The science of hair

Nancy Mendoza
7 November 2002

Almost without exception all humans have hair covering the entire surface of their body (except for the palms of our hands, the soles of our feet, and our eyelids and lips).

There are two distinct types of hair: terminal and vellus. Terminal hair is coloured and visible, and can be divided into two further types: i) Long, soft hair such as that of the head, beard, pubic and axillary (armpit) hair; ii) Short, stiff hair that makes up our eyebrows and eyelashes. Vellus hair (also called Lanugo) is the downy colourless hair that we have everywhere else.

What is hair made of?

The vast majority of the material making up hair is a protein called keratin. This is also the main constituent part of nails, scales and claws and is also prolific in the epithelial cells of skin. The 3D structure of Keratin is maintained by numerous sulphur-to-sulphur covalent bonds. This makes it very strong. It is these sulphur atoms which make hair smell so disgusting when it burns. Hair is also essentially 'dead' being mostly made up of dead cells and protein deposits.

Hair consists of three layers:

  • The outer layer is the cuticle; this is where we find most keratin. This appears a little like the scales on a snake because it is made up of very thin, dead cells that overlap one another in a regular way. The 'scales' all lie in the same direction, which is why when you slide your finger along a hair towards your scalp it feels rough. This provides the inside of the hair with protection and is the most difficult part of the human body to destroy - hair is one of the final tissues to decay after someone dies.

    mummified hair
    mummified Egyptian grave robber- probably buried alive

  • The cortex is the inner part of hair and is made up of dead, elongated cells, which give hair its shape, flexibility and elasticity. It is also the place where small amounts of oil are stored and therefore without it, hair is dry and dull looking. Small pigment granules are found here as well - they show through the outer cuticle layer to give hair its colour.
  • Finally, hair has a central core called the medulla. This can appear hollow because the two rows of cells that fill it are large and vacuous. These cells give hair strength and breadth. Hair that is weak, and the downy Lanugo hair, are both deficient in this layer.
  • How does hair grow?

    Growth begins in the hair bulb where actively dividing cells move up and out to become new hairs. The rate at which this occurs depends on the time of day (it is faster in the light hours), the time of year (faster in the summer), the stage the hair has reached in its lifetime, and the ethnicity, sex and age of the person.


    The Three Ages of Man by Giorgione. c.1510. Oil on canvas. Palatine Gallery, Florence, Italy.

    Actively growing hairs with increasing amounts of pigment are in what is termed the 'anagen' phase of growth. Once active growth is over, the hair enters the 'telogen' phase, during which the follicle shrinks and the hair loses pigment. If brushing or washing does not remove the hair at telogen phase, a new hair emerging pushes it out - either way the hair is lost at this stage.

    The direction in which hair grows is important. If the follicle points toward the surface of the skin and the hair shaft is round, the hair is straight. If the follicle is curved and the hair shaft is oval then the hair will be curly. A very curved follicle and a flat shaft result in 'kinky' hair.

    How can hair have different colours?

    As mentioned before, there are pigment granules in the hair cortex. This pigment is called melanin and comes in two forms: eumelanin and pheomelanin. Eumelanin is the pigment we all associate with a suntan. In the skin it acts as a protective factor against UV rays, but in hair it gives rise to black colour. Ginger and brown hair colours are due to pheomelanin (more pheomelanin than eumelanin occurs in the skin of red-haired people and this is why they often suffer badly from sunburn).

    Unfortunately for some, the secret of growing blonde hair is little understood. The particulars of its pigmentation are yet to be discovered…

    Hair extensions for sale Photo by Flora Roberts

    "Her long loose yellow locks lyke golden wyre, Sprinckled with perle, and perling flowres atweene, Doe lyke a golden mantle her attyre." [Spenser, Epithalamion, st.9] Melanin is produced around the base of the hair by specialised cells called melanocytes for incorporation into the growing shaft. It is the progressive decrease in activity of these cells that causes hair to go 'grey' in old age. The grey colour is in fact the base colour of the hair that is left behind once the melanin ceases to be incorporated. In a somewhat contradictory way, the pigment production by melanocytes actually increases with age and therefore hair can become temporarily darker before turning grey.

    There is hope for those of us who would prefer not to have grey hair as one indication of progressing age. Robert Hoffman of AntiCancer has recently managed to turn the hair of lab mice bright green…

    Hoffman's team used an adenovirus (like the virus that causes colds) as a vector to introduce a jellyfish gene into some of the cells in the hair follicle. The gene incorporated into the cells produces a protein that is very bright green. This protein is put into the mouse hair as it grows, thus turning it green. Using this type of vector could therefore reintroduce the genes responsible for making melanin to the cells that produce grey hairs and turn them brown again. Hoffman plans to experiment with albino mice to try and find a way of reintroducing the gene that produces melanin pigments. Why have hair at all?

    The most significant reason for having hair is to keep warm. Though while other mammals still use their hair for this purpose, humans have lost a great deal of their covering.

    We have not however lost the ability to make our hairs stand up on end. A tiny muscle called the arrector pili is attached to each hair follicle. This muscle contracts when in a cold environment, or when fearful. The contraction of the muscle causes the hair to be pulled downwards so that it stands up straight. This also gives rise to the 'gooseflesh' appearance of the skin.

    In very hairy mammals the hair now traps a significantly larger volume of warm air. The effect is like that of layer of clothing keeping warmth in. When a mammal is scared the purpose of this is to appear bigger by extending the fur outwards as far as possible.


    sexy hair

    Of course, hair is also a useful courting tool. Not only can it be made to look beautiful in order to attract a mate, it is also a useful trap for attractive scent.

    In humans, this scent usually collects in the armpit and groin areas.

    “I loved you in the morning, our kisses deep and warm,
    your hair upon the pillow like a sleepy, golden storm...”
    [Leonard Cohen, Hey that’s no way to say goodbye]

    IMAGE: Love's Shadow by Anthony Fredrick Augustus Sandys (1832-1904)

    Finally, while some animals have evolved to have a colour and pattern of hair on their body which blends in with their background, humans have little need for camouflage.

    Coney Island beach, New York, 1930s, by Weegee (Arthur Felig)

    What does hair say about us?

    A single hair from your head could tell somebody else an awful lot about you. It can reveal information about your genes, your age and even if you are a vegetarian.


    If pulled out, so that the 'bulb' is still attached, hair can provide us with information about a mammal in the form of its DNA. The DNA in the cells surrounding the bottom of the hair contains all the genetic information for that particular organism.

    It is a tiny amount of DNA, but with modern amplification techniques the sample is big enough to be useful. One use for this is in forensic science where hair may be left behind at the scene of a crime and the DNA contained within the hair can be matched to a suspect.

    Also, collecting a few hairs from a child is much less invasive that providing a blood sample to be used in a paternity dispute, making this a convenient sampling technique.

    And if an age ever comes where we all have our genomes registered, or analysed to predict our futures, then a single hair could be used to produce the information necessary to do that.

    "But the very hairs of your head are all numbered." [Bible, Matt. ch.X, v.30] Hair that merely falls out is not suitable to use for a DNA sample as it is completely dead. Dead cells generally have significantly degraded DNA, such that a sample would be hard to obtain and extremely unreliable.

    Yeti seen through Tintin’s binoculars, from Tintin in Tibet, by Herge Copyright Herge Casterman

    The sequenced DNA from hair of unknown origin can tell us about the animal it was originally attached to. This is what Bryan Sykes, Professor of Human Genetics at the Oxford Institute of Molecular Medicine did to try and track down the owner of some hair supposed to belong to the 'Yeti'.

    'We found some DNA in it, but we don't know what it is,' Sykes confessed. 'It's not a human, not a bear nor anything else we have so far been able to identify. It's a mystery and I never thought this would end in a mystery. We have never encountered DNA that we couldn't recognise before.'

    Hair has also been used in this way by Professor Erika Hagelberg, at the University of Oslo, to support the argument that homo sapiens originated in Africa. Her lab is examining hair samples from the Andaman Islands. The results of the study will be published shortly. Already, her findings are causing some controversy.

    Carbon dating

    Hair contains carbon, and carbon can be used to tell how old a hair sample is.

    There are several types of carbon that exist in our environment. They have essentially the same chemical properties but are of a slightly different atomic mass. The main type of carbon found is 'carbon twelve' (having an atomic mass of twelve units). But there are also detectable amounts of 'carbon fourteen', which as its name suggests is slightly heavier (the alternative forms of an atom are called isotopes).

    Carbon fourteen is radioactive. That is, it decays to become carbon twelve over time and gives out ionising radiation as a by-product of the process. The ratio of carbon fourteen isotope to carbon twelve isotope therefore changes over time. By examining the ratio of the two carbon isotopes in a single human hair it is possible to tell how old the hair is. This is of course used in an archaeological context, when hair has often not decayed as much the rest of the body, to pinpoint how long ago the person died.

    "Dear, dead women, with such hair, too - what's become of all the gold
    Used to hang and brush their bosoms?" [Robert Browning, Men and Women - a toccata of Galuppi's st. 15]

    Őtzi, also known as 'The Ice Man' was found in the Austro-Italian Alps in 1993. Carbon-dated using DNA from his hip-bone, he lived c. 3350 - 3100 BC. According to Dr Elspeth Garman, from Oxford's Laboratory of Molecular Biophysics, Őtzi is thought to possess the oldest example of preserved human hair. Spectral analysis has been used to compare the centre and edge of a cross-section of Őtzi's hair and the results show that a significant quantity of copper has been deposited on the outside (showing that he almost certainly smelted the copper for his own axe). His hair, along with clothes and tools found with this incredibly well-preserved body, adds greatly to our picture of the Neolithic period in European pre-history.

    you are what you eat
    Eat your greens?

    Our diet and environment

    Hair has a way of recording the nature of our diets. Isotopes of atoms that do not decay radioactively remain in essentially the same ratio, even after thousands of years. These isotopes occur in food at particular ratios and leave a kind of signature on our hair. Through examination of stable isotopes of carbon, nitrogen and sulphur it was decided that the 'Ice Man' lived on something similar to a modern day vegan diet.

    It seems that certain minerals have a habit of depositing themselves in our hair if they are not needed elsewhere. If humans are exposed to high levels of copper as a result of their occupation, or even in their diet, they sometimes experience 'Green hair syndrome'. The green colour is due to copper deposits in the hair and disappears once the excess copper is removed.

    Why do some men go bald?

    When a man goes bald, it is not that he loses hair follicles. It's just that the hair produced by the follicles gets progressively finer and less coloured until it becomes invisible vellus-type hair. In effect, his hairs have been miniaturised.

    "Marinus, you ... attempt to hide the vast expanse of your gleaming bald pate with the few hair still left growing at your temples. But with every gust of wind the hairs scatter. ... Why not admit you are an old man? Be content to appear as you are, and let the barber shave off your remaining locks. There's nothing worse than a bald man who pretends to have hair." [Marcus Velerius Martial, Epigrams bk X, ep 83]

    It seems that each follicle is programmed to go through a finite number of growth cycles where the usual terminal hairs are produced. After this number of cycles the hair-producing cells are less numerous (or maybe just less productive) and are only able to produce tiny hairs.

    There is a hormone that is derived from testosterone called dihydrotestosterone (DHT). Hair loss in men only occurs where the scalp has a steady supply of DHT. It has been found that follicles in the areas of a man's head that goes bald have higher levels of the enzymes that manufacture DHT from testosterone than areas where hair loss does not occur (or occurs more slowly).

    So why don't all men go bald? Bruno Bernard of the L'Oreal Hair Biology Research Group thinks that the follicles on the heads of men that go bald get through their finite number of cycles much faster than the follicles of hairy men.

    cure for baldness
    Hey presto!

    There are various treatments available to increase hair growth in places where the hairs have become minaturised. This includes the rather drastic process of hair transplant and some less than successful drug treatments.

    There is potential for more successful solutions to hair loss in the form of gene therapies. But this may take some time: scientists with the expertise to carry out the research are more occupied with developing disease therapies.

    A gene that appears to contribute to baldness has been found on the X chromosome. Humans have two sex determination chromosomes each; women have two X chromosomes, and men have one X and one Y. This means that the X chromosome containing the baldness gene can only have been inherited from the man's mother. This corroborates the old wives' tale that men inherit baldness from their mother.

    Two myths explained:

    Does hair grow after death?

    No, definitely not. It may appear to grow but in fact any change in length is due to degredation of the flesh to expose extra hair length that previously existed under the skin (the same goes for finger nails). Hair does get lighter after death though; this is due to oxidation of the melanin pigments. Does shaving, plucking or cutting cause hair to grow faster and coarser?

    No, only hormones can control where, when and how hair grows (perhaps with the exception of surgery). The idea that short hair grows faster is to do with our perception of the change in length. Hair that began at 1cm long and grew 1cm in a month doubles in length; this appears to be a drastic change and is perceived as fast growth. Whereas hair that was 30cm long will not appear very different from hair that is 31cm long a month later and is therefore perceived to have grown more slowly.

    Long hair contest in Los Angeles in the Twenties.

    And finally…

    There are around five million hairs in total on an adult human body. The hair on the scalp accounts for approximately 100,000 of these.

    Hair goes through a growth cycle (which includes periods of rest where no growth occurs). An actively growing hair grows about 0.4mm each day. A single hair's growth cycle can last up to eighteen months - this time period is what determines the maximum length of an ordinary person's hair. There are rare people who have hairs with growth cycles of ten years that can reach almost two metres long.

    Hoo Sateow, a Thai medicine man with a 5.15 metre-long plait, holds the world record for the longest hair.

    A line up of the participants in the 1994 Chinese Long Hair contest. See more eerily long hair pics here

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