The scientific study of hair is called trichology and this field dates to the mid 1800s. Forensic scientists perform three major types of hair analysis. Chemical assays are used to assess the use of illegal drugs, to screen for the presence of heavy metals in the body, and to test for nutritional deficiencies. The root of the hair has cells that contain DNA , which can be used for DNA analyses. Microscopic comparison of hair collected from two different places is used to determine if the hairs are from the same person or animal.
Because hair can be moved from location to location by physical contact, the presence of a specific person's hair can link a suspect or a victim to a crime scene. If hair is transferred directly from the region of the body from which it originates, it is considered a primary transfer. Approximately 100 head hairs are lost per person per day. These hairs usually end up on clothing, furniture, or on other items in the environment. Transfer of hair from these items is called secondary transfer. Secondary transfer of hair is very common with animal hairs, which are commonly found on pet owners and in the environment of pet owners and can be used to link suspects to crime scenes.
Hair grows out of living cells in epidermis of mammals. It is almost entirely made up of the protein keratin. The club-shaped hair root is anchored in a follicle, which has associated muscles, called arrector pili. When these muscles contract, hair becomes oriented nearly perpendicular to the skin. The hair itself is composed of three layers: the medulla, the cortex, and the cuticle. The medulla is the innermost canal that extends through the hair. In humans it can be continuous or discontinuous, interrupted by a series of empty spaces. Surrounding the medulla is the cortex, which makes up the majority of the mass of the hair. The outermost layer is the cuticle, which is a single layer of scales. In humans these scales overlap quite a bit and cling tightly to the cortex.
Pigments are found in both the cortex and the medulla, but they are absent from the cuticle. In humans, the pigments tend to be distributed toward the outer edges of the cortex, but this can vary depending on ethnicity. In human hair, the medulla is generally narrow, taking up less than a third of the diameter of the entire shaft. In hairs from animals, the diameter of the medulla is larger than half the diameter of the entire shaft. The cross section of human hair is most often circular, but occasionally oval.
Using morphologic features, forensic scientists classify six different types of hair on the human body: head hair, eyebrow and eyelash hair, beard and moustache hair, body hair, pubic hair, and axillary hair. Biochemical studies show that there are no significant differences in chemical structure among the hair types. Animals also produce different types of hair. They often have coarse guard hair external to softer fur hairs. They also produce whiskers and longer hairs in such places as the tail and mane.
In humans, hair undergoes cyclical phases of growth (anagen), transition (catagen), and resting (telogen). During the growth phase, the cells of the follicle actively divide and grow upward. The average anagen phase lasts about 1,000 days. During the telogen phase, the cells of the follicle are dormant and hairs naturally fall out. This phase usually lasts for 100 days. At any time, between 10 and 18% of all the hairs on a human head are in the telogen phase; about 2% are in the catagen phase and the rest, between 80 and 90% are actively growing. There is no pattern to determine which hairs on the head are in any phase at a given time.
Because hair grows out of follicles in the skin, materials in the body are incorporated into the hair. Hair grows relatively slowly, so it takes several weeks for materials in the body to be reflected in the composition of the hair. Hair that is collected for the presence of drugs, heavy metals, and nutritional insufficiencies is usually clipped from the nape of the neck. About a spoonful is necessary for analysis.
Results from hair composition analysis are somewhat controversial. A variety of factors impact the results, including the location on the body from where the hair was removed, the color of the hair, and the person's age and race. Standards vary as to methods of washing, cutting, and collecting hair. Standards for analysis also vary and a single lab may report different results from subsamples of the same sample. False-positives for illegal drugs are not uncommon and can occur when someone is in the presence of second hand smoke from marijuana or crack cocaine. External substances such as air pollution, composition of the water used to wash hair, and materials used to treat hair such as shampoo, hairspray, and hair dyes may also skew results. Hair analyses that do report the presence of illegal drugs or heavy metals should be verified with blood or urine tests.
Microscopic hair analysis has two components. The first is to identify characteristic features of the hair in question. The second is to then compare these features in the questioned hair and hair from a known origin. In particular, the hair in question may be collected as evidence from a crime scene and the known hair may be collected from a suspect or from a suspect's possessions. Microscopic hair comparison in forensic laboratories usually involves the use of two compound microscopes that are optically connected so that the hair in question and the hair from a known origin are in the same field of view. The hair is usually magnified between 40x and 400x.
The first step of the examination involves verifying whether the hair in question is that of a human or an animal. If the hair is from an animal, the examiner can potentially identify the species from which it originated, but it is usually impossible to assign the identity of a hair to a particular animal. In the case of dogs, most examiners can attribute hairs to given breeds. If the hair is from a human, the examiner will determine the part of the body from which it originated. Some of the features that the examiner uses include length, shape, size, color, stiffness, curliness, pigmentation, and the appearance of the medulla. The majority of hairs examined in forensic investigations originate from the head and the pubic areas.
Microscopic hair examiners can categorize hair into three different racial groups based on established models. These groups are European ancestry, Asian ancestry, and African ancestry. Hairs from people of European ancestry are generally straight or wavy, have cross-sections that are round or oval and have fine to medium-sized pigment granules that are distributed evenly. Hairs from people of Asian ancestry are straight, have circular cross-sections and have medium-sized pigment granules that are grouped in patches. They may also have a thicker cuticle than in the hairs of other races. Hairs from people of African ancestry are usually curly or kinky and they have an oval cross-section. The pigment granules are large and are found clumped in groups. The hair shaft may twist or buckle and commonly splits. Head hair shows the most distinguishing characteristics for determining race, however other body hairs also evidence identifying characteristics. Complications with the assignment of race involve analysis of hair from infants and from people of mixed race.
The determination of age from hair is usually not possible by microscopic examination. Some general information may be surmised however, as the hair of infants is usually fine and contains few racial indicators. Hair of the elderly shows signs of pigment loss and often has variable diameter. The follicle of hairs contains chromosomes that can be stained to determine the sex of the individual. However, sex is usually determined from DNA testing.
Examination of the root can provide information as to the nature of a crime, especially if violence is suspected. If hairs fall out naturally during the telogen (resting) phase, the root will have a club shape. If hair is pulled out with force, the root will be stretched or broken and may have tissue attached. Examiners can also determine if hair has been burned, cut, or crushed.
A variety of factors influence the microscopic analysis of hair, including the experience and technique of the examiner. Because microscopic hair analysis is subjective, no statistics can be assigned to the probability that a hair belongs to an individual. The analogy often referred to is that an individual can recognize the face of a friend among a group of people even though all of them have the same features: eyes, nose, and mouth. In the same way, an experienced hair examiner can recognize those features of hair that determine whether or not it belongs to a specific individual.
DNA from cells associated with the root can be extracted and used for DNA analysis. Analysis of the DNA in the nucleus of the cell can be used for determining identity and DNA from the y-chromosome focuses on questions of paternity. Mitochondrial DNA is useful for establishing maternity. In theory, a single cell contains sufficient DNA to use for DNA analysis and so a single hair should provide the material required. In practice, a variety of complications make DNA testing of hair more complex. Roots of hair in the anogen (growth) phase contain more DNA than hairs from the telogen (resting) phase. However, hairs in the telogen phase are more likely to fall out passively. In addition, contamination issues are important as dead skin cells, which are also shed passively, contain DNA and may be collected from surfaces along with hair. If a hair from the telogen phase is collected, it may not contain enough nuclear DNA for analysis, but it might contain mitochondrial DNA. If the hair has been forcibly removed, then pieces of tissue may be attached and DNA analyses can usually be run easily on these tissue cells.
see also Crime scene investigation; DNA fingerprint; Microscope, comparison; Mitochondrial DNA analysis; STR (short tandem repeat) analysis.