Hair and Fibers: Nature, Types, Structure and Examination

Hair and textile fibres are class evidence: they place a person or object in a category, not at a single identity. What makes them valuable is transfer. Locard's principle plays out cleanly with fibres because every contact between two textiles leaves shed fibres on both surfaces, and hair sheds in the hundred-per-day range from any scalp. In sexual assault investigations under BNS 64, in hit-and-run reconstructions, in burglaries where the offender brushes past a window grille, and in wildlife seizures where animal hair is the only species marker, this bullet pays for itself.

The forensic question is rarely "did the hair come from this person" (DNA answers that). It is more often "is this hair human or animal", "is this fibre from the suspect's jacket class", "is this transfer consistent with the alleged contact". The book chapter on fibre and textile evidencecovers the transfer and persistence side in depth; this examiners topic focuses on the anatomy, the classification, and the examination workflow that examiners ask.

A hair shaft has three concentric layers, ordered from outside in.

Cuticle. Overlapping flat keratin scales facing tip-ward. Three scale patterns are species-diagnostic.Coronalscales look like a stack of crowns and appear on fine rodent and bat hairs.Spinous(petaloid) scales project outward like petals and appear on seal, mink and some cat hairs.Imbricatescales lie flat in a roof-tile pattern and appear on human and most large-mammal hairs. The pattern is read by casting the hair on a clear nail-varnish slide, peeling, and viewing the negative cast under a compound microscope.

Cortex. The thickest layer, made of long spindle-shaped cortical cells packed with keratin. Embedded inside arepigment granules(melanin, giving hair colour and distributed differently in human, dog, cattle hairs),cortical fusi(air-filled spaces, more abundant in animal hair), andovoid bodies(oval pigment clumps). Cortical features are the workhorse for body-area and treatment determinations.

Medulla. The central canal. Four patterns.Continuous(a single unbroken core, common in animal hair).Interrupted(broken at regular intervals, common in some animals).Fragmentary(occasional patches, frequent in human scalp hair).Absent(no medulla, common in fine human hair, especially blonde). Themedullary index(medulla diameter divided by total hair diameter) is a single-number test: less than 0.33 for human, greater than 0.50 for most animals.

Growth phases. Anagen is the active growing phase, lasts 2 to 6 years on scalp, accounts for about 85 percent of hairs at any moment. Catagen is a 2 to 3 week transition phase, only about 1 to 2 percent of hairs. Telogen is the resting phase, lasts 2 to 4 months, accounts for 10 to 15 percent, ends with the hair shed. The forensic significance is in the root. A telogen hair has aclub-shaped, dry root with no follicular tagit was naturally shed and yields only mitochondrial DNA from the shaft. An anagen hair pulled out forcibly carries afollicular tag of epithelial tissuethe source of nuclear DNA usable for STR profiling.

A hair under the comparison microscope can support a series of determinations, ranked here from most reliable to most contested.

Human versus animal. Medullary index, cuticle scale pattern, pigment distribution and root shape together give a confident answer. Human hair has medullary index less than 0.33, imbricate cuticle, evenly distributed fine pigment and a bulbous or club root. Animal hair has medullary index greater than 0.5, often coronal or spinous cuticle, banded or peripheral pigment and a brush-like or spade root.

Body area. Scalp hair is long, fine, soft, oval cross-section, gradual taper. Pubic hair is shorter, coarse, wiry, more curved, with a triangular cross-section and variable diameter. Axillary hair is bleached at the tip from sweat, often split. Beard hair has a triangular cross-section and a blunt cut tip. Eyelash and eyebrow hairs are short, stubby, sharply tapered, never naturally cut. Body hair is short, fine, gently arched.

Race or ancestry (treat with caution). The historical three-bin scheme (Caucasian straight to wavy hair with oval cross-section and even pigment, African flat ribbon-like hair with coiled or kinked form and dense clumped pigment, Asiatic round cross-section, straight, coarse, peripheral pigment) is still in older Indian textbooks and still appears in forensic practice papers. Modern forensic practice has largely moved away from ancestry calls on hair because of overlap, intermixing and the social misuse risk. Recognise the textbook scheme for purposes; know that it is controversial.

Age. Limited reliability. Juvenile hair tends to be finer with less pigment, geriatric hair shows medullary changes and greying. No precise age call is defensible.

Sex. Barr body (sex chromatin) in the nuclear sheath of a freshly plucked hair root used to be cited as a sex marker. DNA profiling has replaced it entirely.

Treatment, dyeing and bleaching. Bleaching damages the cuticle, removes pigment and leaves a characteristic line where new pigmented growth meets the bleached section. Dyes show as a uniform colour penetrating the cortex, often with a sharp boundary at the time of treatment. Permanent waving distorts the cuticle. These features date the treatment relative to the hair length and growth rate.

Forcibly removed versus naturally shed. The single most useful determination in violent crime. A forcibly pulled hair has an anagen root with an irregular, stretched shape and afollicular tag of clinging tissue. A naturally shed hair has a telogen club root with no tissue. The follicular tag carries enough nuclear DNA for STR work; the shaft alone is restricted to mitochondrial DNA (maternal lineage, lower discrimination).

Textile fibres divide into natural and synthetic, with synthetics splitting again into regenerated cellulose and true polymer fibres. The classification tree is direct material.

Natural animal fibres are protein-based (keratin in wool and hair, fibroin in silk). Wool shows a clear scale pattern under microscopy; silk is triangular in cross-section and has no medulla. Cashmere, mohair and alpaca are specialty animal fibres with distinct scale and diameter signatures.

Natural vegetable fibres are cellulose-based. Cotton shows a flat, twisted ribbon shape (the convolutions) under polarising microscopy. Jute, flax (linen), hemp and sisal are bast fibres with characteristic nodes and lumen profiles; coir is the coarse coconut husk fibre.

Natural mineral fibres are asbestos. The 2011 Supreme Court ruling and subsequent regulation effectively bans chrysotile use in India for new applications; legacy material still appears in arson and occupational disease casework.

Regenerated cellulose synthetics. Rayon (viscose) and acetate are made by dissolving cellulose and re-extruding it. They are cellulose chemically but synthetic in form, so they sit awkwardly in the tree.

Synthetic polymer fibres. Nylon (polyamide), polyester (PET), acrylic (polyacrylonitrile), polypropylene, spandex (polyurethane elastomer) and kevlar (aramid) are the headliners. Each has a characteristic FTIR fingerprint, a distinct refractive-index pair under polarising microscopy, and a pyrolysis GC-MS signature.

Inorganic and specialty. Glass fibre, carbon fibre and metal threads sit outside the protein-cellulose-polymer split. They are identified by SEM-EDX for elemental composition.

The hair and fibre workflow is staged from least to most destructive.

1. Visual and low-power stereomicroscopy. Sort hairs from fibres, count, photograph, record colour and length.
2. Compound microscopy, longitudinal and cross-section. Read cuticle pattern, cortex features, medulla type. Cast the cuticle on nail-varnish for the scale pattern when needed.
3. Polarising microscopy. Measure birefringence (the difference between the two refractive indices). Nylon, polyester and acrylic each fall in their own birefringence band; cotton shows low birefringence with reversing twist sign.
4. Comparison microscopy. Side-by-side dual-stage microscope for known versus questioned hair or fibre comparison.
5. FTIR or micro-FTIR. Infrared spectroscopyreads functional groups: the amide bands separate nylon (polyamide) from polyester (carbonyl ester), the C-H stretches and bend pattern separate acrylic from polypropylene. Micro-FTIR runs on a single fibre under a microscope objective.
6. Raman microspectroscopy. Ramanexcels at dye identification because dye chromophores often have strong Raman cross-sections and weak IR signals. Useful when two fibres match by polymer but might differ by dye.
7. Pyrolysis GC-MS. Pyrolysis coupled to GC-MSthermally breaks the polymer into monomer and oligomer fragments, identifies the polymer unambiguously, and characterises dye breakdown products. The reference technique for synthetic fibre identification.
8. SEM-EDX. Scanning electron microscopy with energy-dispersive X-raygives morphology at high resolution and elemental composition. Used for metal threads, mineral fibres, glass fibres and inorganic loadings in polymers.
9. UV fluorescence. Some dyes and most optical brighteners fluoresce under long-wave UV. A quick screening tool, useful for sorting many fibres at once.

Every sample, transfer and aliquot is tracked on a chain of custodysheet from collection to report. Breaks in custody on trace evidence are the easiest defence target because each fibre is, by definition, small and easy to lose.

The trace-evidence division at CFSL Chandigarh anchors the central system for hair and fibre work, with parallel trace cells in state SFSLs. In sexual assault cases under BNS 64 (replacing IPC 375 and 376), hair and fibre transfer between victim and suspect clothing is part of the standard SOCO collection: tape lifts on the outer garments, combings of pubic and head hair from both parties, and bagging of clothes in paper. In hit-and-run reconstructions, fibre transfer from victim clothing to vehicle and hair transfer to the underside of the bumper or wheel arch are routine yields. Wildlife forensics under the Wildlife Protection Act 1972 leans on hair examination to distinguish protected-species hair (tiger, leopard, snow leopard) from domestic animal hair on seized skins and trophies.

DNA from hair has a clean two-track rule in practice.Nuclear DNA is recovered from a hair with afollicular tagof tissue at the root (forcibly removed anagen hair). This allows STR profiling and individualisation.Mitochondrial DNA is recovered from thehair shaftalone (any hair, including telogen shed hair). Mitochondrial DNA is inherited maternally, has a lower discrimination power than autosomal STRs, and identifies a maternal lineage rather than an individual. Remember both rules and the matching DNA type.