Hairs and Fibers (Lesson 3.01)

Introduction to Trace Evidence: Hairs and Fibers

  • Definition of Trace Evidence

    • Trace evidence includes microscopic materials such as hairs and fibers that are found at crime scenes.

    • Relevant for associating suspects with crime scenes or victims.

  • Importance of Hairs and Fibers in Forensic Science

    • Hairs and fibers are crucial in forensic investigations.

    • They provide vital clues that can lead to identifying suspects or linking victims to crime scenes.

    • Examined under microscopes for identification and matched chemically.

Lesson Goals

  • Identify situations where trace evidence is useful.

  • Describe individual hairs; human vs. animal hair; stages of human hair growth.

  • Explain the technology used in comparing hairs and fibers.

  • Distinguish between types of evidence.

  • Construct arguments from evidence regarding its relevance.

  • Develop models to analyze information obtained from crime scenes.

  • Construct arguments regarding manners of death based on evidence.

  • Identify classes and characteristics of fibers, including chemical characteristics.

  • Establish links between crimes and victims/perpetrators.

Mass Murders in Atlanta Case Study

  • Details surrounding the Atlanta Child Murders (1979-1981).

    • 28 African American males went missing and were found dead, having been strangled.

    • Investigation revealed numerous trace evidence including hairs and fibers.

Structure of Hair

  • Composition of Hair

    • Hair is primarily made of keratin, a protein.

    • Grows outward from the skin.

  • Major Components of Hair

    • Hair Follicle:

    • Located in the skin, contains cells for hair growth.

    • Hair grows from the bottom where keratin is added.

    • Hair Shaft: Visible part of hair, composed of three layers:

    • Cuticle: Transparent outer covering.

    • Cortex: Inner layer containing pigments (melanin) for hair color.

    • Medulla: Innermost core, can be filled or hollow.

  • Associated Structures

    • Sebaceous glands: Near hair follicles, secreting oil.

    • Papilla: Blood vessels and nerve endings located at the base of the follicle.

    • Hair shafts lack nerves, explaining why cutting hair doesn't cause pain.

Hair Growth Phases

  • Hair grows in three stages:

    1. Anagen Phase: Continuous growth lasting up to six years (80-90% of body hairs).

    2. Catagen Phase: Transitional phase lasting 1-3 weeks.

    3. Telogen Phase: Hair remains dormant and eventually sheds after months; about 10% of hairs are in this phase at any time.

Hairs as Evidence

  • Shedding of Hairs

    • Constant shedding provides trace evidence due to Locard's exchange principle: crimes leave evidence.

    • Hairs can be retrieved using forceps, adhesive tape, or vacuuming.

  • Importance of Follicles

    • If pulled, hair may have its follicle attached, containing cellular DNA useful for typing.

Forensic Analysis of Hair

  • Key Questions in Hair Analysis

    • Is the hair animal or human?

    • If human, what part of the body did it come from?

    • Can it indicate racial ancestry?

    • Is it male or female hair?

  • Class Evidence vs. Individual Evidence

    • Hair analysis yields class evidence; while it cannot pinpoint an individual, it can indicate group characteristics.

  • Examination Criteria

    • Microscopic Features: Structure of the shaft (medulla, cortex, cuticle), size, color, shape, and texture.

    • Medullary Index: Ratio of the medulla’s thickness to the shaft’s thickness.

    • Human hairs: index < 0.33

    • Animal hairs: index > 0.50

Human vs. Animal Hair

  • Cuticle Scale Comparisons:

    • Human hairs: flat, overlapping imbricate scales.

    • Animal hairs: various shapes (crown-shaped, petal-shaped, flattened).

  • Origins of Hair:

    • Differentiation based on part of the body:

    • Head Hair: Long, straight, uniform color, circular cross section, may show dye.

    • Eyelash Hair: Coarse with elliptical cross section.

    • Beard Hair: Thicker, triangular cross section with more roughness.

    • Pubic Hair: Oval or triangular, wiry, coarseness.

    • Limb Hair: Shorter, arc-shaped.

Class Characteristics of Hair

  • Differences between male and female hair can be detected chemically or through dyes.

  • Racial Ancestry:

    • European Hair: Straight/wavy, round cross section, small diameter.

    • African Hair: Curly, coiled, flattened cross section, heavily pigmented.

    • Asian Hair: Straight, coarse, thick cuticle, round cross section.

Chemical Analysis of Hair

  • Hair can absorb chemicals, providing chemical markers.

  • Neutron Activation Analysis (NAA):

    • Bombards hair with neutrons; unique gamma emissions identify elements.

    • NAA analysis revealed high arsenic levels in Napoleon Bonaparte’s hair, suggesting poisoning.

Fibers as Trace Evidence

  • Types of Fibers:

    • Natural: Derived from plants or animals (e.g., wool, cotton).

    • Synthetic: Made chemically (e.g., nylon, spandex).

  • Similarities with Hair

    • Fibers can also be exchanged at crime scenes and retrieved like hair.

    • Class evidence in determining links between victims, crime scenes, and suspects.

Differences Between Fiber Types

  • Animal Fibers: E.g., furs, wool; smooth informative scales.

  • Plant Fibers: E.g., cotton (fine, smooth), flax (coarse).

  • Synthetic Fibers: Made from polymers, each with unique properties (e.g., strength, water resistance).

Chemical Analysis of Polymers

  • Different polymers exhibit specific indices of refraction, aiding in identification.

  • Chemical analysis supports identification of specific dyes and polymers—essential for forensic contexts.

Conclusion: Relevance of Trace Evidence

  • Trace evidence like hairs and fibers critically supports criminal investigations by establishing connections.

  • Evidence collected through proper methods can be pivotal in substantiating claims against suspects, providing corroborative links to the crime scenes.