Hair Transfer, Persistence and Recovery

How do hairs come to occur as physical evidence? The answer lies in the concepts of hair transfer, persistence and recovery. In order to properly interpret the significance of forensic hair examinations, it is essential to have a thorough understanding of these concepts combined with knowledge of hair identification and comparison methods.

Hair Transfer

The fundamental principle of hair transfer is the Locard Exchange Principle, first formulated by Edmond Locard who stated: ‘The microscopic debris that cover our bodies are the mute witnesses, sure and faithful, of all our movements and all our encounters’. This principle means that whenever two objects are in contact, there will always be a transfer of material from one to the other, even though in some instances the amount of material transferred may be too small to detect, or may be rapidly lost subsequent to transfer.
The original source of all transferred hairs is the human or animal body area from which they grew. As part of the normal hair cycle, a person loses 75-100 scalp hairs a day. Of course, this loss is not evenly distributed throughout the day. Most occurs during hair grooming – combing, brushing and washing. A struggle including hair pulling (as often occurs during crimes of violence) can greatly accelerate hair loss.
As is the case with transfer of other types of trace evidence, hair transfer is a very complicated process with many mechanisms and involves many variables. Two main types of hair transfer can be identified: direct transfer in which hairs direct from the original source are transferred; and indirect transfers which involve one or more intermediaries such as clothing, bedding, and household or automotive upholstery. Direct transfer is always primary transfer (defined as that transfer which occurs whenever a person transfers his or her own hair to an object, place or other person). Indirect transfer can be either primary or secondary. Secondary transfer arises when a person transfers hair that is not his or her own to an object, place or other person. Unlike primary transfer, secondary transfer does not imply direct association and can lead to falsely incriminating evidence.
Gaudette and Tessarolo conducted several experiments in a preliminary attempt to obtain information on hair transfer mechanisms and the role of some of the variables involved, as well as to obtain a preliminary indication of the extent and importance of secondary hair transfer. Indirect hair transfer mechanisms were found to be much more common than direct ones. They also found that secondary transfer of human scalp hair can and does exist in casework situations. The extent of secondary transfer was found to be extremely variable, being dependent on such factors as texture and fiber type of clothing worn, grooming habits of persons involved, and whether or not horizontal objects (such as upholstered chairs or car seats) used by several people, are involved. Their results showed that when secondary transfer does occur, it is most often first order secondary transfer, with only one source between the place where the hair is found and the original donor. Secondary transfer with two or more intermediates can occur, but was not found to be common.
The following conditions have been found to increase the likelihood of secondary hair transfer: involvement of unclean individuals with poor grooming habits, involvement of individuals with many prior interpersonal contacts, when one or more party is wearing rough textured or wool clothing, when horizontal surfaces are involved, and when there is contact with objects used by several different people. On the other hand, when those involved are clean,well-groomed people, or are wearing smooth textured or tight fitting clothing which is neat and clean, the likelihood of secondary transfer is greatly reduced. Assessment of the chances of secondary transfer having occurred in a given case should also be based on the total number of hairs transferred and whether or not there has been a two-way transfer. In cases where a large number of hairs are transferred, it is unlikely that they would all have been secondarily transferred and it is unlikely that both transfers in a two-way transfer would be secondary.
Quill removed hairs from his clothing at the beginning and end of each work day for a 30-day period. He then compared the hairs removed to known samples from himself, his family and his co-workers. He found that the few hairs recovered from his clothing during the day were contributed by himself and those in his immediate environment.
Peabody, Thomas and Stockdale investigated the shedding of hairs into several types of headgear. They concluded that the number of hairs shed into headgear varies with the type of headgear and the individual, and that hairs from headgear are most similar to hairs from known samples obtained by combing.
In a study of pubic hair transfer during sexual intercourse, at least one transferred hair was found in 17.3% of combings taken from participants immediately after intercourse. About a quarter of these transfers involved multiple hairs (up to four). Transfers from females to males were found to be about twice as prevalent as transfers from males to females.


Hair Persistence

Since it is rare for suspects to be apprehended immediately after hair transfer in a crime, the concept of hair persistence on clothing becomes important. Gaudette and Tessarolo found the persistence of hairs on clothing to be quite similar to that previously found for fibers. A typical decay curve is shown in Fig. 1. Movement and loss of hairs on clothing has been found to be influenced by many variables and it was concluded that ‘it would be quite unjustified to attempt to predict accurately the number of hairs likely to be present in a specific case or to attach great significance to where these hairs are found’. Although most human hairs are removed from items during laundering, some can still be found on items after laundering and hair transfers can occur during laundering.
Through a study of casework received over a six-year period, Mann obtained transfer and persistence results for scalphairs that were consistent with those of the above mentioned studies. However, it was found that forensically significant pubic hair transfer occurred quite infrequently in the cases examined.
In addition to its ubiquitous nature, hair has two other properties that make it a frequently occurring type of physical evidence. First, it is remarkably stable to most environmental conditions and will not easily break down like other biological evidence. Second, since hair is fairly unnoticeable to the untrained eye, a criminal is not likely to make a special effort to destroy it.

Recovery of Hair Evidence

Recovery of hair evidence can occur in two locations: at the crime scene, and in the laboratory. Since the laboratory has the better facilities, as much hair recovery as possible should be conducted there. Accordingly, if an item suspected of having adhering hairs is at all portable, the best method of hair recovery at the scene is to seize and carefully package the entire item. Each exhibit item should be packaged immediately in its own separate container. Clean bags with no exposed inside seams are the best containers for clothing and other large items. Clear plastic vials or metal ointment tins are recommended for small objects. To prevent accidental contamination, exhibit items in an unpackaged state should not be handled by more than one investigator. By placing clear cellulose tape in contact with the area in question, hairs can be recovered from items which cannot practicably be brought to the laboratory.
In the laboratory, the simplest method of recovering hairs is to visually search items under oblique lighting at different angles, using tweezers to pick off any readily visible hairs. The examination table should be of a size sufficient to accommodate large items, have a smooth nonglossy white finish (to assist in finding hairs that fall to the table), be carefully cleaned prior to each use, be located near a good source of natural lighting, and be equipped with a movable light source capable of providing illumination to the entire area.
Hair persistence as a function of time of wear.
Figure 1 Hair persistence as a function of time of wear.
Particularly with dark-colored and rough-textured items, a second method of hair recovery in addition to visual searching should usually be employed. The recommended method is taping. Although originally designed as a method to recover textile fibers, experience has shown that taping can recover many hairs which are not readily visible.
In the taping procedure, a strip of transparent cellulose tape is placed, adhesive side down, on the item and the back surface of the tape is rubbed with a thumb or forefinger. The process is systematically repeated until the entire item or area of interest has been taped. The tape strips are then placed between two layers of clear plastic (such as a clear acetate document protector).
Vacuuming is a method of hair recovery that is not recommended under most circumstances since it has some serious disadvantages. Most notable among these are the potential for contamination from hairs and debris lodged in the vacuum cleaner, and the nondiscriminating nature of the collection. Hairs that were deposited long before the crime are collected simultaneously with hairs associated with the crime, with no way to readily distinguish between them.
Since hairs are readily transferable, it is important to follow a rigid program of contamination prevention. A program similar to that followed for textile fibers or polymerase chain reaction DNA analysis is recommended.

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