Hair & Fiber Analysis Notes
Hair is considered class evidence. A forensic scientist would testify that “the two hairs are very similar”, not that the
“two hairs are the same.”
Examination of hair cannot determine sex or age.
Hair
Hair grows from a pocket of specialized cells called the follicle (root). The presence of follicular tissue on a root suggests it was
removed by force. If a follicle is present, DNA can be extracted. Hair is composed of dead cells, so there is no nuclei in the
hair shaft. Mitochondrial DNA can be extracted from the shaft.
The shaft of the hair is made up of 3 parts: The medulla, the cortex, & the cuticle.
Think of a hair shaft like a pencil: The lead would be the medulla, the wood would be the cortex, and the yellow paint the cuticle.
Cuticle: Layer of cells that cover the surface of the hair shaft. Looks like scales on a fish. Examiners use scale patterns to
determine whether the hair is human or to match one hair to another.
*Scales overlap, point up to the shaft, away from the bulb.
Cuticle Scales are of 3 basic types:
1. Coronal: crown-like, mosaic surface
*Common among rodents
*Rare in human hair
2. Spinous: petal-like, triangular in shape
*Common of cats
*Not found in humans
3. Imbricate: flattened
*Found in humans and other animals
Medulla: Central core of hair
*Contains a collection of cells, but appears as a canal
*Medullary index: width of medulla / width of the shaft
*Greater than 1/3 is usually animal (wide medulla)
*Approximately 1/3 is human (narrow medulla)
*Medulla can exist in patterns
*Continuous
*Interrupted
*Fragmented
*Absent
*Human head hairs generally exhibit no medulla or have fragmented ones (rarely show continuous)
*Exception: Mongoloid race usually has continuous medulla
*Most animals have medullas that are either continuous or interrupted
Cortex: largest portion of the shaft, gives hair its shape
*Contains hair pigment, which gives hair its color (white or gray hair lacks pigment granules)
*Pigment particles vary in color, shape, and distribution
Hair taken from different areas of the body have different cross-sectional shaft geometries.
*Head, eyebrow, and eyelash hairs are round
*Armpit hairs are oval
*Beard hairs are triangular
Criminalists use the following as a checklist for comparing known and unknown hairs.
1. color and width
2. distribution pattern of the medulla
3. color and distribution pattern of the pigment in the cortex
4. cuticle pattern
Determining race
*Caucasians
*tend to have straight or wavy hair
*Cross section is round or oval
*thinner & more evenly distributed pigment pattern in the cortex
*African Ancenstry
*curly hair
*cross section is flat or oval
*cortex pigment pattern is denser & unevenly distributed
*American Indians & Asians
*medulla is usually continuous
*****These are only broad generalizations, and are not considered to be completely reliable!*****
Fibers
*Considered a class evidence
*Lack individuality because they are mass produced in such large quantities
Example: In 2001, over 3.5 billion pounds of cotton yarn was produced in the U.S.
*The more properties or characteristics that can be observed about a fiber, the greater the probability of identification and
matching it to a source
Fiber: any thread-like element of a material
* A fiber is composed of many filaments twisted or bonded together to form a thread or yarn
*Fibers are used in fabric, carpet, rope, and batting.
*Filament: single strand of material of indefinite length, usually twisted with other filaments to make a fiber.
*All apparel in the U.S. must have label listing all fibers that make up at least 5% of the fabric
*Three basic categories
1. Natural fibers: come from animals, plants, and minerals
Examples:
Animal: Wool, mohair, cashmere, & silk
Plant: Cotton, hemp, flax & jute
Mineral: Asbestos
2. Manufactured fibers: created when raw cotton or wood pulp is dissolved and cellulose is extracted
Examples: Rayon, acetate, & triacetate
3. Synthetic (man-made) fibers: come from polymers (petroleum)
Examples: Nylon & polyester
Comparing fibers
1. Diameter, shape, color, shininess, curls & crimps, any attached debris
2. Birefringence: light passes through synthetic fibers, it’s refracted twice and emerges as 2 different wavelengths of light
3. Microspectrophotometry (light transmission, reflection, absorption): Determines a fiber’s true color
4. Polarized light (appears sharper, colors clearer): Determines the reflective index
5. Refractive index: measured by directing a light beam at a fiber and calculating the degree to which light is bent
6. Scanning electron microscope (view magnification up to 100,000 times): used to determine how a fiber has been damaged
7. Gas chromatography/mass spectrometry: chemical composition of fiber, pigments, or chemical treatments (fire retardant,
stain treatment)
8. Density
9. Weave
*There are 3 basic weave patterns
*The warp is the lengthwise yarn
*Usually stronger, smoother, more even with a tighter twist
*The weft is the crosswise yarn
*The warp & weft do not need to be the same material (called a blend)
*The warp & weft can be different diameters to produce special effects
10. Cross Section
*Synthetic fibers are made by passing through a nozzle (similar to a showerhead) called a spinnerette
*Holes in the nozzle can be different shapes, so a cross-section of a fiber can be an important feature
11. Burning Tests
*How a fiber burns, its odor, and the residue that results can aid in identification
12. Decomposition Reaction
*When polymers are gently heated, they often decompose into their monomers (building blocks)
13. Chemical Tests
*Solubility & reactions in acids, bases, & solvents