Hair as a Source of DNA and Toxicological Information

A human hair consists of the shaft (the visible portion) and the root, which is embedded in the follicle within the dermis. The follicle cycles through three phases: anagen (active growth, lasting 2 to 6 years on the scalp), catagen (transition, 2 to 3 weeks), and telogen (resting, 3 to 4 months). The critical point for DNA analysis is that nuclear DNA is available only in anagen-phase roots, where the dermal papilla is actively dividing and the cells at the root sheath still have intact nuclei. Telogen hairs shed naturally and have a small club-shaped root with far fewer nucleated cells, making nuclear DNA extraction difficult.

The shaft itself is composed of three layers: the cuticle (overlapping scales on the outside), the cortex (the main structural body of keratin filaments), and the medulla (a central channel, absent in some hairs). As the shaft keratinocytes differentiate and die, they lose their nuclei and become the compacted keratin of the cortex. This means the shaft, along with its mtDNA-bearing cytoplasm remnants, does not contain nuclear DNA. The cuticle colour scales and cortex pigmentation pattern are used in microscopic comparison; the cortex is the analytic target for both mtDNA extraction and toxicological analysis.

Hair body site also matters. Scalp hair grows the fastest and is the most common submission. Pubic and axillary hair grows more slowly and at variable rates, making segmental timing less precise. Eyebrow and eyelash hair have very short anagen phases and are usually in telogen at time of collection, reducing nuclear DNA yield. Nasal and beard hair growth rates differ from scalp. Analysts should record body site for every sample and apply site-appropriate growth rate assumptions when generating timelines from toxicology.

Extraction of nuclear DNA from hair roots follows the same general path as other biological samples: cell lysis, protein digestion, DNA isolation, and quantification. The challenge is low template quantity. An anagen root from a single hair may yield as little as 0.1 to 1 ng of DNA, and some roots yield sub-nanogram amounts that fall below standard STR kit sensitivity thresholds. Extraction protocols therefore typically include a prolonged digestion step with proteinase K to break down the dense keratin matrix and release the nuclear content.

Once extracted, the DNA is amplified using polymerase chain reaction (PCR) targeting the standard STR loci. In the United States, the CODIS system requires 20 core loci. The United Kingdom uses the National DNA Database profile comprising 16 to 24 loci. The European standard harmonised profile uses 16 loci. Indian guidelines follow the DNA Technology (Use and Application) Regulation Act 2019 framework, which establishes the National DNA Data Bank with similar loci-based profiling. A full match at all typed loci between a questioned hair root and a reference sample can be reported as a match with a statistical weight expressed as a random match probability.

Touch DNA concerns arise when hair is handled. If the outer surface of the shaft has been touched, foreign nuclear DNA from skin cells may contaminate the root end during processing. Analysts use gloves throughout and may wash the root-end segment before digestion to reduce surface contamination. Any mixed profile result should be reported as such, with the complexity of the mixture clearly communicated.

Mitochondrial DNA analysis of hair shafts is particularly valuable for degraded or rootless samples, which are common in crime scenes, mass disasters, and historical investigations. Each cortical cell retains hundreds of mitochondria in its cytoplasm even after the nucleus has been lost during keratinisation, giving the shaft a far higher copy number per unit mass than nuclear DNA. This copy number advantage means mtDNA can often be recovered from hairs that are decades old, heat-damaged, or heavily degraded.

The analytical target is the control region of the mitochondrial genome, specifically hypervariable regions I and II (HVI and HVII). These regions show sufficient sequence variation among individuals to be informative. Sequencing is performed after PCR amplification of the control region. The resulting sequence is compared to the rCRS (revised Cambridge Reference Sequence) and reported as a haplotype. The population frequency of the haplotype is the key statistic: a common haplotype shared by 1 in 100 people in a given population provides far less discrimination than a rare one shared by 1 in 10,000.

The limitation that mtDNA cannot distinguish between individuals on the same maternal line must be communicated explicitly in court. In a case involving siblings, mother and child, or maternal cousins, an mtDNA match does not differentiate between them. This is not a failure of the technique; it is an inherent property of maternal inheritance. Where possible, nuclear DNA typing should be attempted first or in parallel, with mtDNA used as a supplementary or fallback method.

The mechanism by which drugs enter the hair shaft is passive diffusion from blood into the follicular cells during the anagen phase, where they become physically trapped in the keratin matrix as the shaft hardens. Small, lipophilic, basic molecules bind most effectively to melanin granules in the cortex. This has an important consequence: dark hair, which has more eumelanin, binds some drugs, particularly cocaine and its metabolites and amphetamines, at higher concentrations than lighter hair at equivalent blood exposure levels. Interpreting hair drug concentrations without accounting for pigmentation can lead to incorrect conclusions about relative exposure.

Three routes of drug entry into the shaft are recognised: diffusion from blood through the follicle (the main route), diffusion from sweat onto the outer surface, and diffusion from sebum secreted by the sebaceous gland. Sweat and sebum routes introduce a passive surface deposition that may not reflect systemic use, particularly for substances like cannabis, cocaine, and opioids that are commonly present in environmental dust and on the skin of people who have contact with users. These surface contributions are addressed during sample preparation.

The presence of drug metabolites is the strongest evidence of systemic use rather than external contamination. For example, cocaine and its metabolite benzoylecgonine are both found in hair after cocaine ingestion. Benzoylecgonine is not present in cocaine powder and cannot reach the shaft by external exposure to the drug alone. Finding benzoylecgonine in a hair sample is therefore indicative of true systemic exposure. Similar logic applies to other drug classes: specific phase I or phase II metabolites that are not in the parent substance confirm that the drug passed through the body's metabolic processes.

Segmental analysis divides a hair strand into consecutive 1 cm sections, each representing approximately one month of growth. Analysis of each segment individually allows the analyst to map drug concentration against time, showing when use began, whether it was continuous or episodic, when it stopped, and roughly how heavily the person was exposed in each period. This timeline can be correlated with known events: dates of arrest, hospitalisation, treatment programmes, or alleged offence dates.

Interpretation is not straightforward. Drug diffusion within the shaft does not stop once the hair has grown above the scalp; drugs can migrate along the shaft from a zone of high concentration to adjacent segments over time. This causes blurring at the boundary between use and non-use periods. The effect is larger for more polar, water-soluble compounds than for highly keratin-bound lipophilic ones. Reports should always include an uncertainty estimate around the inferred timeline rather than presenting segment boundaries as hard cutoff dates.

Heavy metals behave differently from drugs in hair. Arsenic, lead, mercury, and thallium incorporate into the shaft in proportion to blood concentration, and the segmental arsenic profile has been used in historical poisoning investigations to infer repeated or continuous exposure. The Reinsch test and atomic absorption spectrometry or inductively coupled plasma mass spectrometry (ICP-MS) are standard methods. A classic application is the retrospective analysis of Napoleon Bonaparte's hair, which showed elevated arsenic levels consistent with chronic environmental exposure rather than deliberate poisoning, illustrating both the potential and the interpretive complexity of historical hair toxicology.

Hair for DNA analysis should be collected with sterile scissors or forceps, placed in a paper envelope (not plastic, which promotes moisture-related degradation), and stored dry at room temperature or at 4 degrees Celsius. If a root is needed and the hair has not shed naturally, pulling the hair by hand or with forceps from the scalp will extract the root sheath. Reference samples from a known individual should include at least 25 hairs from different scalp regions to account for growth phase variability.

Hair for toxicology is collected as close to the scalp as possible, bundled in the same orientation (root end marked), and secured with tape or thread. The strand is then cut into segments in the laboratory with clean scissors, starting from the root end. Before any digestion, the sample is washed to remove external contamination. Standard decontamination uses a sequence of organic solvent (methanol or dichloromethane) washes followed by water washes. The wash solutions are themselves analysed: if drug concentrations in the wash are much higher than in the digested shaft, external contamination is the probable explanation. If the shaft digest contains drug and metabolites at meaningful levels and the wash shows little, systemic incorporation is indicated.

Chain of custody documentation for hair samples follows the same principles as any forensic exhibit. The collector signs and dates the collection envelope, the sample is logged into the laboratory information management system on receipt, and every analytical step records the analyst's identity, the date, the instrument used, and the results. Admissibility of hair evidence in court depends on unbroken chain of custody documentation alongside the scientific validity of the method. This applies uniformly across jurisdictions: Daubert standards in the United States, Forensic Science Regulator Codes in the United Kingdom, and accreditation requirements under India's Bharatiya Sakshya Adhiniyam 2023 framework all require documented continuity and validated methodology.

Cosmetic treatment introduces confounders in both DNA and toxicology analysis. Bleaching oxidises melanin and degrades DNA; it also reduces drug concentrations by up to 50% for some compounds. Permanent waving and relaxing treatments alter the keratin matrix. Analysts should record cosmetic history from the donor and apply appropriate caveats when interpreting results from treated hair. For DNA analysis, treated hairs may yield only partial profiles or may fail entirely, and this should be reported as a technical limitation rather than an inconclusive result without explanation.