hair analysis
Human and animal hair is formed from keratin, which is a protein left when the cellular material produced in the hair follicle dies. Although the cells have died, the hair retains
considerable strength. Hair, although obviously biological, is considered in the material evidence category because it is actually a special category of fiber, and the methods used for examining hair are similar to those used for fibers.
considerable strength. Hair, although obviously biological, is considered in the material evidence category because it is actually a special category of fiber, and the methods used for examining hair are similar to those used for fibers.
Hair has a structure consisting of three major components: cuticle, cortex, and medulla. The cuticle, the outer layer, is formed of a very thin sheath of overlapping scales. These scales are usually compared to the shingles on a roof. They overlap and are strong and waterproof. As a result, hair is difficult to destroy. A hurriedly
buried body, perhaps a murder victim, may still have a full head of hair, even after all of the flesh has disappeared and the body is skeletonized. The cuticular scales are what make hair so durable. They keep out the water and bacteria, which fairly quickly decompose the rest of the soft tissue. Some insects can actually take little bites out of the hair, and thereby disrupt the cuticle, but it is otherwise largely impervious to other types of decomposition. The major portion of the hair is called the cortex. It is formed from long strands of keratin. There are some other materials present, such as the little specks of pigment that give the hair its color. |
The third major structure is the central canal, which, when the hair was growing and alive before it emerged from the skin, acted like a canal that transported liquids and nutrients to the growing cells. This area in the center of the hair is called the medulla and is usually an air-filled channel that goes up the middle of the hair. If you look at a hair through a microscope, the medulla usually appears as dark lines going up the center of the hair.
Growth Phases
Hair does not grow continuously, but rather passes through three different phases. The longest is called the anagen phase, which is the active growth phase. When hair is in the anagen phase, new cells are being produced in the hair follicle in the skin and are pushed the older cells out, which lengthens the hair. The second phase is the catagen phase, where the hair follicle stops making new cells and hair growth stops. During the catagen phase, which is relatively short, the little bulb of live cells in the follicle begins to shrink and dry up, destroying the hair’s attachment to the scalp, allowing it to fall out. The third phase is the telogen phase, which is a resting period. After a resting period, which is variable depending on the part of the body and the individual, the hair follicle again enters the anagen phase and a new hair starts to grown. The forensic significance of the cycle is that, at any given moment, a certain percentage of a person’s hair is ready to fall out. Everyone sheds many hairs every day. Further, because telogen hairs are about to fall out, even the slight pressure will remove them. As a result, a struggle or other strenuous activity is likely to cause telogen hairs to be shed. These hairs may become important evidence, particularly if transferred between a victim and an attacker. |
An understanding of the growth phase, and what the root ends looks like during the three phases, allows an analyst to determine whether a hair was forcibly removed or just fell out as a normal part of the cycle.
If someone is hit in the head with a hammer, or hairs are pulled out during a violent altercation, the hairs have a bulb of live tissue clinging to the root end. Thus, a hair examiner can often look at an evidence hair and determine how the hair left the body. This life cycle is also the underlying reason that there are often many loose hairs that an intruder may carry away from a scene on his or her clothing, shoes, or body. |
Laboratory Examination of Hair Evidence
Hair examinations can be broadly divided into two categories. (1) Microscopical comparision and (2) DNA analysis The traditional method, microscopical comparison, is now regarded by experts as valuable primarily as as exclusionary tool. Hair generally cannot be individualized based on its appearance under a microscope. Further analysis is necessary. DNA analysis can be done on hair only if a root is present. So, this is only possible if the the hair was pulled out of the scalp or skin because shed hairs have no root and therefore no nuclear DNA to analyze. A majority of evidentiary hairs are telogen and thus are not amenable to nuclear DNA analysis. In general , hair analysts can tell whether a hair was pulled or cut, and whether certain cosmetic treatments have been applied to it, such as bleaching or dying. |
Characteristics of Racial and Animal Hair
Each of the three “major” races (Caucasoid, Negroid, Mongoloid) generally have significantly different- looking hair, especially head hair. Because these differences are even more apparent by microscopical examination, it is often possible for a hair examiner to “guesstimate” the race of individual whose was the source of a particular hair. We say “guesstimate” because there are many children of mixed-race couples, and because there has been considerable mixing of races over the centuries. Thus, the “racial” characteristics of the hair may not be consistent with the appearance of the hair donor.
A hair examiner might offer a guess of the hair donor’s race to investigators, but most hair examiners would not claim to be able to determine the race of a donor from his or her hairs. However, even a estimation could be useful to the investigation.
A hair examiner might offer a guess of the hair donor’s race to investigators, but most hair examiners would not claim to be able to determine the race of a donor from his or her hairs. However, even a estimation could be useful to the investigation.
Animal hair examination in forensic laboratories is usually primarily to differentiate human and animal hair, and to determine what type of animal an evidence hair could have come from.
There are some good reference collections of hair from many different animals. Although more detailed comparisons between evidence and control hairs from a particular animal can be done, such comparisons are usually restricted to color comparison and consistency of major features. Some work has been done attempting to use DNA technology to make more positive association, but such work is highly specialized and not yet fully validated. |