Trace Analysis: Hair

Trace Analysis: Hair

With some rare exceptions, all humans have hair which is shed with regularity. It may come from the scalp, arms, legs, face, chest, back or pubic region. In addition to normal shedding, hair may be removed from the body by shaving, cutting, waxing, plucking, or it may come off during a fight, either by snatching or pulling it loose, or it may come off when it sticks to a weapon such as when a baseball bat strikes a person’s head. The hair may belong to the victim, the suspect, or some other person who came in contact with either of them. It is the task of the examiner to determine if hair found at the scene is consistent with hair belonging to the victim or suspect. It is also the job of the examiner to determine whether the hair found at the scene was there as a result of primary or secondary transfer, or if it, or any other trace evidence, was planted at the scene.

What does this mean?

When an examiner analyzes hair he, or she, may reach one of three conclusions:

Consistent: This conclusion results when the samples have similar characteristics.

Dissimilar: Non consistent. The two samples do not have similar characteristics.

Inconclusive: When it cannot be determined to any degree of certainty that two samples originated from the same source.

The method of transfer may be:

Primary Transfer: occurs when hair transfers from the part of the body where it is growing to either the victim or the suspect. Specifically if it transfers from the victim to the suspect or from the suspect to the victim. Examples of incidents in which a primary transfer could take place would be during a fight or sexual assault.

Secondary Transfer: This occurs when hair that is on the clothing of the victim or suspect transfers to the other party. For example, picture a hair that has been shed onto the shoulders of a person and then, during a fight, the hair comes off onto the clothing of the other combatant.

The Parts of Hair:

A hair includes a:

Cuticle: The enveloping layer. This is the

layer that also contains the scale pattern

which can be used to identify the species

of animal that the hair came from.

Cortex: An outer layer

Medulla: The inner part of the hair. The medulla may also give clues as to the species of animal that the hair originated from.

An analogy to a pencil is made in “Crime Scene to Court edited by Peter White. The medulla would be the equivalent of the lead in the pencil. The cortex would be the wooden portion of the pencil, and the cuticle would be the paint on the outside of the pencil.

Components of hair include:

Keratin: A type of protein

Melanin: The color or pigment of the hair

Metallic elements: Trace quantities. Note: hair also stores evidence of ingested materials such as drugs and poisons.

What is the value of hair as evidence?

Hair can be said to differ from person to person and accidental or coincidental matches are relatively rare. However, by itself, a hair is not a specific means of identification such as a fingerprint would be. Also, there may be fluctuations in the appearance of the hair at different portions of the shaft of the same hair, or between hairs from different parts of the body. Examination of the cortex may, however, reveal the overall color of the hair, and the size, distribution and density of the Pigment granules within the cortex may be determined and compared with the control hairs to determine if a hair might have come from the same person. “Crime Scene to Court edited by Peter White.

There may also be characteristics of the hair that increase the certainty that it came from a particular person. For example: Hair grows at the average rate of 1 centimeter per month. Suppose a person has their hair dyed on the first of October. The original color of the hair is blond. The new color of the hair is red. On the 15 of November a hair is recovered that has all of the general characteristics of the hair of a missing person, plus its color varies from blond to red over a distance of about a centimeter and a half. The artificial coloring agent may be extracted from the hair, run through a thin layer chromatography process and its chemical components determined and then compared with the known dyes used by the victim. This would add more credence to the theory that a particular hair came from a particular person.

General inferences of racial classifications may be cautiously made from a hair. There are some differences that are attributed to hairs from people of Asian, African or European ancestry, however, in today’s multi-racial society people may have characteristics of several races within their individual makeup.

Blood type may be determined from the hair in question. This is a quick process that may eliminate a person as a possible source of the hair, however, the certainty of the determined blood type is not 100%. Mitochondrial DNA that can be extracted from the hair itself which would demonstrate what family the person belonged to. Mitochondrial DNA is passed down through the mother. Additionally nuclear DNA may be extracted if there are adjacent skin cells attached at the root. In this case a positive identification may be possible from this one piece of evidence.

How Do You Collect Hair as Evidence at the Crime Scene

When examining the crime scene consider where hair is most likely to be. If there was a struggle, or the victim and suspect came into some other form of close contact, the hair of their opponent might be on their shoulders or caught in their clothing. Collecting the victim’s and suspect’s clothing and bagging each item in a separate bag will allow for careful, inch by inch examination of the clothing later in the lab. In a car you will most likely find the hair on the hair rest, possibly shed on the seats or on the floor. This can be collected with tweezers or an adhesive roller. Use one sheet of adhesive for each area searched. For example, one sheet would be for the driver’s headrest. Another would be for the passenger’s headrest. Package each sheet separately. If you are collecting the hair with tweezers, then use a piece of paper in a pharmacy fold to package the hair. Use one pharmacy packet for each area searched for hair.

In some cases a vacuum is used at the crime scene and then dumped on a white sheet and gone through. This is chaotic in my opinion but might be ok for areas like a large rug. Care must be taken that the vacuum is pristine before it is used to avoid contaminating your crime scene with evidence from another scene.

How many hairs must be taken from an individual for comparison purposes?

Saferstein[1] suggests that 50 hairs be removed from the head. The FBI suggests 25 hairs be removed from each area of the body being considered. In The Forensic Casebook by N.E. Genge suggests that up to 100 hairs be taken from the scalp and thirty to sixty hairs be taken from the pubic region in order to have a representative sampling. For this course consider 25 hairs to be sufficient. Note: collected hairs are plucked. (Some people may try to make do with shaving close to the skin.)

Additionally the clothing of a victim or suspect should be gone over with a lint brush and then vacuumed for loose hairs. The head and pubic regions, particularly in sexual assault cases, should be combed for loose strands, before the sample hairs are plucked.

Hair will have specific characteristics that vary from area to area of the body. Head hair is usually the longest hair on the body. It is often dyed, tinted, frosted, cut, and / or permed. Head hair grows at the average rate of 1 centimeter per month.

Pubic hairs are usually more coarse and curly.

Facial hair are also generally coarse. They may be used to corroborate other information such as statements taken during interviews.

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[1] Criminalistics, An Introduction to Forensic Science, Richard Saferstein, Pearson, Prentice Hall, Upper Saddle River, N.J. 2004