Hair analysis
Hair analysis is a forensic technique that involves comparing hair samples from a questioned source to those from a known source to potentially identify the origin of the hair. This analysis can be significant in criminal investigations, helping to determine if hairs found at a crime scene belong to a person or an animal and potentially linking them to victims or suspects. The structure of hair includes the root, which is embedded in the follicle, and the shaft, which is composed of three layers: the medulla, cortex, and cuticle, each providing distinctive characteristics useful for identification.
Different types of hair possess unique features, which allows forensic scientists to differentiate between species and even types of human hair. However, hair analysis has limitations, as it cannot definitively identify an individual source; it can only suggest possible associations. In addition to visual comparisons, toxicological analysis of hair can reveal the presence of drugs or toxins, with hair retaining evidence longer than bodily fluids. When hairs include a root tag, DNA extraction can provide a precise identification, surpassing the potential associations from visual analysis alone. Overall, hair analysis serves as a valuable tool in forensic science, contributing to the understanding of events at crime scenes.
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Subject Terms
Hair analysis
DEFINITION: Comparison of hair from a questioned source to hair from a known source to determine the possible source of the questioned hair.
SIGNIFICANCE: Hair analysis can provide useful information during a criminal investigation, as forensic scientists can determine whether hairs found at a crime scene came from an animal or a person and whether the hairs could have come from the victim of the crime or a possible suspect.
During the commission of a crime, hairs from the perpetrator’s head or clothing are often transferred to the crime scene. Because different types of hair have different characteristics, the analysis of hairs recovered from crime scenes can provide investigators with information on the people who were present at the scene.
Hair Characteristics
Hair is produced from a structure in the skin known as the follicle. The part of a hair embedded in the follicle is known as the root, and the shaft of the hair is what protrudes from the skin, ending in the tip. The shaft generally consists of three layers known as the medulla, the cortex, and the cuticle.
The medulla is a series of cells down the center of the hair shaft that gives the appearance of being a hollow channel. Medullae can be continuous (which means that the channel appears along the entire length of the hair), discontinuous (which means that sections of medulla appear throughout the length of the hair), or absent entirely. The appearance of the medulla can be highly useful to the forensic scientist in differentiating different species of hairs, as various animals have patterned medullae. For example, a cat medulla has an appearance similar to a string of beads. Different hairs from the same person can have different types of medullae.
The cortex is the area surrounding the medulla; it consists of spindle-like cells that contain pigment granules. It is these granules that determine the color of the hair. The granules can vary greatly in shape and distribution throughout the cortex. As is the case with the medulla, the characteristics of the cortex can vary among hairs coming from the same source.
The cuticle, the waxy outermost layer of the hair shaft, consists mainly of flattened cells that form a scalelike pattern on the surface of the shaft. These patterns do not vary greatly between individuals within a species; however, they can provide a wealth of information about the species of the hair.
Limitations of the Technique
Hair analysis can provide information about the type of hair found by means of direct visual comparison. It can be determined with a reasonable degree of certainty the type of animal from which a hair came and, if it is human head hair, the ancestry of the person. It is also possible to differentiate different types of human hair, such as head hair from pubic hair or beard hair, by examining the shape of the cross section. Forensic scientists are also able to determine whether a hair has recently been cut, whether it has been dyed, and whether it was forcibly removed or shed naturally.
If a single hair is found at a crime scene and a forensic scientist wishes to compare it to a known source, it is imperative that many hairs be taken from the known source for comparison purposes. This is because of the high degree of variation within hairs from a single source.
Hair evidence is not individualizing. That is, it is not possible to determine the exact source of a hair by visual examination. If a forensic scientist deems a questioned hair to be consistent with known hairs, the scientist’s report should state that the questioned hair cannot be excluded as having come from a known source.
Other Types of Hair Analyses
In addition to species analysis and visual examinations and comparisons, much research has been done in the area of toxicological analyses on hair, and it has been found that many drugs and toxins can be detected in hair. Drug testing on hair rather than on bodily fluids (urine, saliva, or blood) has gained popularity because a drug retained in the hair is detectable until the part of the hair containing the drug is cut or naturally shed, whereas body fluids retain evidence of drug usage only for a few hours to a few months.
When a hair found at a crime scene includes a root tag, forensic scientists can extract DNA (deoxyribonucleic acid) from the root tag for analysis. DNA comparison can afford an exact match to a rather than the possible association usually afforded by visual hair analysis.
Bibliography
Gardner, Ross M. Practical Crime Scene Processing and Investigation. Boca Raton, Fla.: CRC Press, 2005.
"Hair Analysis." North Carolina Prosecutors' Resource Online, 1 Dec. 2023, ncpro.sog.unc.edu/manual/622-1. Accessed 15 Aug. 2024.
Houck, Max M., and Jay A. Siegel. Fundamentals of Forensic Science. Burlington, Mass.: Elsevier Academic Press, 2006.
Kintz, Pascal, ed. Drug Testing in Hair. Boca Raton, Fla.: CRC Press, 1996.
Robertson, James, ed. Forensic Examination of Hair. Philadelphia: Taylor & Francis, 1999.
Robertson, Sally. "Hair Analysis in Forensic Science." News-Medical, 21 May 2019, www.news-medical.net/life-sciences/Hair-Analysis-in-Forensic-Science.aspx. Accessed 15 Aug. 2024.
Saferstein, Richard. Criminalistics: An Introduction to Forensic Science. 11th ed. Upper Saddle River, N.J.: Pearson Prentice Hall, 2014.