Hair Analysis Using Forensic Microscopy
Hair samples are commonly found as trace evidence at crime scenes. Microscopic evaluation of the hair is key to developing a profile of the individual to whom it belongs.
Hair evidence is found frequently by crime scene investigators due to the ease at which most people and animals shed. Once the hair is collected for evidence processing, the majority of the forensic hair analysis is performed under a microscope. Evaluation through forensic microscopy reveals many characteristics of hair that aid in identifying its source.
Hair is usually found at a crime scene during trace evidence collection. This may be done through vacuuming or using sections of tape to collect debris. Hair can also be collected from the victim or suspect for comparison. Approximately 25 hairs are needed for an appropriate sample. Pubic hair and hair from a person's head retain the most useful characteristics and are preferred for microscopic analysis.
Forensic Hair Analysis Using a Microscope
The first step in microscopic hair evaluation is to determine whether it is animal or human. Hair is made up of a protein called keratin, color and trace minerals. A see-through outer covering called the cuticle is composed of overlapping scales which can be seen under the microscope. The pattern and shape of the scales help to identify the type of animal or human from which the hair originated. Human and animal hairs differ on several physical characteristics including:
- consistent pigmentation of the hair in humans
- a regularly-defined cellular core in animals
- a club-shaped root in humans with variable shapes in animals
- animal coats contain several different types of hairs while humans do not
Once the hair is identified as human, human characteristics are determined which include:
- race
- body region from which the hair was found
- length
- color
- whether the hair was removed forcibly from the body
- damage due to crushing or burning
- chemical treatment such as dyes or bleach
Racial variations include the diameter of the hair shaft, density and distribution of pigment granules and shape of the hair in cross-section. Body hair is differentiated based on length, shaft diameter, texture and condition of the tips. Comparing all of these characteristics allows a forensic technician to determine if hair found at a crime scene matches that of a suspect or victim.
Forensic DNA Hair Testing
It is usually possible to match hairs using microscopy alone, however, sometimes further testing of DNA is needed. DNA testing of hair is not always possible. Sometimes there is not enough material from which to extract DNA from the hair. A hair with a large root will provide enough cells for testing. Hair is destroyed during the process of DNA testing so no further examination of the hair can be performed.
Resources
Deedrick, D. and Koch, S. Microscopy of Hair Part 1: A Practical Guide and Manual for Human Hairs. Forensic Science Communications.