Hair Examination and Microscopy

A hair shaft has three concentric structural zones. The outermost is the cuticle, a single layer of overlapping keratinised scales that protect the inner layers. The middle zone is the cortex, the bulk of the shaft, which houses the pigment granules and the structural proteins that give hair its mechanical properties. The innermost zone is the medulla, a column of loosely packed or hollow cells running along the centre of the shaft, which may be continuous, interrupted, fragmented, or entirely absent.

For forensic purposes, the cuticle scale pattern is the primary species-determination criterion. Three broad scale types are recognised. Coronal scales (crown-like, resembling a stack of cups) are found in the hairs of small rodents and bats. Spinous scales (petal-shaped projections pointing away from the root) are found in cats and some other carnivores. Imbricate scales (overlapping like roof tiles) are found in humans and many other mammals. Within the imbricate category, examiners assess the scale margin shape, the scale frequency (how many scales per unit length), and the degree of scale protrusion, all of which vary between species and can help narrow an unknown hair.

The cortex features most often compared between questioned and reference hairs are: the density and distribution of pigment granules (whether they are dense or sparse, evenly distributed or clumped toward the periphery or centre), the colour of the pigment (eumelanin producing brown-to-black tones, phaeomelanin producing yellow-to-red), the presence and size of cortical fusi, and the overall diameter of the shaft. Hair diameter varies across the scalp of a single individual, so a range of diameters rather than a single value is always reported.

The medullary index is calculated as the diameter of the medulla divided by the total diameter of the hair shaft, both measured at the same point using an eyepiece micrometer or digital image analysis. Multiple measurements are taken along the length of the shaft and averaged. The measurement is most reliable in the mid-shaft region, away from the root (where cortical fusi can be confused with medullary channels) and away from the tip (where wear and cosmetic treatment can alter the apparent diameter).

When a medullary index above 0.33 is found in a questioned hair that is otherwise classified as imbricate (potentially human), the examiner should consider whether the hair may be from a body region other than the scalp (some body hairs have slightly higher indices) or whether it may be an animal hair with imbricate scales. Comparison with a reference collection of known animal hairs is the standard method for resolving ambiguous cases. Many forensic laboratories maintain reference hair collections from domesticated and wild species common in their jurisdiction.

The medullary index alone is not sufficient for species determination. It is used in combination with scale pattern, cortex pigmentation, and overall shaft morphology. A hair classified as human on all criteria is then assessed for body region of origin before any comparison to a reference sample from a named individual.

Once a hair is classified as human, the examiner assesses which body region it most likely originated from. Scalp hairs are the most common questioned hairs and have the widest diameter range and most variation in natural colour. Pubic hairs are typically broader, have irregular diameters along the shaft, may show buckling or curve, have a coarser cortex texture, and often lack a continuous medulla. Eyebrow and eyelash hairs are shorter, have tapered tips, and show a distinctive decreasing diameter pattern from root to tip. Axillary and body hairs occupy intermediate positions.

Forensic hair examiners have historically described hairs in terms of ancestral or racial groupings based on cross-sectional shape and pigment distribution. Hairs described as having Caucasian characteristics tend toward an oval cross-section with evenly distributed, fine-to-medium pigment granules. Hairs described as having African characteristics tend toward a flattened cross-section, tighter curl, and denser, often peripherally distributed pigment. Hairs described as having Asian characteristics tend toward a circular cross-section, straight shaft, and dense, centrally distributed pigment. These are statistical tendencies across populations, not absolute categories: significant overlap exists between groups, mixed ancestry produces intermediate features, and cosmetic treatments alter appearance.

Morphological hair comparison is a class-level technique. A conclusion of "consistent with" means that the questioned hair and the reference sample share a set of microscopic features and that the examiner found no features that would exclude the known individual as a possible source. It does not mean the hair came from that person. Two people may be indistinguishable at the morphological level, and a single person's hairs vary across different scalp regions, at different life stages, and after cosmetic treatments.

The FBI review, which covered testimony given by FBI hair examiners in cases from the 1970s to the 2000s, found that examiners had overstated their conclusions in a large proportion of cases where testimony was reviewed. Common errors included: stating that a hair could be matched to a specific individual to the exclusion of others, citing a probability of a correct match without empirical foundation, and failing to disclose that microscopic hair comparison had never been validated with population frequency data. These findings prompted revision of testimony standards in the US, and similar reviews followed in Canada (the Royal Canadian Mounted Police review) and in the United Kingdom.

The correct formulation of a hair comparison conclusion has three possible outcomes: the questioned hair is consistent with the reference sample (the hairs share features and no exclusionary differences were found); the questioned hair is inconsistent with the reference sample (differences sufficient to exclude the known individual as a source were found); or the questioned hair is unsuitable for comparison (the hair is too damaged, too short, or lacks sufficient features). The first conclusion is meaningfully expressed only when DNA analysis has also been conducted, because without DNA, the class consistency cannot be translated into an individual probability.

A hair with an intact root contains nuclear DNA in the root sheath cells. This DNA can be profiled using short tandem repeat (STR) analysis, producing a full DNA profile that can be compared to a reference profile from a known individual. The statistical weight of an STR match is expressed as a random match probability: the probability that a randomly selected, unrelated person from the relevant population would also match at the tested loci. In a well-typed profile using standard CODIS loci (the US system) or the expanded ESS loci (the European standard), these probabilities are typically in the range of one in billions.

When no root is present, the hair shaft may still contain mitochondrial DNA. mtDNA is present in multiple copies per cell, which makes it recoverable from degraded or rootless samples where nuclear DNA has been destroyed. However, mtDNA is inherited through the maternal line only: a person shares their mtDNA haplotype with their mother, maternal siblings, maternal aunts and uncles, and all other individuals in the maternal line. This means an mtDNA match cannot distinguish between maternal relatives, and the forensic value of an mtDNA result is expressed as an exclusion: if the questioned and reference samples have different mtDNA sequences, the reference individual is excluded as the hair source. If they share the same haplotype, the reference individual is not excluded, but neither is any other person with the same maternal lineage.

The interplay between microscopy and DNA in a hair examination follows a practical sequence. First, all recovered hairs are screened by microscopy to determine species, body region, and suitability for comparison. Hairs classified as human and suitable for comparison are then prioritised for DNA analysis. The microscopy narrows the field and preserves limited DNA resources for the most informative samples. The DNA result then provides the statistical weight that the morphological comparison cannot supply on its own. Where a root is present, STR analysis is the primary method. Where only shaft is available, mtDNA sequencing is used as a supplementary tool.

For cross-links to DNA extraction and profiling methods, see Forensic Biotechnology, and for the serological screening that often accompanies biological trace evidence processing, see Forensic Serology.

Hair evidence is fragile and easily contaminated or lost. Scene hairs are collected individually using clean forceps, or by tape lifting from surfaces where multiple loose hairs are present. Each hair or tape lift is placed in a paper fold (the druggist fold) and sealed inside a paper envelope. Plastic bags are avoided: static charges cause hairs to cling to the inside of the bag and fragments to escape when the bag is opened, and humidity trapped inside promotes mold and DNA degradation. The envelope is labelled with the case number, the item number, the location of collection (which surface, which area of the scene), the date and time, and the collector's name.

Reference samples from a known individual are collected by combing all scalp regions to remove loose hairs, then cutting hairs from at least five scalp regions (frontal, temporal left and right, parietal, occipital) and pulling several hairs from each region to ensure the root is included. A minimum of 25 to 50 scalp hairs is the standard for a reference collection, because natural variation within a single person's head means that a small sample may not represent the full range of morphological features. Reference samples are packaged and labelled separately from questioned hairs, with the same envelope-and-fold method.

Under the Bharatiya Sakshya Adhiniyam 2023 (which replaced the Indian Evidence Act 1872), biological trace evidence including hair must be accompanied by a documented chain of custody to be admissible. This mirrors the requirements under the US Federal Rules of Evidence and the UK Criminal Procedure and Investigations Act 1996 disclosure framework. The chain of custody record tracks every transfer of the exhibit: from the scene collector to the packaging officer, from packaging to the laboratory, from intake to the analyst, and from analysis to secure storage. Any gap in the chain is a basis for a challenge to admissibility or to the weight of the evidence.