Hair

Extended human hair 200 times.

Feline hair, dermis cut.

The hair or hair is a continuation of the scalp formed by a keratin fiber and made up of a root and a stem. It forms in a follicle in the dermis, and is the characteristic feature of thin or thin skin. The difference between the keratin of the horny layer and the keratin of the hair is that in the hair the cells are always attached to each other, giving rise to a very hard keratin. Each of the hairs consists of a root located in a hair follicle and a shaft that projects upwards above the surface of the epidermis. The root is enlarged at its base. The papillary zone or dermal papilla is composed of connective tissue and blood vessels, which provide the hair with the substances necessary for its growth.

Hair is distributed over almost the entire body surface, except for the palmoplantar surfaces, the umbilicus, and the mucous membranes. In an adult, the approximate number of hairs is about five million, distributed unevenly throughout the body. There are about a million on the head, with between 100,000 and 150,000 on the scalp. The hair on the head maintains the body heat of the head and provides the skull with some protection against blows. The eyelashes protect the eyes by reducing the amount of light and dust that can penetrate them; and the eyebrows protect the eyes from sweat that may drip down the forehead.

Hair chemistry

Hair is made up of keratin (fibrous protein), which contains a high amount of cysteine within which is the amino acid cysteine that has a sulfur atom. "The keratin chains are arranged in parallel", which are held together by three types of links:

• Disulfuge bridges
• Hydrogen bridges
• Saline bridges between an acid and a base

Water has the ability to temporarily break saline and hydrogen bonds.

Hair Anatomy

Sebaceous gland and hair follicle.

Anatomically, hair has the same structure as any other type of hair, although implantation in the skin is deeper than in the rest, since the follicle reaches the hypodermis. The sebaceous glands are exocrine secretory organs that produce a fatty substance called sebum and empty into each follicle. They are located in the middle part of the dermis associated with the hair follicle to which they are going to flow. The sebaceous glands are distributed throughout the skin, except in the palmoplantar regions, and are very abundant on the scalp, face and upper chest, pubis, and armpits.

There are smooth muscle fibers associated with each hair (erector pili muscle). The contraction of the muscles causes the hair to stand on end, thus changing its angle in relation to the skin. This process increases the insulating possibilities of the hair cover, thus providing a better coat against the cold.

In the hair follicles of the armpits and genital areas there are also apocrine sweat glands, they are partly responsible for the characteristic body odor of each person.

The stem comprises three layers. The medulla, which consists of loosely packed keratinized cells, and which is present only in the coarsest hairs (guard hair). The intercellular space is filled with air. The medulla is surrounded by the cortex, tightly attached. In the cortex or cortex is the intermediate part of the hair, most of the pigment granules are fixed. Its surface has been covered with an integument, in which the cells may be attached or separated in the terminal portions, forming scales.

Hair undergoes continuous changes. When it finishes growing, the reproduction of the undifferentiated cells at the base of the follicle also stops, the root becomes progressively narrower, and the cells above the papilla undergo a process of cornification. Finally, the root separates from the papilla of the hair, and the hair falls out. Before it is shed, the formation of a new hair begins at the base of the follicle.

Growth phases

Hair does not grow indefinitely, but grows cyclically, which is called the hair cycle. Each follicle has its own cycle, independent of those around it.

• Anagenous phase or anagen: In this phase the hair is glued to the papilla, it is born and grows. It lasts between 2 and 8 years, although occasionally it is much older. The shape of the follicle in this phase is similar to that of an onion, wider in the base than in the stem. The hair grows continuously because the cells of the follicle matrix are constantly divided by mitosis.
• Catalan phase or catagen: It is a phase of transition. It extends about 3 weeks, during which growth stops and separates from the papilla, ceding the activity of the cells of the matrix, including the melanocytes. The bulb takes a cylindrical look.
• Fase Telógena or telogen: It is the stage of rest and hair loss, lasting about 3 months. The root of the hair looks like a match and remains inserted into the follicle.

Every 2 days and 9.5 hours, the hair grows at least one more millimeter.

Functions of hair in humans

In humans, hair has two functions:

• Protection: The hair protects the scalp from the sun and the cold. The eyebrows and eyelashes protect the eyes of the sweat that could fall from the forehead, the hairs of the nose prevent the entry into the dust nostrils, particles that may contain the air. It also serves for the cushioning of beats and tears. Manages insect bites and improves body temperature control by dissipation.
• Aesthetic: In all cultures hair has had a greater or lesser degree of care and importance. From its hairstyle, washed, collected and adorned to its total shave. The very or little presence of hair has been an aesthetic symbol of multiple meanings: strength, wisdom, experience, virility/feminity, freedom, slavery, fashion, religion, purchasing power, social status, political-philosophical ideology, cultural identity, among many others. Modify its appearance through dyes, permanent or smooth, as well as in various hairstyles, has accompanied fashion in a narrow way in all cultures.

Hair Types

In a unitary sense (that is, with regard to each hair), there may be different types of them:

• Cover hair, guard guard or jarra: it is the longest and straightest hair, with a tip ending; this type of hair prevents the heat from leaking.
• Under skin or era: it serves for thermal insulation, since it is very dense; it allows thermal exchange to the outside, or it prevents it.
• Sensory or vibes: is provided with vascular breasts and nerve fibers that wrap the base of the hair follicle, so they have a sensory function.

Hair Types

female hair

Depending on its structure

The structure of the hair is determined by the way in which the disulfide bridges are found, but deep down this is determined by information contained in the DNA:

• Liso, the or lisatric. The shape of the follicle is round and is vertically oriented to the surface of the skin forming a straight angle with it. This structure is because sulfur links are found in parallel.
• Undulated or cinosotric. It has oval shape and is oriented forming a sharp angle.
• Rope or ulotric. It has elliptical shape and the orientation is parallel to the surface of the skin. Sulphur links are diagonally found, leading to a spiral.
• Crespo or ulotric: It has elliptical shape smaller than the curly, usually in black, but it can also be blond in the case of albin people. The spirals that form are very small.

Depending on your epicutaneous emulsion

Hair can be classified according to the epicutaneous emulsion it presents in:

• Normal. Epicutaneous emulsion is balanced. The hair look is bright, soft and velvety.
• Dry. Epicutaneous emulsion contains little fat and little water. The hair looks rough and brittle.
• Graso. Epicutaneous emulsion has high fat content. The hair look is bright and sticky.
• Mixed: it's between an intermediate point between dry and fat. In this case the roots have greater fat and the tips tend to dry. Some fatty hairs suffering from dyes, discolorations (chemicals) become mixed hair.

Hair qualities

The qualities of hair are due to the filamentous structure of keratin. They are: elasticity, resistance, and porosity.

Elasticity

It is the ability of hair to stretch and return to its natural shape without breaking. In most chemical beauty procedures (dyes, bleaching, waving, scaling) the elasticity test is done so as not to overprocess the hair.

Resistance

Hair is strong because the cuticular cells are attached to each other with a complex chemical that acts like cement and also holds them together in the cortex.

Hair is resistant to:

• breakage: a healthy hair can withstand a load of 50-100 g; this can be altered by some chemical agents;
• heat: a hair resists temperatures of 140 °C when dry, and up to 200 °C when wet;
• the putrefaction: the structure of the keratin and its sulfur content make the hair very resistant;
• pH changes: when the hair fiber is subjected to extreme acid or alkaline solutions, it weakens.

Electrical properties

Dry hair is not a good conductor of electricity, and has a high anti-electric resistance. When combing or brushing is applied, the hairs accumulate static electricity and repel each other.

Porosity

It is the ability of hair to absorb liquids. The absorption of water produces a swelling of the fiber with an increase of 15-20% in the diameter and only 0.5-1% in its length.

Wet hair is slightly longer and fluffier.

The absorption of water and the swelling of the fiber depend mainly on the pH of the medium. The alkaline pH favors the swelling of the hair fiber.

Hair Color

The pigments responsible for hair color are melanins, which are produced by melanocytes in the matrix. Melanin accumulates within these cells in spherical organelles called melanosomes, bounded by a lipid membrane. Melanocyte activity is adapted to the hair cycle, and occurs during the anagen phase.

Hair Colors

• Black:

It is the darkest shade of hair, being in turn the most frequent due to a very high amount of eumelanin. It is together with brown hair the most common worldwide.

• Castaño:

Brown hair is characterized by higher levels of eumelanin and lower levels of pheomelanin. Of the two types of eumelanin (black and brown), brown-haired people have brown eumelanin; They also tend to have medium-thick locks.

Brown hair is a hair color that is a reddish shade of brown hair. In contrast to brown hair, the reddish color of the chestnut is darker. Brown hair is common among the native peoples of Western and Eastern Europe.

• Blonde:

Ranges from almost white (platinum blonde) to a dark golden blonde. Strawberry blonde, a mix of blonde and red hair, is a much rarer type that contains more pheomelanin. It can have almost any ratio of pheomelanin and eumelanin, but it has only small amounts of both. More pheomelanin creates a more golden or strawberry blonde color, and more eumelanin creates an ashy or golden color.

Many children born with blonde hair develop darker hair as they age, with most natural blondes developing a darker blonde hair color by the time they reach middle age. Pregnancy hormones speed up this process. Natural light blonde hair is rare in adulthood, with world population claims ranging from 2% of natural blondes

The blonde population predominates in the European continent, and more commonly in the Nordic countries, Russia and Anglo-Saxon countries. Studies conducted in 2012 showed that the natural blonde hair of Melanesians is caused by a recessive mutation in tyrosinase-related protein 1 (TYRP1). In the Solomon Islands, 26% of the population have the gene; However, it is absent outside of Oceania.

• Redhead:

Varies from strawberry blonde shades to light titian, copper, and less commonly pure red. It is caused by a variation in the Mc1r gene and is recessive. Red hair has the highest amounts of pheomelanin, around 67%, and generally low levels of eumelanin. Between 1 and 2% of the population has this hair color, making it the least common. It is most prominently found in the UK countries: Scotland, Northern Ireland, Wales, and England; and outside the kingdom, Ireland. Scotland has the highest proportion of redheads; 13% of the population has red hair and approximately 40% carry the recessive gene.

• Grey or white:

Hair color that generally appears in old age or even earlier, depending on different factors (stress, diet, heredity). They are known as gray hair. Albinism is the genetic condition in which there is a congenital absence of melanin in the eyes, skin and hair in humans and other animals caused by a gene mutation.

Pigments

Depending on the concentration of pigments in each person, hair color can vary, even the absence of specific amounts of melanin give hair color:

• Eumelanins.
  • Black Eumelanine: brings the black color.
  • Brown Eumelanine: provides the brown color.
• Pheomelanins: they bring triguen or blond colors.
• Tricochrome: they bring reddish colors and tones.

The appearance of gray hair is due to the loss of coloration of the hair, a phenomenon that is part of the natural aging process.

Other data

The most important variations in the physical appearance of the head are:

• the thickness varies about 80 μm (0.08 mm).
• accessories of the head
• hair color (original or artificial)
• the type of hair
• the haircut (i.e. the way in which the hair is organized or disorganized in each person, depending on the preferences both individual and cultural: curls, dreadlocks, braids, flakes, wigs, decorative pins).

Hair disorders

• Alopecia. It is a capillary alteration produced by hair loss:
  • Toxic baldness. It may appear as a result of a severe disease with high fever.
  • Alopecia areata. It is a disease in which hair is lost unexpectedly in a specific area, usually in the scalp or beard.
  • Androgenic alopecia. This baldness is hereditary and commonly suffered by men, but it can be commonly given to women during the autumn months. It is usually presented in the head, front, or crown, although because its genetic pattern does not have specific areas.
• Moniletrix. It is a disorder constituted by a regular variation of the thickness of the hair stem, giving the hair a look of rosary with a tendency to break prematurely at the points where it is thinner. It has a dominant autosomal hereditary pattern.
• Tricoptilosis. It is an alteration known for "open marks" (called colloquially "horquetillas"), the hair is opaque and glazed and that division prevents both combing and the hairstyles remaining in place once they have been achieved.
• Invaginated Tricorrexis. Known as bamboo hair. It consists of technical deformations and concaves of the stem that reminds the bamboo. They can occur traumaticly over normal hairs or be congenital. It could be due to a temporary effect of keratinization. This type of hair characteristic of Netherton syndrome.
• Acquired shrinking of the hair. Also known as "Pelo ensortijado compra" and in English as "acquired progressive kinking" is a hair growth disorder of the scalp in which the affected hairs are born from the root encrespados and with rotations on its axis, the hair is very similar in texture and thickness to the pubic hair but has greater length than this.

History

Egyptians

Diodorus of Sicily says that Osiris took an oath not to shave his head until he returned to his homeland. And this, he adds, is the origin of the constant practice among the Egyptians of not cutting their hair or beards from the day they leave their country until the day they return to their homeland. From this passage it can be concluded that the Egyptians habitually shaved their heads. Herodotus affirms this positively of the priests of that nation and adds that they shaved not only the entire head but also all the other parts of the body, fearing that they would profane the worship of the gods with some hidden dirt or with the presence of some insect born among the gods. hair.

As for the Egyptian women, it seems that they kept their hair, which they cut in a square shape on the neck and covered with a kind of very large cap.

The Egyptians also used to offer vows to the gods, like almost all other peoples, for the healing of their sick children and when they were recovered they took them to the temple, where they cut their hair, then putting it on a scale and offering equal weight of gold, the one that they gave to those who took care of the sacred animals.

Jews

Jews were subject to certain particular rules about wearing their hair. They were not allowed to cut it in the round because the Arabs, the Ammonites, the Moabites, the Idumeans and peoples of the Cedan, Tema and Blus wore them in this way in the shape of a crown in imitation of a unco. And to this alludes what Jeremiah says in cap, IX. v. 26, and what we read in Leviticus chap. XIX. v. 27. On certain occasions they were allowed to cut their hair, such as in case of leprosy or other diseases, in mourning, in time of penance, etc. They wore on their heads a kind of tiara like the Persians and Chaldeans.

Ancient Greeks

Among the Greeks, young people of both sexes did not cut their hair until they entered adolescence. The girls cut them off on the eve of their marriage and offered them as a rule to Artemis or the Fates. Young Greeks commonly consecrated their first hair to Apollo or Asclepius, and sometimes to Dionysus.

• the young climbs of both sexes consecrate their hair to Hipólito, who had died single
• the young people of Megara dedicated their first hair to Ifinoe daughter of Alcatoo, who died a virgin
• Sition, Aegean
• the island of Delos, Ecaergeo and his sister Opi
• Argos and Athens, Athena, etc.

Theseus first offered his hair to the God of Delphi, whose example was followed by all the distinguished young men of Athena. The Assyrians had a similar custom, offering the young the hair and the older the beard. The poor sometimes consecrated it to Hercules, or to any other god worshiped in Athens.

In early times this use was not constant and we see several heroes consecrate their first hair by a private vote to the deities who had taken special care of their childhood and many times, to the gods of the rivers. So it is that Achilles promised his to the river Esperchaeus if he returned safe and sound from the Trojan war: but having learned afterwards that he must perish in that place, Homer says that he cut off his hair and threw it on the pyre of his friend Patroclus. Memnon offered his to the Nile.

This use of the Greeks was imitated by the young Romans, who offered their first beard and hair to some deity. Dion tells of Augustus and Suetonius reproaches Caligula for having admitted this religious ceremony. Juvenal talks about the parties and invitations that were held in such cases.

Many times the young were satisfied with tying the first hair to the statue of the divinity to which they had been consecrated and Pausanias says that the statue of Igia was almost entirely covered with the hair that the women had hung or tied to it. of Sicion.

Baccantes in a Greek relief

Only the bacchantes among the Greek women were those who wore their hair wavy and without any restraint. Girls tied it on the forehead or behind the head: but married women usually tied it on the nape of the neck in a single braid that billowed over the shoulders.

The Greeks believed that the infernal gods cut a hair from mortals at the moment in which the Fates also cut the yarn of their life. Thus it is that death in Euripides appears armed with a sword in an attitude of cutting the fatal hair of the generous Alceste to make a victim consecrated to the infernal gods. Macrobius recognizes a similar imitation of this passage from Euripides in Virgil, which indicates the usage among the Greeks of cutting the hair of the dying.

The Greeks also used to cut their hair in mourning to throw it on the corpses and on the pyre of the people who had been dear to them: and when they had not been present at the funeral they would lay their hair on the tomb of relatives or friends, of whose use we could cite countless examples. Sometimes, not only the relatives and friends of the deceased gave this external proof of pain, but in certain cases an entire town cut their hair as a gift to some person whose loss was very sensitive to them. The Thessalians, according to Plutarch, all cut their hair at the death of Pelopidas, and the Persians after that of Masistius. Other towns went even further, cutting the manes of their horses in order to show that even those animals participated in their pain. Alexander was not content with making the Macedonians and their horses wear mourning for the death of his friend Hephaestion, but even wanted inanimate beings to show their feelings by having the battlements of the towers and walls torn down.

Ancient Rome

The Romans adopted this usage from the Greeks and Dionysius of Halicarnassus recounts that the Roman virgins and matrons who attended the funerals of the Daughter of Virginio threw their hair and the bandages on the funeral bed of that virtuous and unfortunate Roman. they were used to tie them.

Those who had been shipwrecked and lost all their belongings offered their hair to the gods of the sea as the only offering they could still present. We see in the Anthology that Lucilo after his shipwreck offers his hair to the numina of the Ocean, because he had nothing left but to make an offering. For this reason Petronius called the action of cutting one's hair the last vow of those who are close to being shipwrecked or who have been shipwrecked. For the same reason, that is, to acknowledge the gods, they cut the hair of those who had come out of serious illnesses or imminent danger and for this reason, those who were in this case let their hair grow until they were quite long. long to offer them.

The use of cutting one's hair and offering it to the gods of the sea, throwing it into the waves when they were in a storm, gave rise among sailors to the superstitious opinion that it was a bad omen to cut one's nails and hair in a ship unless it is in imminent danger.

Among the Romans we see that those accused of great crimes and those who resorted to the people against some powerful oppressor let their beards and hair grow as a sign of pain, which were cut off the day they were acquitted or on which they had obtained justice. Some also, in addition to wearing beards and long hair in times of affliction, covered it with ash or earth, like the Jews and other peoples.

Philologists disagree about the way slaves wore their hair. Some believed that all their hair was cut off, based on that Greek proverb of Suidas: are you a slave, and do you have hair?, while others believe that they also shaved the heads of those who slaves became freedmen, that is, before giving them the cap of freedom called pileus. This last custom was based on the religious offering that they made of their hair to the gods in recognition of having improved their luck. Ovid says that the hair of the slaves was used to make the false scalps.

The victors used to shave the heads of the prisoners as a sign of slavery and there were some so barbarous, like the Scythians when they invaded Palestine, that they not only shaved the hair, but also ripped the skin off the heads of many Hebrews: whose inhumanity the cruel Antiochus also executed against two of the Maccabees.

Through history we see that the ladies of Carthage cut their hair to make ropes that were needed to move the war machines and the Roman matrons made the same sacrifice in honor of their homeland and their freedom in another similar predicament.

The ancients used a hot iron called calamistrum to curl their hair. Among the Greeks and Romans, only married women and girls followed this usage. But among the Phrygians and among the other peoples famous for their effeminate customs, this fashion was common to both sexes. The Sicambrians and the Germans formed a single knot in her long hair, which was, according to Tacitus's testimony, her characteristic attribute. This way of tying hair became a proverb and Marcial indicates it with the words nodus rheni. The Armenians, the Saracens and some other Asian peoples tied their hair around their heads with various bandages or ribbons, forming a mitre. Parthians and Persians wore long flowing hair, as seen on some of their medals. The Arabs, the Abantians and the Mysians, as well as the Curetti and the Aetolians, cut their hair in front so that their enemies would not catch them in combat. The Gauls, according to Diodorus of Sicily, wore long hair that they washed very often with lime water. The Athenians who served in the cavalry let their hair grow, and so did all the Lacedaemonians, both soldiers and citizens.

The Romans, as can be deduced from their monuments, wore short hair, which they let grow in time of mourning. The Lacedaemonians wore them long and perfumed them with essences when going to battle.

The Medes and the Assyrians, according to Herodotus, and after them the Persians wore them ringed in front and stretched out on both sides on their shoulders. The numidas wore them ringed from the top of the head to the bottom. Athenian women and effeminate men from the same city curled and perfumed their hair and covered it with a kind of yellow powder. Lucio Vero, brother of the Emperor Marcus Aurelius, sprinkled some gold dust on his men.

Roman matrons used a kind of needle to part their hair, with which married women were distinguished from the maidens who wore it together. Not only did the ancients curl their hair with a hot iron, but they also sometimes sprinkled gold dust over them and tied them with threads or sheets of the same metal.

The Athenians put some golden cicadas in their hair. In the lower empire, men adorned themselves no less than women, because just like the latter they adorned themselves with gold pendants and precious stones, which they put on their hair.

The wise men and philosophers of Athens and Rome very often rebuked the use of curling their hair and declaimed highly against men who disgraced themselves with this effeminate luxury. Thucydides himself did not want young men to curl their hair, nor to wear it raised on their foreheads in a knot, as virgins or maidens used to do. Cicero, in his harangue delivered after his return to the Senate, designated Piso as a man given over to vice with the words cincinnatum ganeonem, kinky-haired libertine. He reprimanded the consul Gabino for the same defect, calling him saltator calamistratus , noting on his forehead the marks of the hot iron that had been used to form the rings of his hair: frontem calamistri vestigis notatum . Suetonius, describing Nero's vices, does not omit the great care that he put into composing his hair.

The ancients also knew the use of wigs or false hair, and they were called by the Romans galeri and galericuli. They were sometimes adorned with another kind of wig called corymbio, which imitated the headdress of virgins. They not only used wigs to hide their lack of hair, but also to present themselves with hair of a different color from the natural one or to disguise themselves. Caligula wore a wig and a long tunic to frequent the brothels: and the infamous wife of Claudius Messalina hid her black hair under a blonde wig when she spent the nights in houses of prostitution.

Blond wigs were highly appreciated in Rome and came from Germany and the northern countries of Europe. It is said that the art of dyeing hair was invented by Medea. The Goths and other northern peoples valued good hair and took great care to preserve it. Among women it was a sign of virginity, so the maidens wore their heads bare and their hair flowing, while the married women wore their heads covered.

The ancient Gauls kept their hair as a distinctive sign of honor and freedom, for which reason Caesar ordered them to be cut off after having subjugated them. The slaves had their heads shaved. The ecclesiastics and those who abandoned the world had their hair cut and offered their hair to God to prove their spiritual slavery and to show that they renounced all worldly honors, promising absolute subjection to God and his superiors. Abbe Fleuri says that at first the nuns of Egypt and Syria had their hair cut for cleanliness, while in other parts they kept them, the practices of antiquity being various in this particular.

In other times they swore by the hair, the same as now by the honor and to cut it to someone was contempt and ignominy. The accomplices in a conspiracy were condemned to cut each other's hair.

Middle Ages

The Frankish kings were given the epithet hairy because they used to wear a long head of hair, and earlier the same was given to young Romans who had not reached the age of puberty, until which At that time they let their hair grow, the same as the eunuchs and the priests of Cibeles; but not those of Bellona, nor of Ceres.

The ancient Franks cut their hair all the way around the head, keeping only a few on top of it. Only the princes of the royal family were allowed to wear their hair long and flowing over their backs, and some authors add that the different degrees of nobility of each one were known by their hair. To cut the hair of a prince or any Frank was not only to degrade him and separate him from his family, but also to exclude him from the nation or from the class of citizen, since only slaves wore their heads shaved.

It seems that part of these uses were not exclusive to the Franks, since in Spain we see that cutting a prince's hair also disqualified him from reigning. For this reason, the story says that when Ervigio tried to overthrow Wamba from the throne, he gave him a soporific drink that deprived him of consciousness for some time, during which he made him cut his hair and that when he came to, Wamba renounced the crown and withdrew. to the Monastery of Pampliega where he died in holy peace in the year 687.

Sajo the first breed of French kings, when greeting some person of consideration you could not give him a finer and more respectful gift than tearing out a hair and presenting it to him, with which action he manifested to be his most devoted slave, since a man when passing from the free state to that of slavery cut his hair and presented it to his master or lord.

Scientific applications

Hair samples are currently used for toxicological analysis and provide much more information than can be obtained from traditional matrix analysis, such as blood and urine samples. The hair sample gives us the possibility of establishing a chronological profile of the moment in which drugs are consumed or a person has been exposed to a certain toxin. It is used for medicines, drugs of abuse and also for some metals, which has been a great help and revolution in the field of forensic toxicology.

History

In 1858 Casper was the first researcher to use a hair sample for a toxicological analysis. In this case, he analyzed a corpse 11 years after his death and found traces of possible exposure to arsenic in it. Later, more information came to light about the death of other well-known historical figures such as Napoleon Bonaparte, who was found to have toxic substances including compounds such as arsenic 125 years after his death or the analysis of the well-known musician's own hair and composer Ludwig van Beethoven, where traces of lead were found years after his death.

Hair as a toxicological matrix

Traditionally, biological samples of blood and urine have been used for the evaluation of the presence of drugs of abuse for many decades. This type of sample offers a series of limitations regarding the extrapolation of the results, due to the fact that, when analyzing blood or urine, they only allow to know the values of drugs existing at the moment in which said samples are taken, so they do not the data obtained are correlated with the clinical status of the investigated subject. These new alternatives for the analysis of non-conventional matrices, such as hair, can give us information about the knowledge of a chronic drug addiction or a long exposure to certain toxins.

If, for example, a heavy metal is analyzed:

-If it is found on the tip of the hair only, it means that the individual was exposed to heavy metal in the past.

-If the heavy metal is in the root, it means that the individual is exposed at the present time.

-If it is seen throughout the hair, it means that the individual has been exposed to the xenobiotic for a significant period of time, intoxication called chronic.

For this reason, hair analysis is of great interest in the forensic branch of toxicology, since it can be useful in cases of divorce, corpses in a state of decomposition where one wants to know the substance to which they were exposed, doping control in athletes, cycling and other types of athletes or in cases of sexual assault or crimes.

Incorporation of the drug into the hair

All substances found in the environment or around us, whether they are medicines, drugs, pollutants or toxins, are susceptible to entering our body, passing directly into the bloodstream from the hair follicle. In order to spread the toxic to the blood circulation, when passing through the hair, it binds to its keratin matrix, so that as the hair grows, the toxic continues to accumulate in it.

Knowing that its approximate growth is more or less than 1 cm per month, segmenting it from the roots to the tips, we will be able to obtain data on the period during which drugs or toxins were added to it.. This method of analysis must take into account limitations such as irregular hair growth that varies from 0.7 to 1.5 cm depending on the individual.

The incorporation of substances into hair can be explained through a complex model, in which three different mechanisms are accepted, which are:

1. Incorporation from the bloodstream that nourishes the dermal papilla into the hair fiber or growing follicle.

2. Incorporation through sweat or secretions from the sebaceous gland that are located near the follicle.

3. Passive exposure to substances found in the environment of the intoxicated individual.

Hair Analysis Procedure

To carry out a correct toxicological analysis of the hair, the steps developed below must be followed, always taking into account the possible variations depending on the sample.

The hair sample is collected to carry out the analysis of the xenobiotic in question. The growth of each of the hair types must be taken into account to select the segment to be analyzed. Capillary hair regularly grows 0.33 to 0.60mm per day, pubic and armpit hair grows 0.25mm per day, beard hair 0.27mm per day and hair 1cm per month. The posterior vertex of the head is the ideal point to collect the lock of hair since it is here where there is less variability of the hair follicle. The samples will be stored in dry environments, without contact with direct sunlight and avoiding keeping them in refrigerators or freezers. It is recommended to select a thick lock the size of a pen.

Next, the segmentation of the selected strand is carried out. Measured segments of between 10 and 30 mm are cut, and in some cases this is followed by mechanical action such as crushing or pulverizing.

Next, the external contamination is eliminated by means of successive washings in the sample with solvents for a marked time. This is done to avoid interference from endogenous contaminants, as well as to avoid false positive results due to the possible existence of exogenous contaminants.

The resulting sample undergoes an extraction process that removes interferents and concentrates substances for further analysis (usually by liquid-liquid extraction and solid-phase extraction).

Finally, the instrumental analysis and/or analysis of scanning immunochemical techniques is carried out to obtain results in quantitative terms. The procedure is protocol, but it is always necessary to assess the representativeness of the sample and the improvement of its adaptation to the process of analysis of xenobiotic substances present in the hair.

Prospects for the future

Hair has some very valuable characteristics when analyzing the presence of a toxin in the body compared to other biological matrices such as blood, urine or tissue samples, but despite this it has a series of limitations related to the complexity of the analytical process, the time it takes to analyze it, the lack of precision in the determination of strands smaller than 1 centimeter, or the need to know more information about the subject being investigated. In addition to the impossibility of knowing the specific day of consumption or exposure to the drug or if the sample was externally contaminated with agents such as sebum or sweat. That is why in the future work is being done to overcome all these limitations.

Forensic toxicology is a field of study that must continue to grow and develop continuously to improve the reliability of the results of the analysis for the detection of toxic substances or metabolites, generating higher quality results.