Forensic hair analysis – Trichology is the study of hair. Hair grows at the rate of 0.4 mm/day and nails at 0.1 mm/day. The examination of the hair is undertaken to find out:

(1) Is it Hair or is it Some other Fiber? Hair consists of bulb or root and a shaft. Considerable force is required to pluck out a lot of healthy growing hair from scalp. An adult can be lifted or dragged by the hair and the scalp may even be torn from the skull. In most hair, there are three well-defined layers.

  • CUTICLE: This is the outer layer and consists of thin, non-pigmented scales.
  • CORTEX: This is the middle layer and consists of longitudinally arranged, elongated cells without nuclei. Within these cells are fibrils on which theme may be granules of pigment.
  • MEDULLA: This is the inner layer composed of keratinised remains of cells.

FIBRES: COTTON fibres are flattened and twisted tubes. They consist of long tubular cells, with thickened edges and blunt, pointed ends. LINEN fibres show cross lines or folds about which the fibre is often swollen and has a narrow lumen. Fibres are straight and taper to a point. JUTE fibres are smooth without transverse lines. The cell cavity is not uniform. The ends are blunt.

Forensic hair analysis
Forensic hair analysis

SILK consists of long clear threads without any cells. They are smooth and finely striated. Wool fibres show an outer layer of flattened cells and overlapping margins. The interior are composed of fibrous tissue but sometimes medulla is present.

(2) Human or animal: Colour changes along the hair shaft called “banding” is seen in some animals.

Medullary Index of Hair: It is the ratio of diameter of medulla and diameter of the whole hair shaft. In humans it is less than 0.3 and in animals more than 0.5 The value varies in the hair of different parts of the body and as such it is also helpful to know the part of the body from which it is derived.

(3) FROM WHAT PART OF THE BODY DERIVED? Hair from the head is usually long and soft and taper gradually from the root to the tip. The beard and moustache hair are usually thicker than the hair of any other part of the body. Hair of the eyebrows, eyelashes and nostrils is stiff, thick and taper to a point. The hair on the chest, axillae and pubic region is short, stout and curly. Hair from the axillae and pubic region also show split ends. The hair on the other parts of the body is fine, short, and flexible and does not show pigment cells in the cortex.

(4) Sex: Sexing of human hair is difficult, except that of the beard and moustache. Male hair is usually thicker, coarser and darker. In human head hair, Barr bodies are found in hair follicles in a proportion of 29 ± 5 percent in females and 6 ± 2 percent in males.

(5) Age: Age can be determined sometimes from the hair, but only within wide limits, as between that of an infant or an adult. Roots of hair from children will dissolve rapidly in a solution of caustic potash, but in older people roots will resist the treatment.

Age                                       Diameter

Twelve days                        0.024 mm

Six months                          0.036 mm

Fifteen years                       0.053 mm

Adults                                   0.07 mm

The body hair of the human foetus and the newly born child is fine, soft, non-pigmented (colourless) and non-medullated. This lanugo hair is replaced by hair which is coarser, pigmented, medullated, and has a more complex scale pattern. At puberty axillary and pubic hair grows which is at first fine, soft and curly and later becomes coarse, and pigmented.

Adult hairs have maximum pigmentation. Loss of scalp hair in men starts from the third decade. In women, there is often loss of axillary hair and an increase of facial hair, at about the menopause. Grey hair usually appears after forty years.

(6) Has the hair been altered by dyeing, bleaching or disease? Bleached hair is brittle, dry, and straw-yellow. If the hair is coloured, the colour will not be uniform, the roots are of different colour and the hair rough, brittle and lustreless. The scalp will also be coloured. The colour of head hair will be different from the colour of hair on other parts of the body. The length of extra-follicular part of an uncoloured zone is used to determine the time of the colour last applied.

Scalp hair grows at the rate of two to three mm. a week, average being two and half mm; beard hair has a slightly faster and other hair a slightly slower growth rate. Some hair can be examined chemically to find out any metal contained in the paint. Dyed hair shows characteristic fluorescence with ultraviolet light. With polarised light microscope, the undyed part appears much brighter than the rest.

Blood Groups: ABO groups can be determined in a single hair if hair blub is present, from any part of the body by a modified absorption-elution technique or mixed agglutination technique with hundred percent accuracy.

Identification: Hair cannot provide a permanent record for identification, because the distribution and concentration of trace elements along the shaft of a hair varies as the hair grows. The colour of the hair may alter with disease. It is lighter in patients with malnutrition, ulcerative colitis, and Kwashiorkor; the normal colour appears when health is restored.

The colour of hair becomes green from copper, blue from cobalt, and yellow from picric acid poisoning. The hair of copper smelters may be greenish, indigo workers and cobalt miners blue and aniline workers bluish. The colour of hair alters sometimes after burial.

(7) Is the hair identical with the hair of the victim or the suspect? By careful comparison, one can say that the hair could have come from a particular person. Debris, grease, etc., adherent to the hair is very important.

Hair is usually mounted on a glass slide for examination in a comparison microscope. For preparing cross-section, it is embedded in a wax or resin block and sliced finely. The impressions of the cuticle scales are made on cellulose acetate. Microscopically, the intimate structure of the dyed hair appears hazy, and shows uniformity in general shade which is not seen in hair of natural colour.

Because of diet and drug intake and atmospheric conditions, traces of eighteen elements are deposited in our hair in proportions quite different from other persons, which can be measured through neutron activation analysis. Only three out of one lakh persons will have comparable amount of the nine major trace elements.

When hair is irradiated in a nuclear reactor, elements are converted to radioactive isotopes. Comparison of the radiation emitted from the hair with known standards provides quantitative comparison. Electrophorctic and electrofocusing methods to study proteins and enzymes in hair root sheath and matrix proteins are of considerable importance.

(8) Did it fall naturally or was it forcibly removed? The base must be examined to see whether the root is present. If the hair has fallen naturally, the root will be distorted and atrophied, and the root sheath absent. If the hair is forcibly pulled out, the hair bulb will be larger, irregular and the sheath will be ruptured.

(9) What is the cause of the injury? If the hair has not been cut, the tip is pointed and non-medullated, but repeated injury to the tip damages the cuticle, due to which the exposed and unprotected cortex splits and frays. The hair of axilla, pubis and frequently brushed hair has ragged ends. A blow with blunt objects crushes the shaft with flattening and splitting. A sharp weapon produces a clean uniform cut surface.

Recently cut hair shows a sharply cut edge with a projecting cuticle. After a week, the end becomes square, smooth and later rounded but blunt. After three to four months, the end becomes elongated, but the medulla is absent. Hair may get singed due to burns or firearm injury. Singed hair is swollen, black, fragile, twisted or curled and has a peculiar odour; carbon may be found deposited on it. The tip is swollen like a bulb.

Medicolegal Importance:

(1) Hair is important in crime investigation, for it remains identifiable on the clothes, body and the alleged weapons in crimes committed long before. It often provides the only connection between a weapon or even the accused and the victim of an assault.

  • Motor vehicles responsible for injuries may be identified by the detection of hair on the vehicle.
  • In rape and sodomy, the pubic hair of the accused may be found on the victim or vice versa. In bestiality, animal hair may be found on the body or underclothing of the accused, or his pubic hair may be found about the genitals of the animal.
  • Stains on the hair may indicate the nature of the assault, e.g., mud stains in struggle, seminal stains in sexual offences, salivary stains in asphyxial deaths, blood stains in injury, etc. Stains may be got from the walls, doors, furniture, etc., and may indicate the scene of crime.

(2) Nature of weapon can be made out from the injuries to the hair, and hair bulb.

(3) Hair is useful in identification especially when there has been some known peculiarity of the hair, dyeing, bleaching or artificial waving.

(4) Age of a person may he determined from the growth of hair on different parts of body.

(5) Sex may be determined from their distribution on body, texture and from Barr bodies.

(6) Singeing of the hair indicates burns or a close range firearm injury.

(7) It is helpful in differentiating scalds from burns.

(8) In chronic poisoning with heavy metals, e.g., arsenic, the poison can be detected in the hair.

(9) The time of death can sometimes be determined from the length of the hair on the face.