by Andrea Campbell
I often get forensic science questions from readers. This one is about hair analysis.
A.: A true forensic approach to hair examination will reveal that it is not yet possible to individualize a human hair to any single head or body. Over decades, scientists have tried to find a way to pinpoint the physical and chemical properties of hair, so that it could be used as an individual characteristic of identity. These efforts fail because there is no one property that remains consistent with time or is uniform throughout the head or body. It is said that the color and structure of hair are its most characteristic features, because its chemical properties are not relevant for forensic distinction. Saying that does not mean that hair has no value as physical evidence; it does. When hair is properly collected at a crime scene, and the laboratory tests are accompanied by an adequate number of controls, hair provides strong corroborative evidence for placing an individual within the vicinity of a crime. It is usually placed in the category called “trace evidence,” along with fibers and other minutiae.
In the question as posed above, hair is a smashing good witness over time because its best two features are: its capacity to hold up against chemical decomposition, and its ability to retain structural features over a long duration.
Animal or Human?The forensic goal for hair evidence in a criminal case usually involves two questions. Is it human or animal hair? And, secondly, how does the hair found at a crime scene compare with hair from a particular individual?
The first question—animal or human—is answered with the help of the medulla. The medulla is a collection of cells resembling a canal running through a hair. In most animals, this canal is a predominant feature, occupying more than half of the hair’s diameter. The medullary index measures the diameter of the medulla relative to the diameter of the hair shaft and is normally expressed as a fraction. For humans, the index generally has a value less than 1/3. For animals, the value is 1/2 or greater.
Not all hairs have medullae—head hairs generally exhibit none, or have fragmented ones with one exception, the Mongoloid race (this is to say, Asians, American Indians, Eskimos, etc). Their head hairs usually have continuous medullae. The shape and form of medullae, referred to as medullation, is different for different species as well. Under a microscope, they look like distinct patterns: a cat’s, for example, resemble a string of pearls. There are reference standards that scientists use to differentiate, and, of course, experience helps in their evaluations.
A more common request in a forensic investigation is whether or not scalp or pubic hair from the crime scene compares to a suspect’s hair. The evidentiary value boils down to the degree of probability with which the examiner can associate the questioned hair with the known sample.
Hair's Characteristics A comparison microscope is an invaluable tool, in that it allows the technician to view the questioned hair alongside the known sample. Because hair from any part of the body exhibits a range of characteristics, it is necessary to have an adequate number of known hairs that are representative of all its features when making a comparison.
The criminalist is particularly interested inmatching the color, length and diameter. Other important features are the presence or absence of a medulla and the distribution, shape, and color intensity of the pigment granules present in the cortex. Under a microscope you will also be able to distinguish dyed or bleached hair, and detect the presence of fungal or nit infections. It is pretty easy to figure out the body area of a hair because scalp hairs show little diameter variation and have a more uniform distribution of pigment color. Pubic hairs (yes!) are short and curly; with wide variations in shaft diameter and mostly have a continuous medullae. Beard hairs are coarse and customarily triangle in cross section, with blunt tips from all the cutting and shaving. The age of a person cannot be determined from hair other than with babies who have fine hair and fine pigment, whereas, a gray hair is fairly characteristic on its own. But it is possible to determine if hair was forcibly removed. If a hair root is found to have follicular tissue (root sheath cells), that hair may have been pulled or brushed out. Naturally falling hair will show a bulbous-shaped root but will be free of any adhering tissue. The current examination techniques of hair specimens still rely on mostly morphological characteristics, although the feasibility of recovering DNA from hair and typing a segment is now possible. This method utilizes a technique called polymerase chain reaction (PCR). It amplifies the small amount of DNA found in the human hair root (follicle tissue). The actual success rate for DNA typing is in the area of 34% for case samples. In 1996, the FBI initiated a program to compare human head hair and pubic hairs through DNA analysis. This type of DNA analysis is called mitochondrial DNA and is located within the nuclei of body cells. The DNA is transmitted only from mother to child. It can be used when the amount of DNA present is small or degraded. Hair is class evidence and it is not possible except in rare cases to determine that a questioned hair sample came from a particular individual to the exclusion of all others. The examiner to a large extent must rely on his experience and a statistical probability. They may say something like, “In my opinion, the representative hairs revealed that the odds of two similar pubic hairs originating from two different individuals are about 800 to 1." Collecting Hair To collect hair evidence, one usually collects about 50 full-length hairs from all areas of the scalp for a representative sampling. A minimum collection of two dozen full-length pubic hairs should cover the range of characteristics present in that portion of the body. In rape cases, a clean comb is used to capture all loose foreign hair present in the pubic area before the victim is sampled for control hair. The comb is then packaged in a separate envelope. And, a criminalist would not collect, nor compare, head hair with pubic hair—separate is the watchword here.
As a routine procedure, hair samples are collected from a victim of suspicious death during an autopsy. Failure to make this simple collection at an opportune time may result in complicated legal problems at a later date.
Test your knowledge with a mini-quiz at this interesting Web site.