Fundamentals of Criminal Investigations
Adam J. McKee, Ph.D.
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Section 3.5: Hair Evidence
During the course of a criminal investigation, many types of physical evidence are encountered. One of the most common is hair evidence. The identification and comparison of human and animal hairs can be helpful in demonstrating physical contact with a suspect, victim, and crime scene. Hairs can provide investigators with valuable information for potential leads.
Until recently, the comparison microscope was considered the only reliable tool for the identification and comparison of the microscopic characteristics found in hair. Today, nuclear and mitochondrial DNA (mtDNA) testing can provide additional information that can influence the value of microscopic examinations. When the microscope is coupled with DNA technologies, the combination of these technologies profoundly affects the way forensic scientists, investigators, and prosecutors view hair evidence.
Although DNA technologies may add significant information to hair evidence recovered at a crime scene, the first step necessary in the analytical process is the identification and comparison of human and animal hairs.
A hair can be defined as a slender, thread-like outgrowth from a follicle in the skin of mammals. Composed mainly of keratin, it has three morphological regions—the cuticle, medulla, and cortex.
The cuticle is a translucent outer layer of the hair shaft consisting of scales that cover the shaft. Cuticular scales always point from the proximal or root end of the hair to the distal or tip end of the hair. There are three basic scale structures that make up the cuticle—coronal (crown-like), spinous (petal-like), and imbricate (flattened). Combinations and variations of these types are possible.
Animal Versus Human Hairs
Human hairs are distinguishable from hairs of other mammals. Animal hairs are classified into the following three basic types.
- Guard hairs that form the outer coat of an animal and provide protection
- Fur or wool hairs that form the inner coat of an animal and provide insulation
- Tactile hairs (whiskers) that are found on the head of animals provide sensory functions
Other types of hairs found on animals include tail hair and mane hair (horse). Human hair is not so differentiated and might be described as a modified combination of the characteristics of guard hairs and fur hairs.
Human hairs are generally consistent in color and pigmentation throughout the length of the hair shaft, whereas animal hairs may exhibit radical color changes in a short distance, called banding. The distribution and density of pigment in animal hairs can also be identifiable features. The pigmentation of human hairs is evenly distributed, or slightly more dense toward the cuticle, whereas the pigmentation of animal hairs is more centrally distributed, although denser toward the medulla.
The medulla, when present in human hairs, is amorphous in appearance, and the width is generally less than one-third the overall diameter of the hair shaft. The medulla in animal hairs is normally continuous and structured and generally occupies an area of greater than one-third the overall diameter of the hair shaft.
Human Hair Classification
Hair evidence examined under a microscope provides investigators with valuable information. Hairs found on a knife or club may support a murder and/or assault weapon claim. A questioned hair specimen can be compared microscopically with hairs from a known individual, when the characteristics are compared side-by-side.
Human hairs can be classified by racial origin such as Caucasian (European origin), Negroid (African origin), and Mongoloid (Asian origin). In some instances, the racial characteristics exhibited are not clearly defined, indicating the hair may be of mixed-racial origin.
The region of the body where a hair originated can be determined with considerable accuracy by its gross appearance and microscopic characteristics. The length and color can be determined. It can also be determined whether the hair was forcibly removed, damaged by burning or crushing, or artificially treated by dyeing or bleaching.
The characteristics and their variations allow an experienced examiner to distinguish between hairs from different individuals. Hair examinations and comparisons, with the aid of a comparison microscope, can be valuable in an investigation of a crime.
Hairs that have been matched or associated through a microscopic examination should also be examined by mtDNA sequencing. Although it is uncommon to find hairs from two different individuals exhibiting the same microscopic characteristics, it can occur. For this reason, the hairs or portions of the hairs should be forwarded for mtDNA sequencing. The combined procedures add credibility to each.
Although nuclear DNA analysis of hairs may provide an identity match, the microscopic examination should not be disregarded. The time and costs associated with DNA analyses warrant a preliminary microscopic examination. Often it is not possible to extract DNA fully, or there is not enough tissue present to conduct an examination. Hairs with large roots and tissue are promising sources of nuclear DNA. However, DNA examinations destroy hairs, eliminating the possibility of further microscopic examination.
Hair Recovery and Sampling Methods
Because of the nature of trace evidence, when processing evidentiary items, care should be taken to minimize the possibility of contamination and cross-transfer. Examinations should be sequenced to maximize the potential value of the submitted evidence.
Hairs can be recovered from evidentiary items using a number of different techniques. Some of the methods used to collect hairs from clothing and bedding items are scraping, shaking, taping, and picking. Debris from large carpeted surfaces might be vacuumed into a filtered canister. If the specific location of a hair on a clothing item is important, it might be necessary to pick off the hair or tape the item and record where the hair was removed.
Whichever method is used, it should be done in a location designed for that purpose to avoid the possibility of contamination and cross-transfer. Special lighting and magnification may facilitate the location and recovery.
After trace debris has been removed from items of evidence, it is necessary to select the appropriate types and number of hairs for examination. Sometimes when removing a large quantity of debris (e.g., vacuuming), it may be necessary to select only a representative sample. This process includes selecting samples such as hairs of different lengths, racial groups, body area, and color. Another method is to select hairs that are similar in appearance to a target group (e.g., known hairs from a suspect or victim). The combination of random and target sampling ensures a representative sample.
Selecting hairs for microscopic analyses takes place during the initial processing as well as during low-power microscopy at the bench. The microscopic characteristics of hairs are viewed and selected with the intention of providing an examiner with a good range of the hair types present.
Head hairs and pubic hairs exhibit a greater range of microscopic characteristics than other human hairs; therefore, head and pubic hairs are routinely forensically compared. An adequate selection of known hair samples includes both random pullings and combings. The number of hairs necessary to represent a suitable known sample varies with the individual. Twenty-five randomly selected head hairs are generally considered adequate to represent the range of hair characteristics of that individual. It is recommended that the same number of hairs be collected from the pubic region. The selection of hairs to be mounted from a known hair standard may be random, but representative, especially when the known standard consists of many hairs.
The collection of known head hair standards from a suspect might take place many months, possibly years, after the crime. In these instances, the characteristics of the known head hair sample may look quite different from hairs that were shed when the crime occurred. Some hair examiners have indicated that a one-year time span is the outside limit, and environmental conditions or cosmetic alterations could make it shorter. Pubic hairs seem to retain their characteristics for a longer period of time.
Hair Comparison Characteristics
Certain physical features such as sex, size, age, shape, eye color, hair texture, and color can distinguish individuals. None of these features is peculiar to only one individual, but the general appearance and arrangement of these features serves as criteria for identification. There are, likewise, a number of features or characteristics that may be present in a given hair sample that, when considered collectively, provide a basis for association.
There is no criterion for the importance assigned to a particular characteristic. Such a determination can be made only by the individual examiner and must be based on experience. Hair characteristics are not frequently studied because of all the variations in a single sample and the inherent difficulty in assigning standard values for the variations. If, however, particular characteristics are seen in hair samples that appear with regularity throughout the sample, they must be considered as significant.
The process of identification or association involves distinct stages in the course of an examination. The following 15 different features or characteristics should be considered in the comparison of hair specimens. There are other lists that identify 25 or more hair characteristics, but those lists generally do not disagree in substance with the following, only in the manner of organization.
Race: Features that serve to determine racial origin have been discussed previously. Hairs of a particular racial group can exhibit a significant range in the distribution of microscopic characteristics.
Body area: Body area characteristics have been discussed previously. As a general rule, most comparisons are conducted using head and pubic hair samples. Hairs from other body areas may be of limited comparative value.
Color: There are many variations among individuals in hair color. The particular hue (color shade), value (lightness or darkness), and intensity (saturation) of a specimen are enhanced through microscopy so that even subtle differences may be distinguished. The range in color of a particular hair sample and the variations in color that exist along the length of hairs are important comparison characteristics.
Length: Length is considered, although hairs may have been cut between the time of deposition of the questioned specimen and the collection of a known sample. In addition, there may be a significant difference in the lengths of the shortest and longest hairs on an individual’s head.
Tip: The tip can be cut, broken, split, abraded (rounded), or finely pointed as illustrated by Figures 51-55. An individual’s grooming, hygiene, health, and nutrition can affect these features.
Root: The mature hair root will be hardened, have a bulbous shape, and have little or no follicular tissue adhering to it. Pigment is sparse or absent, and there is frequently an abundance of cortical fusi. A root that has been plucked prior to maturation will be soft, have a distorted appearance, and may have tissue adhering to it. Pigment is present, and there are rarely cortical fusi. A catagen root may exhibit the bulbous shape with a tag attached. Hairs are naturally sloughed from the body after a period of growth. The life cycle includes a growing or anagen phase, a transition or catagen phase, and a resting or telogen phase.
Diameter: The overall shaft diameter can range from very fine (40-50um) to very coarse (110-120um). Consideration is given to the range of variation in a particular sample and the variation in a single hair shaft. Consideration is also given to the degree of shaft diameter variation as well as the rate of change between variations. The phenomenon of abrupt and radical changes is referred to as buckling. The shape of the hair shaft and how the hair lies on the glass microscope slide influences the apparent shaft diameter variations that exist in hair. What appears to be diameter variation may be different viewing angles of a constant diameter. The twisting of flat to oval hairs on a glass microscope slide also influences the interpretation of diameter variation.
Cuticle: The cuticle is comprised of an outer layer of scales that may vary in thickness and color. There may even be variation in thickness and color throughout the length of a single hair. The inner margin of the cuticle may be clearly defined or may be without a sharp delineation
Scales: A scale cast is not necessary to observe the features of scales. The scales may be undisturbed and closely aligned with the hair shaft, or they may protrude outward from the shaft. Scale damage and protrusion are associated with mechanical action such as backcombing or harsh chemical action such as dyeing and bleaching. The scales may protrude out from the hair shaft, then recurve back to the shaft, giving a looped appearance
Pigment: The pigment granules may be absent as in gray hair or may be so dense that they obscure the inner structural detail of the hair specimen. Granule size can range from very fine to coarse. Consideration is given to local distribution of the pigment across the hair shaft, as well as to variations in distribution and density along the shaft from proximal to distal. The granules can be regularly arranged in streaks or clumps, with consideration given to the size, distribution, and density of these groupings.
Medulla: The structure of the medulla can vary from continuous throughout the center of the hair shaft to fragmentary, or absent altogether. It can be opaque, translucent, vacuolated, or completely amorphous in appearance. When the medulla is fragmentary, the cell structures may appear fusiform, or spindle-shaped. The width of the medulla in relation to the overall shaft diameter should be considered.
Cortex: The general appearance of the cortex should be considered. The margins of the elongated cells comprising the cortex may be poorly defined or may be distinct (Figure 69). These cells are prominent, particularly in hairs that have been bleached and have resulted in a straw-like appearance.
Artificial treatment: Bleaching removes pigment from the hair and can give the hair a characteristic yellow cast. The cortical cell margins may become more prominent and cortical fusi may develop. In addition, harsh or repeated treatments can make the hair shaft brittle, and the scales will appear disturbed. Artificial bleaching can be distinguished from solar bleaching by a sharper line of demarcation between the bleached and unbleached regions. To the experienced examiner, dyed hairs possess an unnatural cast or color. In addition, the cuticle will take on the color of the dye. If hair generally grows at the rate of one half-inch per month, the distance can be measured from the root to the line of demarcation of the dyed portion to estimate the time since dyeing. Repeated dyeing or bleaching results in several lines of demarcation. This would serve to further individualize a particular hair specimen.
Damage: Cutting with scissors produces a sheared or square-cut end, whereas a razor cut is angular and very straight or clean. The length of time since cutting is subject to many variables; hence, no reliable determination can be made. Crushed hairs exhibit a widening of the hair shaft, and the cortical cells appear ruptured and separated. Broken hairs exhibit a square tip with elongated fragments. Burned or singed hairs are charred and brittle and exhibit round vacuoles at the point of burning.
References and Further Reading
Modification History File Created: 05/02/2019 Last Modified: 05/15/2019
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