Trace Evidence: Hair

As a whole a hair is made up of a root (bulb), shaft and tip. Investigators use the shape of the root to indicate the stage of growth and whether the hair was pulled out or shed naturally. The root may also contain follicular tissue which is used for D.N.A. testing.

The shaft can be examined, using a compound microscope and backlighting, for unique characteristics within the shaft. Characteristics include the shape and type of the medulla, the presence and dispersal patterns of pigment granules and the shape and pattern of the external scales. The overall condition of the shaft can show damage such as insect bite marks indicating a hair not recently shed. Burning or crushing will cause the shaft to curl and bubble ( Chayco & Patreco, 2003). Hair grows at a fairly constant rate of 1 mm per day (Crocker, 1999). With this knowledge an investigator can estimate the time since a dye, permanent wave or other exposure to chemicals occurred (Crocker, 1999).

The tip of a hair may reveal chemical or heat treatment indicating head hair. It may also be blunt ended indicating beard hair that has been shaved or clipped.

A hair in cross section can be visualized as being like a pencil. The medulla is the lead. The cortex is the wood. The cuticle is the paint covering the wood (Bisbing, 2002) The medulla may be viewed microscopically by dry mounting the entire hair or by embedding the hair in paraffin wax and slicing it into thin sections. The medullary index is used to distinguish animal hair from human hair. It is expressed as a ratio of the shaft diameter to the diameter of the medulla (Saferstein, 2004). In animals the medulla will make up more than 1/2 of the total diameter of the hair. In humans the ratio is usually less than 1/3 (Saferstein, 2004) The medulla can be classified as appearing either absent, fragmented, interrupted or continuous (Lane, 1992) (Saferstein, 2004). Most human head hair with the exception of that of the Mongoloid race has no medulla or a fragmented one. People of the Mongoloid race have a continuous medulla.

Most animals have a continuous or interrupted medulla. Hair of animal origin may exhibit specific patterns such as a uni or multiserial ladder (rabbits) or a lattice (Deer) (Saferstein, 2004)( Lane, )1992 The shape of the medulla as well as the pattern is exhibits can be used to determine species, and when human, racial origin (Saferstein, 2004) (Lane, 1992).

The cortex surrounds the medulla as does the wood of a pencil around the lead. Microscopic structures within the cortex such as pigment granules and fusi (air bubbles) are used to compare one hair to another (Saferstein, 2004) (Kubic & Patraco, 2003).

The cuticle is like the outer paint of the pencil. The cuticle is used mainly to observe the scale patterns present which indicate species (Saferstein, 2004) (Lane, 1992) (Crocker, 1999) (Kubic & Petraco, 2003) .

Scale patterns are observed by embedding the hair in a liquid medium often clear nail polish and allowing it to set. Once the polish has air dried the hair is removed leaving a cast of the outer scales (Crocker, 1999).

Scale structure is used to determine species. The patterns may be coronal, petal, or umbricate. Umbricate scales are overlapping and exhibit no apparent pattern. Umbricate scales are found in humans (Saferstein, 2004) Petal scales resemble the scale of a reptile and are not found in humans. Coronal scales are symmetrical and overlap one and other. They are not usually found in humans (Cheyko & Petreco, 2003).