How Do Scientists Use Hair in Crime Scene Investigations?

We’ve all watched television programs like “CSI: Crime Scene Investigation” and “Law and Order.” Shows like these simplify the investigative process that investigators conduct in trying to solve the mysteries behind crimes. If you’ve paid close attention, you may have noticed investigators collecting and examining hair as evidence in crime cases. How is the hair used to solve the crime? The study of the hair follicles found at the scene goes beyond the hair color.

Three layers of the hair shaft are used in determining how a crime was committed and who the criminal may be. The medulla, the innermost core of the hair, can be used to determine the race or ethnicity of the potential perpetrator.

The cortex is the layer over the medulla and is used to compare one hair to another. Microscopic air bubbles and pigment granules help scientists distinguish between the hair types. The outermost layer, the cuticle, can be used to determine the species (if the hair belongs to an animal) through analysis of the patterns of scales that cover the cortex.

In terms of investigation, the root of the hair can be one of the most useful pieces of evidence. By the root (or lack thereof), scientists can tell if the hair was shed naturally, pulled out, or cut off. The root is where investigators can find DNA to help precipitate their analysis. If a hair is pulled out during the anagen phase of the hair growth cycle a root bulb will appear with follicular tissue. This tissue has nuclear DNA which helps create a DNA profile unique to the hair’s owner.

During the catagen stage, the root of the hair pulls away from the shaft, and the hair is easily pulled from the head. The root bulb is less round and more elongated in this phase. In the final stage, the telogen phase, the hair sheds naturally and has a club-shaped root bulb. These two phases do not produce a follicular tag, which means nuclear DNA cannot be harvested from hairs in the catagen and telogen phases. However, mitochondrial DNA may be found in just the hair and can help to narrow down subjects, though it cannot distinguish between siblings.

Hair is also useful in chemical tests since it is often the only evidence still remaining after other bodily evidence has disappeared. Scientists can determine the presence of chemicals, poisons, heat treatments, and more. The important thing to note about the use of hair in investigations is that it is only one piece of an investigative puzzle. Hair cannot be used as evidence on its own. However, it can be very useful along with other pieces of evidence. Who knew your hair held so much information?

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Bertino, Anthony J., and Patricia Nolan. Bertino. Forensic Science: Fundamentals & Investigations. Mason, OH: South-Western Cengage Learning, 2009.

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