactory threshold during the ovulatory phase of the menstrual cycle | journal = Biological Psychology | volume = 63 | issue = 3 | pages = 269–279 | date = July 2003 | pmid = 12853171 | doi = 10.1016/s0301-0511(03)00076-0 | s2cid = 46065468 | doi-access = free }}</ref> Pheromones can be used to deliver information about the [[major histocompatibility complex]] (MHC).<ref name="Grammer_2005"/> The
Studies have suggested that people might be using odor cues associated with the immune system to select mates. Using a brain-imaging technique, Swedish researchers have shown that [[homosexuality|homosexual]] and [[heterosexuality|heterosexual]] males' brains respond in different ways to two odors that may be involved in sexual arousal, and that homosexual men respond in the same way as heterosexual women, though it could not be determined whether this was cause or effect. When the study was expanded to include lesbian women, the results were consistent with previous findings – meaning that lesbian women were not as responsive to male-identified odors, while responding to female odors in a similar way as heterosexual males.<ref>{{cite journal |vauthors=Berglund H, Lindström P, Savic I |date=May 2006 |title=Brain response to putative pheromones in lesbian women |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=103 |issue=21 |pages=8269–8274 |bibcode=2006PNAS..103.8269B |doi=10.1073/pnas.0600331103 |pmc=1570103 |pmid=16705035 |doi-access=free}}</ref> According to the researchers, this research suggests a possible role for human pheromones in the biological basis of [[sexual orientation]].<ref>{{cite news |date=9 May 2005 |title=Gay Men Are Found to Have Different Scent of Attraction |work=The New York Times |url=https://www.nytimes.com/2005/05/09/science/09cnd-smell.html |vauthors=Wade N}}</ref>
Furthermore, recent advances in technology, such as the use of electronic noses, have expanded our understanding of how odor influences sexual selection. A study from Airlangga University demonstrated that electronic noses could classify human gender with high accuracy by analyzing sweat odors, showing that sweat contains distinct chemical components that differ between genders. This discovery not only enhances our understanding of the role of scent in human interactions but also opens up potential applications in fields like biometric identification and personalized product development. The ability of electronic noses to detect and classify these subtle chemical signals could eventually lead to new insights into the role of odor in sexual selection and attraction, reinforcing the idea that human scent plays a crucial role in reproductive success and mate selection.<ref>{{cite book | chapter-url=https://scholar.unair.ac.id/en/publications/classification-of-human-gender-from-sweat-odor-using-electronic-n | doi=10.1109/APWiMob51111.2021.9435205 | chapter=Classification of Human Gender from Sweat Odor using Electronic Nose with Machine Learning Methods | title=2021 IEEE Asia Pacific Conference on Wireless and Mobile (APWiMob) | date=2021 | pages=109–115 | isbn=978-1-7281-9475-2 | vauthors = Sabilla IA, Cahyaningtyas ZA, Sarno R, Al Fauzi A, Wijaya DR, Gunawan R }}</ref>
==== Kinship communication ====
Humans can olfactorily detect blood-related kin.<ref name="Porter_1985"/> Mothers can identify by body odor their biological children, but not their stepchildren. Preadolescent children can olfactorily detect their full siblings, but not half-siblings or step-siblings, and this might explain [[inbreeding avoidance|incest avoidance]] and the [[Westermarck effect]].<ref>{{cite journal | vauthors = Weisfeld GE, Czilli T, Phillips KA, Gall JA, Lichtman CM | title = Possible olfaction-based mechanisms in human kin recognition and inbreeding avoidance | journal = Journal of Experimental Child Psychology | volume = 85 | issue = 3 | pages = 279–295 | date = July 2003 | pmid = 12810039 | doi = 10.1016/s0022-0965(03)00061-4 }}</ref> Babies can recognize their mothers by smell while mothers, fathers, and other relatives can identify a baby by smell.<ref name="Oxford2007" /> This connection between genetically similar family members is due to the habituation of familial pheromones. In the case of babies and mothers, this chemosensory information is primarily contained within breastmilk and the mother's sweat.
In terms of biological functioning, olfactory signaling allows for functional [[breastfeeding]] to occur. In cases of effective latching, breastfed infants are able to locate their mother's nipples for feeding using the sensory information enclosed in their mother's body odor.<ref name="Varendi_1994">{{cite journal | vauthors = Varendi H, Porter RH, Winberg J | title = Does the newborn baby find the nipple by smell? | language = English | journal = Lancet | volume = 344 | issue = 8928 | pages = 989–990 | date = October 1994 | pmid = 7934434 | doi = 10.1016/S0140-6736(94)91645-4 | s2cid = 35029502 }}</ref> While no specific human breast pheromones have been identified, studies compare the communication to that of the rabbit mammary pheromone 2MB2.<ref>{{Citation | vauthors = Schaal B |title=Pheromones for Newborns |date=2014 |url=http://www.ncbi.nlm.nih.gov/books/NBK200997/ |work=Neurobiology of Chemical Communication | veditors = Mucignat-Caretta C |series=Frontiers in Neuroscience |place=Boca Raton (FL) |publisher=CRC Press/Taylor & Francis |isbn=978-1-4665-5341-5 |pmid=24830031 |access-date=2022-11-27}}</ref><ref>{{Cite web |title=Pheromone From Mother's Milk May Rapidly Promote Learning In Newborn Mammals |url=https://www.sciencedaily.com/releases/2006/10/061010022813.htm |access-date=2022-11-27 |website=ScienceDaily |language=en}}</ref> The perception and integration of these signals is an evolutionary response that allows newborns to locate their source of nutrition. Signaling contains a level of precision that allows babies to differentiate their mother's breasts from that of other women.
Beyond a biological function, a mother's body odor plays a role in developing a baby's social capabilities. The ability of an infant to evaluate the properties of human faces stems from the olfactory cues given from their mother.<ref name="Damon_2021"/> Frequent exposure to the [[pheromone]]s exuded by their mother allows the connection between vision and smell to form in infants.<ref name="Endevelt-Shapira_2021" /> This type of connection is only found between mothers and babies and over time it socializes the ability to recognize the features that distinguish human faces from inanimate objects.<ref name="Damon_2021" />
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'''Age-Related Differences'''
As seen in non-human animals such as mice, black-tailed deer, rabbits, otters, and owl monkeys, body odor contains age-related signals that these animals can detect and process. Similarly, humans have been seen to distinguish age-related information from body odor, particularly relating to odors of those of old age. In a study determining if there is a difference between the body odor of individuals of various ages, three groups were studied: those aged 20-30, aged 45-55, and aged 75-95, corresponding to young age, middle-aged, and old age, respectively. This study determined that individuals could distinguish between odors of various ages and group odors of old age, suggesting that there are certain chemical differences in age resulting in “age-dependent odor characteristics”.<ref>{{cite journal |last1=Mitro |first1=Susanna |last2=Gordon |first2=Amy R. |last3=Olsson |first3=Mats J. |last4=Lundström |first4=Johan N. |title=The Smell of Age: Perception and Discrimination of Body Odors of Different Ages |journal=PLOS ONE |date=May 30, 2012 |volume=7 |issue=5 |pages=e38110 |doi=10.1371/journal.pone.0038110 |doi-access=free |pmid=22666457 |pmc=3364187 |bibcode=2012PLoSO...738110M }}</ref>
Another study evaluated the components of body odor in participants aged 26 through 75 using headspace gas chromatography and mass spectroscopy. This study demonstrated that in individuals 40 years or older, 2-Nonenal, an unsaturated aldehyde producing a greasy and grassy odor, was detected in increasing concentrations of those individuals. The detection of increasing amounts of 2-Nonenal in individuals 40 years or older suggested that 2-Nonenal contributes to the deteriorating body odor seen with aging.<ref>{{cite journal |last1=Haze |first1=Shinichiro |last2=Gozu |first2=Yoko |last3=Nakamura |first3=Shoji |last4=Kohno |first4=Yoshiyuki |last5=Sawano |first5=Kiyohito |last6=Ohta |first6=Hideaki |last7=Yamazaki |first7=Kazuo |title=2-Nonenal Newly Found in Human Body Odor Tends to Increase with Aging |journal=Journal of Investigative Dermatology |date=December 8, 2015 |volume=116 |issue=4 |pages=520–524|doi=10.1046/j.0022-202x.2001.01287.x |doi-access=free |pmid=11286617 }}</ref>
'''Body Odor and Disease'''
In mammals, body odor can also be used as a symptom of disease. One's body odor is completely unique to themselves, similar to a fingerprint, and can change due to sexual life, genetics, age and diet. Body odor, however, can be used as an indication for disease. For example, typically, human urine contains 95% water,<ref>{{cite
{| class="wikitable"
|+Frequencies of ABCC11 allele c.538 (One nonsynonymous SNP 538G > A)<ref name="pmid23316210">{{cite journal | vauthors = Ishikawa T, Toyoda Y, Yoshiura K, Niikawa N | title = Pharmacogenetics of human ABC transporter ABCC11: new insights into apocrine gland growth and metabolite secretion | journal = Frontiers in Genetics | volume = 3 | issue = | pages = 306 | date = 2012 | pmid = 23316210 | pmc = 3539816 | doi = 10.3389/fgene.2012.00306 | doi-access = free }}</ref><ref>{{cite journal |last1=Miura |first1=Kiyonori |last2=Yoshiura |first2=Koh-ichiro |last3=Miura |first3=Shoko |last4=Shimada |first4=Takako |last5=Yamasaki |first5=Kentaro |last6=Yoshida |first6=Atsushi |last7=Nakayama |first7=Daisuke |last8=Shibata |first8=Yoshisada |last9=Niikawa |first9=Norio |last10=Masuzaki |first10=Hideaki |title=A strong association between human earwax-type and apocrine colostrum secretion from the mammary gland |journal=Human Genetics |date=June 2007 |volume=121 |issue=5 |pages=631–633 |doi=10.1007/s00439-007-0356-9 |pmid=17394018 |s2cid=575882 |url=https://pubmed.ncbi.nlm.nih.gov/17394018/ |issn=0340-6717}}</ref>
! style="text-align:left;"|Ethnic groups
! style="text-align:left;"|Tribes or inhabitants
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|-
|[[Vietnamese people|Vietnamese]] ||People from multiple regions ||53.6% ||39.2% ||7.2%
|-
|[[Dravidian people|Dravidian]]|| Inhabitants of southern India ||54.0%||17%||29%
|-
|[[Indigenous peoples of the Americas|Native American]] || ||30% ||40% ||30%
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===Industry===
As many as 90% of Americans and 92% of teenagers use antiperspirants or deodorants.<ref>{{cite news | vauthors = Pomeroy R |title=Antiperspirants Alter Your Armpit Bacteria and Could Actually Make You Smell Worse |url=http://www.realclearscience.com/blog/2014/08/antiperspirants_alter_your_armpit_bacteria_and_could_actually_make_you_smell_worse.html |work=RealClearScience |date=10 August 2014 }}</ref><ref>{{cite news | vauthors = Considine A |title=Genetically, Some of Us Never Have Body Odor, But We Still Think We're Smelly |url= https://www.vice.com/en_us/article/wnnyz9/even-if-you-dont-smell-you-probably-use-deodorant |work=Vice |date=17 January 2013 }}</ref> In 2014, the global market for deodorants was estimated at US$13
==Medical conditions==
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