Hair

Hair
Scanning electron microscopy image of Merino wool (top) and human hair (bottom) showing keratin scales
Details
System	Integumentary system
Identifiers
Latin	capillum
MeSH	D006197
TA98	A16.0.00.014
TA2	7053
TH	H3.12.00.3.02001
FMA	53667
Anatomical terminology [edit on Wikidata ]

Hair is a protein filament that grows from follicles found in the dermis. Hair is one of the defining characteristics of mammals. The

.

Attitudes towards different forms of hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person's personal beliefs or social position, such as their age, gender, or religion.^[1]

Overview

The word "hair" usually refers to two distinct structures:

1. the part beneath the skin, called the hair follicle, or, when pulled from the skin, the bulb or root. This organ is located in the dermis and maintains stem cells, which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound.^[2]
2. the hair shaft, which is the hard filamentous part that extends above the skin surface. A cross section of the hair shaft may be divided roughly into three zones.

Hair fibers have a structure consisting of several layers, starting from the outside:

1. the cuticle, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles
2. the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like
3. the medulla, a disorganized and open area at the fiber's center^[3]

Description

Each strand of hair is made up of the

Hair growth begins inside the

.

Root of the hair

Root of the hair
sebaceous glands.
Details
Identifiers
Latin	radix pili
MeSH	D006197
TA98	A16.0.00.014
TA2	7053
TH	H3.12.00.3.02001
FMA	53667
Anatomical terminology [edit on Wikidata ]

The root of the hair ends in an enlargement, the hair bulb, which is whiter in color and softer in texture than the shaft and is lodged in a follicular involution of the

Natural color

All natural hair colors are the result of two types of hair pigments. Both of these pigments are melanin types, produced inside the hair follicle and packed into granules found in the fibers.

is white hair (and often the skin to which the hair is attached), typically in spots that never possessed melanin at all, or ceased for natural reasons, generally genetic, in the first years of life.

Human hair growth

Hair grows everywhere on the external body except for mucus membranes and glabrous skin, such as that found on the palms of the hands, soles of the feet, and lips.

The body has different types of hair, including

, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly, warmth and protection.

The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The growth rate of hair varies from individual to individual depending on their age, genetic predisposition and a number of environmental factors.^[11] It is commonly stated that hair grows about 1 cm per month on average; however reality is more complex, since not all hair grows at once. Scalp hair was reported to grow between 0.6 cm and 3.36 cm per month. The growth rate of scalp hair somewhat depends on age (hair tends to grow more slowly with age), sex, and ethnicity.^[12] Thicker hair (>60 µm) grows generally faster (11.4 mm per month) than thinner (20-30 µm) hair (7.6 mm per month).^[13][14]

It was previously thought that Caucasian hair grew more quickly than Asian hair and that the growth rate of women's hair was faster than that of men.

The world record for longest hair on a living person stands with Smita Srivastava of Uttar Pradesh, India. At 7 feet and 9 inches long, she broke a Guinness World Record in November 2023, having grown her hair for 32 years.^[22]

Texture

Hair exists in a variety of textures. Three main aspects of hair texture are the curl pattern, volume, and consistency. All mammalian hair is composed of

The flatter the hair shaft becomes, the curlier hair gets, because the shape allows more cysteines to become compacted together resulting in a bent shape that, with every additional disulfide bond, becomes curlier in form.^[23] As the hair follicle shape determines curl pattern, the hair follicle size determines thickness. While the circumference of the hair follicle expands, so does the thickness of the hair follicle. An individual's hair volume, as a result, can be thin, normal, or thick. The consistency of hair can almost always be grouped into three categories: fine, medium, and coarse. This trait is determined by the hair follicle volume and the condition of the strand.^[24] Fine hair has the smallest circumference, coarse hair has the largest circumference, and medium hair is anywhere between the other two.^[24] Coarse hair has a more open cuticle than thin or medium hair causing it to be the most porous.^[24]

Classification systems

There are various systems that people use to classify their curl patterns. Being knowledgeable of an individual's hair type is a good start to knowing how to take care of one's hair. There is not just one method to discovering one's hair type. Additionally it is possible, and quite normal to have more than one kind of hair type, for instance having a mixture of both type 3a & 3b curls.

Andre Walker system

The Andre Walker Hair Typing System is the most widely used system to classify hair. The system was created by the hairstylist of Oprah Winfrey, Andre Walker. According to this system there are four types of hair: straight, wavy, curly, kinky.

• Type 1 is straight hair, which reflects the most sheen and also the most resilient hair of all of the hair types. It is hard to damage and immensely difficult to curl this hair texture. Because the
  sebum
  easily spreads from the scalp to the ends without curls or kinks to interrupt its path, it is the most oily hair texture of all.
• Type 2 is wavy hair, whose texture and sheen ranges somewhere between straight and curly hair. Wavy hair is also more likely to become frizzy than straight hair. While type A waves can easily alternate between straight and curly styles, type B and C Wavy hair is resistant to styling.
• Type 3 is
  curly hair known to have an S-shape. The curl pattern may resemble a lowercase "s", uppercase "S", or sometimes an uppercase "Z" or lowercase "z". ^{[citation needed}
  ] Lack of proper care causes less defined curls.
• Type 4 is kinky hair, which features a tightly coiled curl pattern (or no discernible curl pattern at all) that is often fragile with a very high density. This type of hair shrinks when wet and because it has fewer cuticle layers than other hair types it is more susceptible to damage.

Andre Walker hair types

Type 1: Straight
1a	Straight (Fine/Thin)	Hair tends to be very soft, thin, shiny, oily, poor at holding curls, difficult to damage.
1b	Straight (Medium)	Hair characterized by volume and body.
1c	Straight (Coarse)	Hair tends to be bone-straight, coarse, difficult to curl.
Type 2: Wavy
2a	Wavy (Fine/Thin)	Hair has definite "S" pattern, can easily be straightened or curled, usually receptive to a variety of styles.
2b	Wavy (Medium)	Can tend to be frizzy and a little resistant to styling.
2c	Wavy (Coarse)	Fairly coarse, frizzy or very frizzy with thicker waves, often more resistant to styling.
Type 3: Curly
3a	Curly (Loose)	Presents a definite "S" pattern, tends to combine thickness, volume, and/or frizziness.
3b	Curly (Tight)	Presents a definite "S" pattern, curls ranging from spirals to spiral-shaped corkscrew
Type 4: Kinky
4a	Kinky (Soft)	Hair tends to be very wiry and fragile, tightly coiled and can feature curly patterning.
4b	Kinky (Wiry)	As 4a but with less defined pattern of curls, looks more like a "Z" with sharp angles

FIA system

This is a method which classifies the hair by curl pattern, hair-strand thickness and overall hair volume.

FIA hair classification

Curliness
Straight
1a	Stick-straight.
1b	Straight but with a slight body wave adding some volume.
1c	Straight with body wave and one or two visible S-waves (e.g. at nape of neck or temples).
Wavy
2a	Loose with stretched S-waves throughout.
2b	Shorter with more distinct S-waves (resembling e.g. braided damp hair).
2c	Distinct S-waves, some spiral curling.
Curly
3a	Big, loose spiral curls.
3b	Bouncy ringlets.
3c	Tight corkscrews.
Very ("Really") curly
4a	Tightly coiled S-curls.
4b	Z-patterned (tightly coiled, sharply angled)
4c	Mostly Z-patterned (tightly kinked, less definition)
Strands
F	Fine Thin strands that sometimes are almost translucent when held up to the light. Shed strands can be hard to see even against a contrasting background. Fine hair is difficult to feel or it feels like an ultra-fine strand of silk.
M	Medium Strands are neither fine nor coarse. Medium hair feels like a cotton thread, but is not stiff or rough. It is neither fine nor coarse.
C	Coarse Thick strands whose shed strands usually are easily identified. Coarse hair feels hard and wiry.
Volume by circumference of full-hair ponytail
i	Thin	Circumference less than 2 inches (5 centimetres)
ii	Normal	... from 2 to 4 inches (5 to 10 centimetres)
iii	Thick	... more than 4 inches (10 centimetres)

Composition

Hair is mainly composed of keratin proteins and keratin-associated proteins (KRTAPs). The human genome encodes 54 different keratin proteins which are present in various amounts in hair. Similarly, humans encode more than 100 different KRTAPs which crosslink keratins in hair. The content of KRTAPs ranges from less than 3% in human hair to 30–40% in echidna quill.^[25]

Functions

Many mammals have fur and other hairs that serve different functions. Hair provides thermal regulation and camouflage for many animals; for others it provides signals to other animals such as warnings, mating, or other communicative displays; and for some animals hair provides defensive functions and, rarely, even offensive protection. Hair also has a sensory function, extending the sense of touch beyond the surface of the skin. Guard hairs give warnings that may trigger a recoiling reaction.

Warmth

While humans have developed clothing and other means of keeping warm, the hair found on the head serves primarily as a source of heat

This is more effective in other mammals whose fur fluffs up to create air pockets between hairs that insulate the body from the cold. The opposite actions occur when the body is too warm; the arrector muscles make the hair lie flat on the skin which allows heat to leave.

Protection

In some mammals, such as hedgehogs and porcupines, the hairs have been modified into hard spines or quills. These are covered with thick plates of keratin and serve as protection against predators. Thick hair such as that of the lion's mane and grizzly bear's fur do offer some protection from physical damages such as bites and scratches.

Touch sense

Displacement and vibration of hair shafts are detected by hair follicle nerve receptors and nerve receptors within the skin. Hairs can sense movements of air as well as touch by physical objects and they provide sensory awareness of the presence of

Eyebrows and eyelashes

The

by displaying emotions such as sadness, anger, surprise and excitement. In many other mammals, they contain much longer, whisker-like hairs that act as tactile sensors.

The

sensation

.

Eyebrows and eyelashes do not grow beyond a certain length (eyelashes are rarely more than 10 mm long). However, trichomegaly can cause the lashes to grow remarkably long and prominent (in some cases the upper lashes grow to 15 mm long).

Evolution

Hair has its origins in the common ancestor of mammals, the

.

Some modern mammals have a special gland in front of each orbit used to preen the fur, called the harderian gland. Imprints of this structure are found in the skull of the small early mammals like Morganucodon, but not in their cynodont ancestors like Thrinaxodon.^[39]

The hairs of the fur in modern animals are all connected to nerves, and so the fur also serves as a transmitter for sensory input. Fur could have evolved from sensory hair (whiskers). The signals from this sensory apparatus is interpreted in the

The high interspecific variability of the size, color, and microstructure of hair often enables the identification of species based on single hair filaments.[43]^[44]

In varying degrees most

.

The naked mole-rat (Heterocephalus glaber) has evolved skin lacking in general, pelagic hair covering, yet has retained long, very sparsely scattered tactile hairs over its body.^[45] Glabrousness is a trait that may be associated with neoteny.^[47]

Human hairlessness

Evolutionary variation

Primates are relatively hairless compared to other mammals, and

Another factor in human evolution that also occurred in the prehistoric past was a preferential selection for

, women do not, leaving their vellus hair visible.

Texture

Curly hair

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Jablonski

or 'kinky' (i.e. tightly coiled). This argument assumes that curly hair better impedes the passage of UV light into the body relative to straight hair (thus curly or coiled hair would be particularly advantageous for light-skinned hominids living at the equator).

It is substantiated by Iyengar's findings (1998) that UV light can enter into straight human hair roots (and thus into the body through the skin) via the hair shaft. Specifically, the results of that study suggest that this phenomenon resembles the passage of light through fiber optic tubes (which do not function as effectively when kinked or sharply curved or coiled). In this sense, when hominids (i.e. Homo erectus) were gradually losing their straight body hair and thereby exposing the initially pale skin underneath their fur to the sun, straight hair would have been an adaptive liability. By inverse logic, later, as humans traveled farther from Africa and/or the equator, straight hair may have (initially) evolved to aid the entry of UV light into the body during the transition from dark, UV-protected skin to paler skin.

Jablonski's assertions^[52] suggest that the adjective "woolly" in reference to Afro-hair is a misnomer in connoting the high heat insulation derivable from the true wool of sheep. Instead, the relatively sparse density of Afro-hair, combined with its springy coils actually results in an airy, almost sponge-like structure that in turn, Jablonski argues,^[52] more likely facilitates an increase in the circulation of cool air onto the scalp. Further, wet Afro-hair does not stick to the neck and scalp unless totally drenched and instead tends to retain its basic springy puffiness because it less easily responds to moisture and sweat than straight hair does. In this sense, the trait may enhance comfort levels in intense equatorial climates more than straight hair (which, on the other hand, tends to naturally fall over the ears and neck to a degree that provides slightly enhanced comfort levels in cold climates relative to tightly coiled hair).

Further, it is notable that the most pervasive expression of this hair texture can be found in sub-Saharan Africa; a region of the world that abundant genetic and paleo-anthropological evidence suggests, was the relatively recent (≈200,000-year-old) point of origin for modern humanity. In fact, although genetic findings (Tishkoff, 2009) suggest that sub-Saharan Africans are the most genetically diverse continental group on Earth,

approaches ubiquity in this region. This points to a strong, long-term selective pressure that, in stark contrast to most other regions of the genomes of sub-Saharan groups, left little room for genetic variation at the determining loci. Such a pattern, again, does not seem to support human sexual aesthetics as being the sole or primary cause of this distribution.

The EDAR locus

A group of studies have recently shown that genetic patterns at the EDAR locus, a region of the modern human genome that contributes to hair texture variation among most individuals of East Asian descent, support the hypothesis that (East Asian) straight hair likely developed in this branch of the modern human lineage subsequent to the original expression of tightly coiled

Specifically, the relevant findings indicate that the EDAR mutation coding for the predominant East Asian 'coarse' or thick, straight hair texture arose within the past ≈65,000 years, which is a time frame that covers from the earliest of the 'Out of Africa' migrations up to now.

Disease

See also:

Hair diseases

Premature greying of hair is another condition that results in greying before the age of 20 years in Europeans, before 25 years in Asians, and before 30 years in Africans.^[60]

Hair care

Hair care involves the hygiene and cosmetology of hair including hair on the scalp, facial hair (beard and moustache), pubic hair and other body hair. Hair care routines differ according to an individual's culture and the physical characteristics of one's hair. Hair may be colored, trimmed, shaved, plucked, or otherwise removed with treatments such as waxing, sugaring, and threading.

Removal practices

.

Shaving

. Stubble typically appears to grow back thicker because the shaved hairs are blunted instead of tapered off at the end, although the hair never actually grows back thicker.

Waxing

Waxing involves using a sticky wax and strip of paper or cloth to pull hair from the root. Waxing is the ideal hair removal technique to keep an area hair-free for long periods of time. It can take three to five weeks for waxed hair to begin to resurface again. Hair in areas that have been waxed consistently is known to grow back finer and thinner, especially compared to hair that has been shaved with a razor.

Laser removal

Laser hair removal is a cosmetic method where a small laser beam pulses selective heat on dark target matter in the area that causes hair growth without harming the skin tissue. This process is repeated several times over the course of many months to a couple of years with hair regrowing less frequently until it finally stops; this is used as a more permanent solution to waxing or shaving. Laser removal is practiced in many clinics along with many at-home products.

Cutting and trimming

Because the hair on one's head is normally longer than other types of body hair, it is cut with

. People with longer hair will most often use scissors to cut their hair, whereas shorter hair is maintained using a trimmer. Depending on the desired length and overall health of the hair, periods without cutting or trimming the hair can vary.

Cut hair may be used in wigs. Global imports of hair in 2010 was worth $US 1.24 billion.^[61]

Social role

Uffizi Gallery

. It shows a plucked hairline that gives a fashionably noble brow.

Hair has great social significance for

.

The world's longest documented hair belongs to Xie Qiuping (in China), at 5.627 m (18 ft 5.54 in) when measured on 8 May 2004. She has been growing her hair since 1973, from the age of 13.^[64]

Indication of status

Healthy hair indicates health and youth (important in

was a male hairstyle in which the hair at the front and top was shaved every 10 days in a style mimicking pattern baldness, while the remaining hair at the back was braided into a long pigtail.

A hairstyle may be an indicator of group membership. During the English Civil War, followers of Oliver Cromwell cropped their hair close to their head in an act of defiance against the curls and ringlets of the king's men, which led to them being nicknamed Roundheads.^[65] Recent isotopic analysis of hair is helping to shed further light on sociocultural interaction, giving information on food procurement and consumption in the 19th century.^[66] Having bobbed hair was popular among the flappers in the 1920s as a sign of rebellion against traditional roles for women. Female art students known as the Cropheads also adopted the style, notably at the Slade School in London. Regional variations in hirsutism has caused practices regarding hair on the arms and legs to differ. Some religious groups may follow certain rules regarding hair as part of religious observance. The rules often differ for men and women.

Many subcultures have hairstyles which may indicate an unofficial membership. Many

, and younger hipsters in the 2000s and early 2010s.

Heads were shaved in

In the time of Confucius (5th century BCE), the Chinese grew out their hair and often tied it, as a symbol of filial piety. Regular hairdressing in some cultures is considered a sign of wealth or status. The

Rastafari movement

were despised early in the movement's history. In some cultures, having one's hair cut can symbolize a liberation from one's past, usually after a trying time in one's life. Cutting the hair also may be a sign of mourning.

Tightly coiled hair in its natural state may be worn in an

) dominant culture are not absolute. African Americans as a whole have a variety of hair textures, as they are not an ethnically homogeneous group, but an ad-hoc of different racial admixtures.

The film

Religious practices

Women's hair may be hidden using

madhabs, the donning of a kufi or topi is a form of sunnah.^[75]

In Arabic poetry

Since ancient times, women's long, thick, wavy hair has featured prominently in

Ibn al-Mu'tazz, who compared a lock of hair and a birthmark to a polo stick driving a ball.^[76]

See also

• List of hairstyles
• Body hair
• Chaetophobia
  – the fear of hair
• Hair analysis (alternative medicine)
• Hypertrichosis – the state of having an excess of hair on the head or body
• Hypotrichosis
  – the state of having a less than normal amount of hair on the head or body
• Lanugo
• Seta – hair-like structures in insects
• Bristle sensilla – tactile hairs in insects
• Trichotillomania – hair pulling

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External links

Wikimedia Commons has media related to Hair.

• Quotations related to Hair at Wikiquote
• The dictionary definition of hair at Wiktionary
• How to measure the diameter of your own hair using a laser pointer
• Instant insight outlining the chemistry of hair from the Royal Society of Chemistry
• PUIU, TIBI (23 August 2018). "How fast hair grows, and other hairy science". ZME Science. Retrieved 30 August 2018.