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
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. This revision of the
1977 Microscopy of Hairs: A Practical Guide and Manual by John W. Hicks
is intended to introduce hair evidence to the forensic examiner and to provide
a foundation for its proper identification and comparison.
Basic Structure of Hair
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 regionsthe cuticle, medulla, and cortex. These regions are illustrated in Figure 1 with some of the basic structures found in them. The illustration is a diagram used to emphasize structural features discussed in this guide. Certain structures may be omitted, and others enhanced for illustrative purposes.
Figure 1. Hair Diagram
A hair grows from the papilla and with the exception of that point of generation is made up of dead, cornified cells. It consists of a shaft that projects above the skin, and a root that is imbedded in the skin. Figure 2 diagrams how the lower end of the root expands to form the root bulb. Its basic components are keratin (a protein), melanin (a pigment), and trace quantities of metallic elements. These elements are deposited in the hair during its growth and/or absorbed by the hair from an external environment. After a period of growth, the hair remains in the follicle in a resting stage to eventually be sloughed from the body.
Figure 2. Diagram of Hair in Skin
The cuticle is a translucent outer layer of the hair shaft consisting of scales that cover the shaft. Figure 3 illustrates how the cuticular scales always point from the proximal or root end of the hair to the distal or tip end of the hair.
Figure 3. Scanning Electron Photomicrograph of Hair
There are three basic scale structures that make up the cuticlecoronal (crown-like), spinous (petal-like), and imbricate (flattened). Combinations and variations of these types are possible. Figures 4-9 illustrate scale structures.
- The coronal, or crown-like scale pattern is found in hairs of very fine diameter and resemble a stack of paper cups. Coronal scales are commonly found in the hairs of small rodents and bats but rarely in human hairs. Figure 4 is a diagram depicting a longitudinal view of coronal scales, and Figure 5 is a photomicrograph of a free-tailed bat hair.
Figure 4. Diagram of Coronal Scales
Figure 5. Photomicrograph of Bat Hair
- Spinous or petal-like scales are triangular in shape and protrude from the hair shaft. They are found at the proximal region of mink hairs and on the fur hairs of seals, cats, and some other animals. They are never found in human hairs. Figure 6 is a diagram of spinous scales, and Figure 7 is a photomicrograph of the proximal scale pattern in mink hairs.
Figure 6. Diagram of Spinous Scales
Figure 7. Photomicrograph of Proximal Scale Pattern (Mink)
- The imbricate or flattened scales type consists of overlapping scales with narrow margins. They are commonly found in human hairs and many animal hairs. Figure 8 is a diagram of imbricate scales, and Figure 9 is a photomicrograph of the scale pattern in human hairs.
Figure 8. Diagram of Imbricate Scales
Figure 9. Photomicrograph of Scale Pattern (Human)
The medulla is a central core of cells that may be present in the hair. If it is filled with air, it appears as a black or opaque structure under transmitted light, or as a white structure under reflected light. If it is filled with mounting medium or some other clear substance, the structure appears clear or translucent in transmitted light, or nearly invisible in reflected light. In human hairs, the medulla is generally amorphous in appearance, whereas in animal hairs, its structure is frequently very regular and well defined. Figures 10 through 13 are photomicrographs of medullary types found in animal hairs. Figure 10 exhibits a uniserial ladder, and Figure 11 exhibits a multiserial ladder, both found in rabbit hairs. Figure 12 exhibits the cellular or vacuolated type common in many animal hairs. Figure 13 exhibits a lattice found in deer family hairs.
Figure 10. Photomicrograph of Uniserial Ladder Medulla
Figure 11. Photomicrograph of Multiserial Ladder Medulla
Figure 12. Photomicrograph of Animal Hair
Figure 13. Photomicrograph of Deer Medulla
When the medulla is present in human hairs, its structure can be described asfragmentary or trace, discontinuous or broken, or continuous. Figure 14 is a diagram depicting the three basic medullary types.
Figure 14. Diagram of Medullas (Trace, top; Discontinuous, middle; Continuous, bottom)
The cortex is the main body of the hair composed of elongated and fusiform (spindle-shaped) cells. It may contain cortical fusi, pigment granules, and/or large oval-to-round-shaped structures called ovoid bodies.
Cortical fusi in Figure 15 are irregular-shaped airspaces of varying sizes. They are commonly found near the root of a mature human hair, although they may be present throughout the length of the hair.
Figure 15. Photomicrograph of Cortical Fusi in Human Hair
Pigment granules are small, dark, and solid structures that are granular in appearance and considerably smaller than cortical fusi. They vary in color, size, and distribution in a single hair. In humans, pigment granules are commonly distributed toward the cuticle as shown in Figure 16, except in red-haired individuals as in Figure 17. Animal hairs have the pigment granules commonly distributed toward the medulla, as shown in Figure 18.
Figure 16. Photomicrograph of Pigment Distribution in Human Hair
Figure 17. Photomicrograph of Pigment Distribution in Red Human Hair
Figure 18. Photomicrograph of Pigment Distribution in Animal Hair
Ovoid bodies are large (larger than pigment granules), solid structures that are spherical to oval in shape, with very regular margins. They are abundant in some cattle (Figure 19) and dog (Figure 20) hairs as well as in other animal hairs. To varying degrees, they are also found in human hairs (Figure 21).
Figure 19. Photomicrograph of Ovoid Bodies in Cattle Hair
Figure 20. Photomicrograph of Ovoid Bodies in Dog Hair
Figure 21. Photomicrograph of Ovoid Bodies in Human Hair
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 more dense 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.
The root of human hairs is commonly club-shaped (Figure 22), whereas the roots of animal hairs are highly variable.
Figure 22. Photomicrograph of Human Hair Root
The scale pattern of the cuticle in human hairs is routinely imbricate. Animal hairs exhibit more variable scale patterns. The shape of the hair shaft is also more variable in animal hairs.
Human Hair Classifications
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.
Methods of Hair Recovery
Sir Edmund Locard's principle (1930) states that "whenever two objects come into contact, a transfer of material will occur. Trace evidence that is transferred can be used to associate objects, individuals, or locations" (Scientific Working Group on Materials Analysis 1999). 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.
It may be necessary to make a scale cast of the hair specimen in order to see the scale pattern more clearly, particularly in the identification of some animal hairs. Ogle and Mitosinka (1973) devised a quick and easy method of making a scale cast with the use of a Polaroid film-print coater. A thin layer is applied to a glass microscope slide with two or three passes of the Polaroid print coater. The hair specimen is lightly pressed onto the film and allowed to stand until the film is dry. The hair is then pulled from the film, and the cast remains.
A method developed by Crocker (1998) at the Centre of Forensic Sciences in Toronto, Canada, uses clear tape as a mounting medium and coverslip together, which allows for quick observation of such surface features as the scale pattern.
Scale casts may also be prepared using clear nail polish. A thin coat is painted on a glass microscope slide or, if the lacquer is thinned with acetone, a drop may be allowed to run down the surface of the slide. The hair is placed on the slide and allowed to dry. When the surface has dried, the hair is removed to reveal the scale pattern.
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.
Glass Microscope Slide Preparation
Hair specimens are prepared for microscopic examination by mounting them in a semipermanent medium, such as Permount®. Whatever mounting medium is selected, the refractive index of the medium should approximate that of hair (1.52) in order to visualize the internal microscopic characteristics.
Positioning a hair on the glass slide is made easier by first applying a thin film of solvent on the slide surface. Longer hairs are configured in a figure eight in order to fit it under the cover slip. This enables the examiner to view the entire hair from root to tip. One or more hairs can be mounted on a slide, depending on their thickness and curl. Too many hairs on one slide can cause excessive overlapping that may obscure the viewing of characteristics of underlying hairs. Excess solvent can be removed with a small square of blotter paper. Several drops of mounting medium are applied on top of the hair(s), and a cover slip is carefully lowered to prevent the presence of air bubbles. Figure 23 diagrams this process. It may be necessary to apply some weight to the cover slip in order to ensure a thin mount. The thinner the mount, the easier it is to examine the hairs.
Figure 23. Illustration of Slide Preparation
In most cases, hairs can be mounted directly onto the slide; however, occasionally, in order to observe structural detail, it may be necessary to clean debris from the hairs. If covered with blood, hairs can be cleaned with a saline solution, but because water is not miscible in Permount®, the sample must be dried completely before applying the mounting medium. Oily or other debris-contaminated hairs can be cleansed with xylene or an ether-alcohol solution. Before cleaning the hairs, consider if any blood and other materials on the surface may have evidentiary value.
Conducting a reliable hair examination involves maintaining a reliable microscope. It must be kept in good working condition and adjusted for proper illumination. General care and cleaning procedures should be followed to prevent dust, dirt, fingerprints, or other contaminants from affecting the use of the microscope.
The specimen field is illuminated with a low-voltage tungsten filament lamp. A color-correcting blue filter is used to approximate white light. If each side of the comparison microscope uses a separate light source, it is essential to color-balance the light sources before starting any comparison. Calibrating for uniform illumination between the sides of the comparison microscope and in each field of view should be done daily.
Köhler illumination ensures that the light path is optimized. A modified Köhler illumination calibration process is as follows:
- Open the field and aperture diaphragms
- Adjust the interpupillary distance of the oculars
- Place the specimen on the stage and focus using the nonadjustable eyepiece, then use the adjustable eyepiece to focus that eyepiece
- Close the field diaphragm by half
- Focus and center the condenser
- Open the field diaphragm until it is just out of view
- Remove the eyepiece and close the aperture diaphragm by 1/3
- Replace the eyepiece
The microscope is ready to use.
The field diaphragm protects the specimen against unnecessary heating. As part of Köhler illumination, the field diaphragm is closed, and then opened until it just clears the field of view. If it is opened too far, the excess light will cause the image to lose its sharpness and contrast.
Aperture Diaphragm (condenser)
The aperture diaphragm determines the resolution and contrast of the microscopic image. To observe specimens under normal contrast, remove the eyepiece and reduce the objective aperture by one-third.
The condenser lens concentrates the light on the specimen. It should be in position with the objective at a numerical aperture (N.A.) larger than 0.25. The condenser lens may be swung out of position with the objective having an N.A. of less than 0.25.
The objective lens forms an inverted and side-reversed intermediate image. On each objective are sets of numbers. The first set refers to the mechanical tube length, which includes the thickness of the cover glass for which the objective was designed (in mm). The next set indicates the magnification of the objective lens and the numerical aperture. Letters before the magnification identify the type of system. Apo refers to apochromatic, Fl stands for fluorite, or Oel refers to oil immersion. If nothing is listed before the magnification, the system is achromatic.
The numerical aperture is an important aspect of the microscope; it determines the resolving power or the ability of the objective to focus on the separate features in a sample. It is the relationship between the widest angle of the path light travels to the objective, and the refractive index of the mounting medium that the light passes through. This relationship determines the maximum resolution of the objective. The ability of the objective lens to make fine structural detail in the specimen distinct is the main purpose for using a microscope, thus its dependence on the numerical aperture is important to understand.
Resolving power = λ/(2 N.A.)
where N.A. = n • sinα
α = angle formed from the outer ray of light admitted by the objective and optical axis
α = A/2
The useful magnification of a light microscope is approximately 1000 times the numerical aperture of its objective. Generally the range of magnification used is between 40 and 400x. Calculating the maximum useful magnification for each objective used requires multiplying the figures printed on the objective by 1000 and comparing the result to the magnification.
The revolving nosepiece allows the objective lenses to be conveniently rotated. The different objectives are designed to be parfocal, and therefore require only fine adjustment of focus when rotated.
The binocular tube allows the use of both eyes in observing the specimen. It can be adjusted according to the interpupillary distance of the observer.
Eyepiece or Ocular
The eyepiece gathers the intermediate image produced by the objective and magnifies the image. The product of the magnification of the objective and the magnification of the eyepiece yields the resulting or total magnification.
The glass microscope slide is placed on the stage for observation.
Analysis and Comparison
Individual hairs must be visually separated into their component parts or characteristics. The color, size, and configuration of these characteristics, as well as their relationship to each other, are observed. Examiners may use a check-off sheet that lists the various macroscopic and microscopic characteristics during this process.
The characteristics of the questioned hair determined through analysis are compared with characteristics present in hair samples of known origin for consistencies or inconsistencies.
Human Hairs Identification
Human hairs can generally be identified by racial origin, body area, and other comparison characteristics.
Key characteristics serve as racial indicators. These indicators are generalities and apply primarily to head hairs. The examiner may encounter hairs that cannot easily be associated with a particular racial model because of poorly defined characteristics, limited size, or inconsistent indicators. These hairs can be identified as apparent racial mixtures or as not classifiable. In spite of an inability to substantiate race, the hair may still be of value for comparison purposes. This racial admixture may serve to further individualize the hair and its source, particularly if the same mixed racial characteristics are observed in both the questioned and known samples.
Caucasian (Figures 24 and 25)
- Shaft diameter: moderate with minimal variation (mean diameter for human head hairs - 80um)
- Pigment granules: sparse to moderately dense with fairly even distribution
- Cross-sectional shape: oval
Figure 24. Photomicrograph of Cross-section of Caucasian Hair
Figure 25. Photomicrograph of Caucasian Head Hair
Negroid (Figures 26 and 27)
- Shaft diameter: moderate to fine with considerable variation
- Pigment granules: densely distributed (hair shaft may be opaque) and arranged in prominent clumps
- Shaft: prominent twist and curl
- Cross-sectional shape: flattened
Figure 26. Photomicrograph of Cross-section of Negroid Hair
Figure 27. Photomicrograph of Negroid Head Hair
Mongoloid (Figures 28 and 29)
- Shaft diameter: coarse and usually with little or no variation
- Pigment granules: densely distributed and often arranged in large patchy areas or streaks
- Medulla: prominent (often broad and continuous)
- Cuticle: thick
- Cross-sectional shape: round
Figure 28. Photomicrograph of Cross-section of Mongoloid Hair
Figure 29. Photomicrograph of Mongoloid Head Hair
Certain features of individual hairs identify the region of the body where it originated. The features listed are generalities and align themselves with racial models derived from known samples. Body area can be made with considerable accuracy; however, variations occur that can make this determination difficult. Hairs that fall into this category include those that are immature, transitional, and fragmentary.
- Long with moderate shaft diameter and diameter variation
- Medulla absent to continuous and relatively narrow when compared to the structure of hairs from other body areas
- Often with cut or split tips
- Can show artificial treatment, solar bleaching, or mechanical damage
- Soft texture, pliable
- Shaft diameter coarse with wide variations and buckling (Figure 30)
- Medulla relatively broad and usually continuous when present (Figure 31)
- Root frequently with tag (Figure 32)
- Tip usually tapered, rounded, or abraded
- Stiff texture, wiry
Figure 30. Photomicrograph of Pubic Hair Buckling
Figure 31. Photomicrograph of Pubic Hair Medulla
Figure 32. Photomicrograph of Pubic Hair Root
- Diameter fine with little variation
- Gross appearance of hair is arc-like in shape
- Medulla is discontinuous to trace with a granular appearance (Figure 33)
- Tips usually tapered, often blunt and abraded, rounded scale ends due to wear (Figure 34)
- Soft texture
Figure 33. Photomicrograph of Limb Hair Medulla
Figure 34. Photomicrograph of Limb Hair Tip
Facial Hairs (Beard/Mustache)
- Diameter very coarse with irregular or triangular cross-sectional shape (Figure 35)
- Medulla very broad and continuous, may be doubled (Figure 36)
Figure 35. Photomicrograph of Beard Hair (Shape)
Figure 36. Photomicrograph of Beard Hair Medulla (Doubled)
- Shaft diameter moderate and variable
- Tip often darker in color, long and fine, arc-like
- Medulla may be granular
- Stiff texture
Axillary or Underarm Hairs
- Resemble pubic hairs in general appearance, but less wiry
- Medullary appearance similar to limb hairs
- Diameter moderate and variable with less buckling than pubic hairs
- Tips long and fine, frequently with bleached appearance
Other Body Hairs
- Eyebrow: Stubby, some diameter fluctuation, saber-like in appearance
- Eyelash: Short, stubby with little shaft diameter fluctuation, saber-like in appearance
- Trunk: A combination of features of limb and pubic hairs, a transitional hair
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 a 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. The photomicrographs of Figures 37-45 illustrate some of the variation that can be seen in the head hairs of different individuals from different racial groups.
Figure 37. Photomicrograph of Head Hair of Caucasian Individual
Figure 38. Photomicrograph of Head Hair of Caucasian Individual
Figure 39. Photomicrograph of Head Hair of Caucasian Individual
Figure 40. Photomicrograph of Head Hair of Negroid Individual
Figure 41. Photomicrograph of Head Hair of Negroid Individual
Figure 42. Photomicrograph of Head Hair of Negroid Individual
Figure 43. Photomicrograph of Head Hair of Mongoloid Individual
Figure 44. Photomicrograph of Head Hair of Mongoloid Individual
Figure 45. Photomicrograph of Head Hair of Mongoloid Individual
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. The photomicrographs of Figures 46-50 illustrate the variation in color that can exist in one hair.
Figures 46-50. Five regions of a Single Head Hair (Proximal to Distal)
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,
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.
Figure 51. Photomicrograph of Rounded (Limb) Hair Tip
Figure 52. Photomicrograph of Glass-Cut or Broken Hair Tip
Figure 53. Photomicrograph of Abraded Hair Tip
Figure 54. Photomicrograph of Cut Hair Tip
Figure 55. Photomicrograph of Worn Razor-Cut Tip
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 (Figures 56-58).
Figure 56. Photomicrograph of Telogen Hair Root
Figure 57. Photomicrograph of Anagen Hair Root
Figure 58. Photomicrograph of Catagen Hair Root
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 (Figures 59 and 60).
Figure 59. Photomicrograph of Inner Cuticle Margins
Figure 60. Photomicrograph of Inner Cuticle Margins
Scales: A scale cast is not necessary to observe the features of scales. The scale margins in Figure 61 are visible in the cuticle in whole mount, so their overall length can be considered. 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 as illustrated by Figure 62.
Figure 61. Photomicrograph of Scale Protrusion
Figure 62. Photomicrograph of Scale Looping
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 (Figure 63), 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.
Figure 63. Photomicrograph of One-Sided Pigment Distribution
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 (Figures 64-68). 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.
Figure 64. Photomicrograph of Continuous Clear Medulla
Figure 65. Photomicrograph of Continuous Opaque Medulla
Figure 66. Photomicrograph of Wafer Medulla
Figure 67. Photomicrograph of Trace Medulla
Figure 68. Photomicrograph of Bubbly or Cellular Medulla
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.
Figure 69. Photomicrograph of Coarse Cellular 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 (Figure 70). 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.
Figure 70. Photomicrograph of Dyed Human Hair
Damage: Cutting with scissors produces a sheared or square-cut end (Figure 71), whereas a razor cut is angular and very straight or clean (Figure 72). 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 (Figure73). Burned or singed hairs are charred and brittle and exhibit round vacuoles at the point of burning (Figure 74).
Figure 71. Photomicrograph of Scissor-Cut Hair
Figure 72. Photomicrograph of Razor-Cut Hair
Figure 73. Photomicrograph of Broken Hair
Figure 74. Photomicrograph of Burned Hair
Special Characteristics: There are other structures or characteristics that may be encountered and should be considered in a comparison of hair specimens.
- The presence, abundance, and distribution of ovoid bodies, the dark, oval to round shaped structures of varying sizes, can be significant comparison points (Figure 75).
Figure 75. Photomicrograph of Ovoid Bodies
- The presence, size, distribution, and density of cortical fusi should be considered (Figure 76).
Figure 76. Photomicrograph of Cortical Fusi
- Certain diseases or deficiencies may result in changes in the appearance of hair. These include ringed or banded hairs (pili annulati) (Figures 77 and 78), conspicuous nodes (trichorrhexis nodosa), or regular diameter fluctuations (monilethrix). Egg sacks of parasitic lice (Figure 79) may be attached to the base of the hair shaft. All of these serve to further individualize the hair specimen.
Figure 77. Photomicrograph of Pili Annulati
Figure 78. Photomicrograph of Pili Annulati
Figure 79. Photomicrograph of Lice Egg Case
- A double medulla (Figure 80) is encountered on occasion (usually in beard hairs, but also in red Caucasian head hairs). However, unless it is a regularly occurring feature in a sample, it is of little value for individualization.
Figure 80. Photomicrograph of Double Medulla
- The presence of a dark band (Figure 81) at the root end may indicate it was shed postmortem.
Figure 81. Photomicrograph of Postmortem Root Band
There are three basic conclusions that can be reached from a microscopic examination and comparison of hairs.
- The hairs from the questioned (Q) source exhibit the same microscopic characteristics as the hairs in a known (K) hair sample and can be associated to the source of the known hairs.
- The hairs from the questioned source are microscopically dissimilar to the hairs in a known hair sample and cannot be associated with the source of the known hairs.
- The questioned hairs exhibit both similarities and slight differences to hairs found in a known hair sample, and no conclusion can be reached whether they could have originated from the known source. It may be that, in the opinion of the examiner, the differences are not sufficient to eliminate the source of the known hairs as being a possible source of the questioned hairs. At the same time, the presence of these differences precludes an association being made between the questioned and known hairs.
In the first conclusion, it is stated that the questioned hairs can be associated with the source of the known hairs. Hairs are biological specimens and subject to variation. During the analysis of hair, the examiner must establish the range of variation in the sample, and then determine whether the questioned hair fits in that range. It has been found that when two hair samples are randomly selected from different individuals and compared microscopically, it is very unusual that they cannot be distinguished. However, the possibility cannot be dismissed that there may be two hair samples whose ranges of variation overlap and distinguishing between the samples is not possible.
The information contained in the report should be limited to a factual statement of findings concerning the examinations conducted. An interpretation of the evidence is saved for court testimony that includes an explanation about the basis for the examinations. The report should be clear, concise, and easily understood. Technical terminology that is foreign to a layperson or contributor does not serve a useful purpose. The report should contain information pertinent to the requests made by the contributor of the evidence and to the examinations performed. A listing of the items of evidence and their origin, either a description or the contributor's reference number, should be included. Results of examinations should be set out clearly, followed by a statement of the examiner's conclusions. A statement may follow to clearly state the limiting factors of hair examinations.
- Item 1 = item from crime scene
Q1 knit cap
- Item 2 = item from suspect
K1 head hair sample from John Doe
- Results of examinations: Caucasian head hairs that exhibit the same microscopic characteristics as hairs in specimen K1 were found on the Q1 knit cap.
Accordingly, these hairs can be associated to John Doe, the identified source of the K1 hairs.
Hair comparisons are not a means of absolute personal identification. The statement of results sets forth, fairly completely, those determinations that can be made (i.e., that the hairs came from the head, that they exhibit Caucasian characteristics, and that the Q hairs are consistent with the K hairs in microscopic appearance). The resulting conclusion is that given these results, the Q hairs can be associated with John Doe, the source of the K hairs. The last paragraph is optional and is given so that a reader who is not generally familiar with hair examinations can better understand the limits of hair identification.
The introduction of mtDNA sequencing can result in an addendum to the report:
The comparison of the microscopic characteristics in hairs does not constitute a basis for absolute personal identification. The probative value of hair comparisons may be affected by the results of mtDNA analysis.
It must be understood that microscopy and mtDNA sequencing are two different forensic toolseach providing separate and distinct pieces of information. Two hairs can exhibit the same microscopic characteristics and be shown to be different in mtDNA sequence. Conversely, two hairs that have the same mtDNA sequence can have very different microscopic characteristics, as in the case of two individuals with the same mother.
Expert witness testimony should include an education component on hair evidence for the jury and judge and a statement of the results as reported. The witness should be prepared to discuss the procedures used in reaching the conclusion(s) and to defend opinions. An expert witness should endeavor to promote a better understanding of the methods of examination, the theory of the transfer of trace materials, and the strengths and limitations of the science.
Significance and Value
The forensic analysis of
hair has been accepted in courts of law for many years, but this does not
necessarily validate the science. The reliability of hair examinations must
be weighed with the education and training of the examiner, as well as with
used in the analysis. The examinations must be objective and impartial, and
the weight placed on the results must be in accordance with the experience
and training of the examiner.
Human hair identifications are subjective interpretations of objective criteria. The variability and distribution of the microscopic characteristics are useful in determining whether or not a questioned hair could have originated from a particular individual.
It is recognized that hair comparisons do not constitute a basis for absolute personal identification. Whereas hairs cannot be positively identified as originating from a particular individual, it is unusual to find different people having the same hair characteristics. This is based on evidentiary samples received in casework and on proficiency tests prepared in the laboratory.
Studies (Bisbing and Wolner 1984; Gaudette and Keeping 1973) have been conducted to determine the significance of hair associations. Some of these studies attempted to establish a mathematical probability of a match. The FBI Laboratory does not use the mathematical calculations of other researchers nor does it support the feasibility of establishing a numerical probability of a hair match.
The ability to analyze and interpret hair characteristics is a skill gained by training and testing. New examiners must study hair samples from different racial groups and body areas and take hair-matching tests to demonstrate the ability to correctly associate hairs with a particular source.
Hair associations in casework should be subject to a confirmation process when another qualified examiner reviews the match before a report is issued. The significance of a hair match is dependent on the distinct qualities of the hairs and the experience of the examiner. The greater the number of associations found in casework and the greater the number of cross matches, the greater the chance of association. Mitochondrial DNA sequencing testing on matched hairs has shown a high degree of conformity.
Achromat: An objective lens system constructed of glass with limited quality of correction for color and spherical aberration.
Anagen: Actively growing root stage.
Apochromat: A complex objective lens system that produces brilliant images and is corrected to the highest degree for color and spherical aberration.
Catagen: A transitional root stage between the actively growing anagen stage and the resting telogen stage.
Comparison microscope: Two microscopes joined by an optical bridge with a split screen to see both fields of views at the same time.
Cortex: Middle portion of hair extending from the cuticle to the medulla and containing the pigment granules, cortical fusi, and ovoid bodies.
Cortical fusi: Air spaces located in the cortex of hairs.
Cuticle: Translucent outer layer of the hair shaft consisting of overlapping scales.
Fluorite system: An objective system constructed of fluorspar that provides improved color and spherical correction over achromatic lens systems.
Fur hairs: Fine hairs that make up the undercoat of mammals and provide warmth.
Guard hairs: Coarse hairs that provide protection and are usually longer than fur hairs.
Keratin: A fibrous protein forming the chemical basis for hair, nails, and feathers.
Medulla: The central portion of hair, the core area.
Melanin: A pigment that gives hair its color.
Miscible: Able to be mixed together.
Numerical aperture: A calculation that shows the ability of an objective
lens to make fine structural detail in the specimen distinct (N.A. = n • sinα).
Oil immersion: A system that uses oils of a known refractive index to test the refractive index of other materials and provide greater resolution than air-mounted specimens.
Ovoid bodies: Dark bodies of unknown origin that are a useful discriminatory characteristic in their pattern of appearance.
Papilla: Connective tissue from which hair is generated from the follicle.
Parfocal: A method of setting up the microscope in order to have similar focal distances for objectives of different magnifications.
Pigment granules: Melanin granules whose size, shape, density, and distribution vary.
Refractive index: The ratio of the velocity of light in air to the velocity of light in a medium.
Resolving power: The ability of the microscope to make visible the individual parts of an image [R = λ/(2 N.A.)].
Scales: Outermost portion of the cuticle, flattened and imbricate in humans, pointing toward the distal end of the hair shaft.
Tactile hairs (Vibrissae): Whiskers only found on animals.
Telogen: Resting stage when the root takes on club shape and is ready to be naturally shed.
Vellus: Fine body hair.
Bisbing, R. E. and Wolner, M. F. Microscopical discrimination of twins' head hair, Journal of Forensic Sciences (1984) 29:780-786.
Crocker, E. J. A new technique for the rapid simultaneous examination of medullae and cuticular patterns of hairs, Microscope (1998) 46(3):169-173.
Gaudette, B. D. and Keeping, E. S. An attempt at determining probabilities in human scalp hair comparison, Journal of Forensic Sciences (1973) 19:599-606.
Hicks, J. W. Microscopy of Hairs: A Practical Guide and Manual, Federal Bureau of Investigation, U.S. Government Printing Office, Washington, DC, 1977.
Locard, E. The analysis of dust traces, American Journal of Police Science (1930 )1:276-291.
Ogle, R. R. and Mitosinka, G. A. A rapid technique for preparing hair cuticular scale casts, Journal of Forensic Sciences (1973) 18(1):82-83.
Scientific Working Group on Materials Analysis, Trace evidence recovery
guidelines, Forensic Science Communications [Online]. Available: