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
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. The three basic patterns are:
Coronal
Spinous
Imbricate
The cuticle scales in animals tend to resemble petals (spinous) or they give the appearance of 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
The cortex gives the hair its shape.
It 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.
For animal hair, it is usually greater than 1/2
Humans vs. Animal Hair
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.

Collection of hair
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
Hair Comparison-what to look for?
Color
Dyed hair has color in cuticle and cortex
Bleaching removes pigment and gives a yellow tint
Grey hair has no pigmentation
Length
Diameter
Distribution, shape and color intensity of pigment granules
Scale types
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
Terminology
Anagen—hair that is actively growing; lasting 3 to 5 years (80-90% of a person’s hair will be in this phase)
Catagen—hair is not growing; a resting phase
Telogen—hair that is dying and ready to fall out; lasting two to six months
Grows about 1 centimeter per month; approximately one half inch per month
The Root
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
Advantages:
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.