Hair is part of human hariness. Hair covers the top, sides, and back of the head and is made of keratin.
We have 100 000 to 150 000 hair; light-haired people have more than dark or red-haired individuals. Hair density is 150 to 200 hair per square centimeters.
On average, we lose 40 to 50 hair per day.
The follicle shape is a genetic characteristic, and it impacts the repartition of the keratin layers:
- A round stretched out perpendicular to skin surface follicles produces round and straight hair.
- An oval and comma-shaped hair follicle produces flat and curly hair.
- An elliptic and not perpendicular to the skin follicle makes afro hair.
Hair types depend on genetics. Asians have straight and thick hair, while Europeans have straight and thin hair. African have curly, frizzy, and thick hair.
In humans, hair color depends on follicles pigmentation. It is naturally determined by two types of melanin: eumelanin and pheomelanin.
Genetics and biochemistry
Pheomelanin causes red hair. L’eumelanin has two subtypes: black or brown. It determines hair color depending on its concentration; the more melanin, the darker the hair is. A low brown eumelanin concentration gives blond hair, while a low black eumelanin concentration produces grey hair. A higher concentration makes black hair. In some cases, a combination of brown and white hair gives the impression of grey hair when the hair is not grey. A better qualificative is salt and pepper hair.
Pheomelanin is chemically more stable than black eumelanin and less than brown, which oxides slower. It explains why hydrogen peroxide gives darker hair a reddish shade during an artificial decoloration process, then orange, yellow, and finally white.
Hair color genetics is not a sure thing yet. According to a theory, at least two pairs of genes are responsible for human hair color:
- A phenotype (brown/blond) has a brown dominant allele and a recessive blond allele. Someone with a brown allele has brown hair, while someone without it is blond. It explains why a brown-haired couple may have a blond kid.
- The other phenotype (red-haired/-not red-haired) has a dominant non-red-haired allele (which blocks pheomelanin production), while the red-hair allele is recessive. Someone with two copies of the red-hair allele is red-haired, but they would have auburn or red-oranged hair depending if the pair of the first gene has a brown or blond phenotype.
This model does not take all the possible shades into account. Blond people may be platin blonde, dark blond, or light brown. This theory neither explains why some people get darker hair when they grow older. Many gene pairs could produce a cumulative effect of light color versus dark color. Hair color is a multifactorial effect, and an individual genotype may interact with the environment to produce different phenotypes.
Dutch researchers developed a technique to determine someone’s hair color according to their DNA. This technique has a 90% liability rate on whether someone’s hair is black or red and 80% brown or blond. It may also tell if it is dark blonde, auburn, or Venitian blond. This technique helps know someone’s hair color from DNA collected in the bones in archeology.
Natural hair colors
Depending on the two melanin pigment concentration, it exists eight natural colors.
- Black. It is the most common color around the world.
- Brown. It is the second most common color.
- Venitian blond
In Hairstyling, hair color is classed according to the Eugene Schueller scale. It counts ten shades, from the darkest to the lightest.
- 2 Brown
- Dark chestnut
- Light chestnut
- Dark blond
- Medium blond
- Light blond
- Very light blond
- Platine blond
According to this scale, the shades of red hair are not base colors. There are also golden, cobber or ash reflections depending on the eumelanin pt pheomelanin concentration. However, there is a big difference between chestnut hair depending on their shade. Eumelanin and pheomelanin mix makes different shades depending on the balance of each molecule. A low concentration of eumelanin makes light chestnut hair, while a high quantity makes it dark. The same applies to blond hair.
Hair color repartition in the world
The most common hair color in the world is black, sometimes with brown reflections. This color is standard in Africa, Occidental Asia, and the mixed-race population of South America. Jet black is widespread in Asia (China, India, Japan) and Native Americans.
The Mediterranean population has brown hair with black. In Northern Europe and Central Asia, hair is lighter: chestnut, red and blond. They are even lighter in Russia, Scandinavia, and Baltic countries, where most people are blond.
This repartition does not consider migration, which brings all the colors worldwide.
Conditions changing hair colors
- Aging makes achromotrichia (hair depigmentation), which may be caused by Bcl2 and Bcl-w genes and the melanocytic stem cells.
- Canitie (Latin for grey hair)
- Gender. A study on 288 people showed no difference between men on women on eye and hair coloration.
- Medical condition: albinism, vitiligo, malnourishment, Werner syndrome, pernicious anemia.
Artificial hair color
Artificial hair coloration has existed since prehistory. Products to color skin and hair are the same as paint in the cave (blood, animal fat, coal, crushed rocks such as ocher). Egyptian women used henna leaves to color their hair and nails. Greek women used mineral pigments such as colored mud or vegetal ones such as henna. Roman cleaners sold lighting creams made of lead or cobber salts, nut husk, indigo, goat fat, or beech ashes. To darken their hair, they used lead combs. Roman wig makers made wigs with light hair from Gaulish women.
Bright hair coloration became popular with Emo, Punk, and Visual trends.
It is possible to dye hair with a chemical process. In a hair salon, a colorist is in charge of performing it. Colorists have a hairstylist education. Dying may be permanent or semi-permanent. Hair dye is also possible using natural products such as henna. Depending on how much time the product sits in the hair, the shades will be lighter or darker, going from light reddish reflections to red/auburn share on naturally chestnut hair. With current techniques, it is possible to get any shade, including blue and green. The most common hair dyes are European shades such as chestnut, blonde, red, auburn, or brown. Hair dye is now widespread worldwide, even among non-European populations.
Results depend on used techniques (complete dye or balayage, reflection), natural hair color, and nature (afro hair is more sensitive).
The natural aspect of the dye depends on the color: hair tends to be darker on the back of the head and in the neck, so keeping this specificity would make the result look more natural, especially for lighter shades. The complexion is also crucial in the results of hair dye. Blond looks better on fair complexion, auburn or red to milky skin, chestnut to medium complexion, and brown to someone with darker skin. Black sharpens face features and is harder to wear for Europeans. Blond hair looks unnatural on darker or Asian skins.
Oxidation dye, also known as a permanent dye, is the long-lasting hair pigmentation transformation (swapping natural hair pigments for artificial ones).
There is vegetal hair dye such as henna.
A couple of hair dye techniques exist: lock by lock, applying the color with a brush on the locks set on sticking papers, or the balayage without the sticking paper. In all cases, after the application, the hair is left to sit under a heating device, then the hair is washed with a scale sealer product.
A newborn has more than 1 000 hair follicles per square centimeter. This density decreases over time and is less than 500 between 30 and 50 years old.
Hair does not constantly grow, but it follows a cyclic rhythm that varies depending on the individual, age, and season.
The growing phase (anagen), regression (catagen), and rest (telogen) follow. The last phase ends with the death and the fall of the hair, quickly followed by the growth of a new hair.
The first hair growth cycle starts on the fifth month of pregnancy and lasts until the last hair loss.
In women, the cycle lasts for 4 to 7 years. In men, it only lasts for 2 to 4 years. Hair grows faster in women.
Hair counts two parts:
- The root is the non-visible, living part.
- The stem is the visible and biologically dead part, made of three parts: cuticle, cortex, and medulla.
A hair falls only two to three months after the death of the producing cell (spontaneous or pathological). Mechanic factors (traction, friction, or shampoo) may shorten this time.
Dead hair has a whole bulb, atrophied, white, and dry. Many people wrongly believe it is the hair root.
Hair growth speed depends on age, season, and genetics. On average, hair grows 0.7 to 2 cm per month. Straight hair grows faster, and afro hair slower.
In life, hair grows 10m, 1000 km if we put them all in a line.
Many causes, either physiological or pathological, may change this evolution. Male hormones determine the abundant growth of male hairiness (mustache and beard), but they slow hair growth on the head. It is why many men suffer from alopecia. This condition also affects women taking androgens or suffering from hormonal imbalance. In the Antiquity, eunuchs were never balds. Since then, scientists have proved testosterone’s responsibility in hair loss.
Hair does not grow from the tip but the roots. Cutting split ends helps the hair not break, so it looks like they grow faster and healthier.
The hair which falls grows back about 25 times in life (their average life span is about four years).
Repeated aggression, a poor diet, sickness, stress, air pollution may mess with the hair cycle.
Washing hair every day damages the hair and the scalp. It increases sebum production to protect the scalp. The more we wash hair, the more we have to.
Hair diameter depends on age, genetics, race. It goes from 40 to 100 µm.
We lose about 30 to 60 hair per day. In some cases, this number goes up to 100.
Hair loss is not only due to stress or a poor diet. Hair loss causes maybe both intern and extern. There are two categories: androgenic alopecia, also called baldness (hormonal and genetics cause), and telogen effluvium, also known as diffuse baldness (frequent hair loss caused by an organic disorder). Generally, genetic hair loss causes are permanent, while non-genetic ones are temporary.
The most used treatment to slow hair loss is mesotherapy, with vitamins and minerals mixes or platelet-rich plasma (PRP) and LED. In the case of baldness, the only effective solution is a hair transplant.
Hair, environment, and biomonitoring
Hair may suffer from sea salt and UV. It may also absorb or adsorb many air pollutants: metallic pollutants such as mercury or lead, especially fat-soluble pollutants such as volatile organic components, organochloride, and organophosphorus. They accumulate some toxins the body evacuated during their growth, such as heavy metals. It is how we can follow the miners’ intoxication or Guyana Natives’ intoxication with mercury analyzing their hair. Hair is easier to take, stock, and transport than biopsies or urine or blood samples. Their mercury content reflects the individuals’ contamination, and often the environment’s one.
Long after his death, scientists analyzed the heavy metals content in Beethoven’s hair and found he suffered from Saturnism.
Like sheeps’ wool, hair is an excellent natural absorbent; it is possible to know if the person consumed alcohol or drug analyzing their hair. Hair is used for biomonitoring of some pollutions (HAP metals, pesticides).
Hair, hygiene and health
In humans, hair is the louse’s habitat, spreading pathogens and playing a crucial role in immunity. Demodex, small acarids live in hair follicles. Trichology can measure hair health and its exposure to some conditions.
Significant stress or dramatic even is known for causing brutal hair loss except on white hair. It looks like the hair has become white.
Society and culture
A seduction element
In almost all civilizations, hair is significant.
Hair is often linked to intimacy, seduction, modesty, and sexuality. Interpersonal relationships through hair (combing, delousing, cutting) is a mark of affection.
A symbol of power
Depending on the period and the places, hair symbolized strength or/and virility (Samson’s hair) or feminity, showed once hidden. There was a time people believed stealing someone’s hair could make a love elixir or bewitch them. Rapunzel’s hair has healin power.
During various ceremonies worldwide, many ethnical groups dyed hair (using annatto in Amazonia and henna in the Middle East), covered with ashes or mud.
Long hair symbolized royalty for the Francs from the early Middle Ages.
In Europe, wealthy people often wore extravagant wigs. Long hair was a royal thing, while regular people started cutting their hair. In France, men could have long unpinned hair in public while women were socially expected to wear updo and headgear.
Later, young women could wear their hair unpinned. This custom could be from Great Britain. In 1855, English women launched a new trend of unpinned hair and gave up fat cream made of beef marrow and almond oil, more or less scented. Those creams greased the hair and made them look dirty and flat. At the time, European believed light curls were the “most beautiful races’ privilege.”
Modern armies request short hair or shaved head to prevent lice spreading, while cutting a samurai’s air would have been the ultimate offense. Hairstyle and hair length was significant for military and administrative casts in China, Korea, and Japan. Mandchous invaders imposed Chinese people to wear braids. Native Americans used their enemies’ scalps as trophies.
Relic and keepsake
Saints’ relic cult and love promises are often based on hair locks. People also believed if someone would find a hair, he could use it for witchcraft.
Some families kept baby teeth and hair locks of their children. In China, there was a time when cutting an adult’s hair was considered mutilation. Short hair was incompatible with some functions. Some monks cut their hair to symbolize renouncement and distancing to everyday life.
Hair and religion, mourning or will
In many cultures, not caring for hair was a sign of mourning. In many cultures, women were and are requested to hide their hair in public.
First haircut rituals
Some populations organized parties and rituals for the first haircut. It would happen either in early childhood, like for Jews, or later in life, after significant events such as marriage.