The Study of Hair

Human Hair

is one of the most frequently found pieces of evidence at the scene of a violent crime. It can provide a link between the criminal and the crime

From hair one can determine

If the source is human or animal; Race (sometimes); Origin of the location on the source’s body; Whether the hair was forcibly removed; If the hair has been treated with chemicals; If drugs have been ingested

Cuticle, Cortex, Medulla

Parts of a hair shaft

Cuticle

outside covering, made of overlapping scales

Cortex

inner layer made of keratin and imbedded with pigment; also contains air sacs called cortical fusi

Medulla

inside layer running down the center of the cortex

The cuticle

is the outermost layer of hair which is covered with scales. The scales point toward the tip of the hair. Scales differ among species of animals and are named based on their appearance.

*Three basic cuticle scale patterns are*

Coronal, Spinous, Imbricate

The cuticle scales in animals tend to

resemble petals (spinous) or a stack of crowns (coronal). 

The cuticle scales in humans commonly are

flattened and narrow (imbricate). 

Cortex: 

inner layer made of keratin and imbedded with pigment; also contains air sacs called cortical fusi. Gives the hair its shape

Melanin, Cortical fusi

has two major characteristics

Melanin

pigment granules that give hair its color

Cortical fusi

air spaces, usually found near the root but may be found throughout the hair shaft

Medulla:

inside layer running down the center of the cortex

The medulla is the

hair core that is not always visible. The medulla comes in different types and patterns.

Human medulla may be

continuous, fragmented or absent

Medullary Index

Determined by measuring the diameter of the medulla and dividing it by the diameter of the hair

Medullary Index for human hair is generally

less than 1/3

Medullary Index for animal hair

is usually greater than 1/2

Pigmentation in animal hair

is denser toward the medulla. In humans it tends to be denser toward the cuticle.

Unlike human hair, animal hair

abruptly can change colors in banded patterns. 

The medullary index is different.  In animals the medulla

is much thicker than it is in humans. 

Questioned hairs must be accompanied by an adequate number of control samples.

From victim, from possible suspects, from others who may have deposited hair at the scene

Control Sample

50 full-length hairs from all areas of scalp 24 full-length pubic hairs

Color

Dyed hair has color in cuticle and cortex. Bleaching removes pigment and gives a yellow tint. Grey hair has no pigmentation

Hair Comparison

Color, length, diameter, Distribution, shape and color intensity of pigment granules, Scale types, Presence or absence of medulla

* Presence or absence of medulla

Medullary type, Medullary pattern, Medullary index

Hair shape based on cross sections

Hair examiners have identified some physical characteristics that generally can be associated with broad, racial groups. These characteristics, however, will not apply to all individuals in these groups. In addition, at times, it will be impossible to assign specific hairs to any of these groups because their characteristics are poorly defined or hard to measure. 

 Round (Straight hair)

Asian & Native American descent

Oval (Wavy hair)

Caucasian, Hispanic, Middle Eastern descent

Flat or Crescent (Curly)

African, African-American descent

Hair Growth: Anagen

hair that is actively growing; lasting 3 to 5 years (80-90% of a person’s hair will be in this phase)

Hair Growth: Catagen

hair is not growing; a resting phase

Hair Growth: Telogen

hair that is dying and ready to fall out; lasting two to six months

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Hair Growth:

Grows about 1 centimeter per month; approximately one half inch per month

Human roots look different based on whether they have been

forcibly removed or if they are telogen hairs and have fallen out.

Animal roots will vary,

but in general will have a spear shape.

DNA from hair

The root contains nuclear DNA. If the hair has been forcibly removed, some follicular tissue may be attached containing DNA. 

The hair shaft

contains abundant mitochondrial DNA, inherited only from the mother.  It can be typed by comparing relatives if no DNA from the body is available. This process is more difficult and costly than using nuclear DNA.

Testing for substances in the hair shaft

Chemicals that are ingested or absorbed by the skin often can be detected by analysis of the hair shaft. 

A forensic scientist can perform chemical tests for the presence of various substances. 

The hair shaft can be examined in sections to establish a timeline for exposure to toxins. 

Advantages of Hair Toxicology

Easy to collect and store, Is externally available. Can provide information on the individual’s history of drug use or of poisoning. Collections must be taken from different locations on the body to get an accurate timeline