Hair Toxicology: Core Lecture Notes (copy)

Biological Basis

• Hair as a long-term biological archive

  • Keratinized shaft traps parent drugs and their metabolites during growth.

  • Once locked in, chemicals remain for the life of the hair, offering a retrospective calendar of exposure.

• Typical compounds mentioned

  • Cocaine, alcohol (via ethyl glucuronide, fatty-acid ethyl esters).

  • GHB; particularly relevant in drug-facilitated sexual assault (DFSA) investigations.

Time-Stamping Drug Intake

• Principle: Distance from the scalp ≈ time since ingestion.

• Average human hair growth rate

  • 6\,\text{mm}\;\text{month}^{-1} (slow) to 33.6\,\text{mm}\;\text{month}^{-1} (fast).

  • Classroom rule of thumb: \approx 10\,\text{mm}\ (1\,\text{cm})\;\text{month}^{-1}.

• Estimating event date

  • \text{Time (months)} = \dfrac{\text{Distance from root (mm)}}{\text{Growth rate (mm month}^{-1})}

• Application example

  • DFSA case: Collect hair 1–3 months after assault to target the segment formed during/just after the event, maximizing likelihood of finding GHB markers.

Laboratory Workflow

• Segmentation

  • Cut strand into consecutive 1–2 cm sections to create a chronological series.

• Decontamination (washing)

  • Removes sweat, sebum, and environmental residues so that only endogenous incorporation is measured.

• Analysis considerations

  • Each drug/metabolite incorporates at different efficiencies; must calibrate for matrix effects and potential external contamination.

Interpretive Caveats & Practical Notes

• Individual variability in growth rate (age, nutrition, ethnicity) introduces uncertainty in back-calculation.

• External contamination vs. true ingestion remains a central forensic debate; thorough washing and metabolite/parent-drug ratios help resolve.

• Ethical implications: Long-term monitoring may infringe on privacy; informed consent and clear legal mandates are essential.

Quick-Reference Cheat Sheet

• Growth rate: 6–33.6\,\text{mm month}^{-1}.

• Average window captured by a 3 cm lock: \approx 3 months.

• Critical formula: \text{Time (mo)} = \dfrac{\text{Segment centroid distance (mm)}}{\text{Growth rate}}.