Hair Medullary Index: Calculation Method, Normal Ranges & Forensic Use

The medullary index (MI) quantifies the medulla's relative width in a hair shaft:

MI = (Mean medullary diameter ÷ Mean shaft diameter) × 100

Measurement Protocol

1. Cross-section hair; view via light microscopy or digital imaging.
2. Measure medulla and total shaft widths at multiple points.
3. Average values; apply formula.

Typical Ranges

• Human scalp hair: ~35%
• Human body hair: ~20%
• Animal hairs: often >50% (key forensic discriminator)

Applications

• Species identification (forensic, wildlife)
• Hair health assessment (high MI ↔ coarse, resilient)
• Cosmetic research