HAIR AND FIBERS
During the course of a criminal investigation, many types of physical evidence are encountered. One of the most common is hair evidence. The identification and comparison of human and animal hairs can be helpful in demonstrating physical contact with a suspect, victim, and crime scene. Hairs can provide investigators with valuable information for potential leads.
Anagen - Active growth period of a hair when materials are deposited in the hair shaft by the follicle
Telogen – Resting or dormant phase of hair growth cycle when hair loss is most common
Catagen – The transition period between the anagen and telogen phases
Pluckings – A collection method where attached hairs are forcibly removed with sheath attached which insures the sample is from the subject being examined. Normally 25 – 50 hairs are collected for each sample.
Racial identification - The shape of the hair follicle determines the shape and structure of the hair. Variations are used to determine race.
Caucasoid – blonde to black color ranges, round to oval cross sections, fine to medium sized evenly distributed pigment granules
Mongoloid – Dark with reddish casts, round cross section, thicker cuticle, thick and continuous medulla with large pigment granules
Negroid – Predominantly black coloration, largest pigment granules, flattened shape cross section, twisting or buckling frequently seen
Age cannot be determined by examination
Sex can be determined from DNA and chromatin in follicle tissue if attached to the hair
Human hairs are distinguishable from hairs of other mammals. Animal hairs are classified into the following three basic types.
- Guard hairs that form the outer coat of an animal and provide protection
- Fur or wool hairs that form the inner coat of an animal and provide insulation
- Tactile hairs (whiskers) that are found on the head of animals provide sensory functions
Hair comparisons are not a basis for absolute personal identification. It should be noted, however, that because it is unusual to find hairs from two different individuals that exhibit the same microscopic characteristics, a microscopic association or match is the basis for a strong association.
Cuticle – The outer layer of the hair comprised of overlapping scales that vary in shape and color along a single hair.
Cortex – The main body of the hair which may contain ovoid bodies and pigment granules.
Dermis - (also called the cutis) the layer of the skin just beneath the epidermis.
Hair follicle - a tube-shaped sheath that surrounds the part of the hair that is under the skin. It is located in the epidermis and the dermis. The hair is nourished by the follicle at its base (this is also where the hair grows).
Hair shaft - The part of the hair that is above the skin.
Hair erector muscle - a muscle is connected to each hair follicle and the skin - it contracts (in response to cold, fear, etc.), resulting in an erect hair and a "goosebump" on the skin.
Medulla - The structure of the medulla can vary from continuous throughout the center of the hair shaft to fragmentary, or absent altogether. It can be opaque or translucent.
Melanocyte - a cell in the epidermis that produces melanin (a dark-colored pigment that protects the skin from sunlight).
Sebaceous gland - a small, sack-shaped gland that releases fatty liquids onto the hair follicle. The oil lubricates and softens the skin. These glands are located in the dermis, next to hair follicles.
Sweat gland - a tube-shaped gland that produces eccrine sweat. The gland is located in the epidermis; it releases sweat onto the skin.
Subcutaneous tissue - fatty tissue located under the dermis.
Natural fiber – A fiber that is produced by an organism such as hair, in fibrous plants like cotton, or in materials produced by living things including silk worms and spiders. Natural fibers may contain levels of the element sulfur which causes the release of a strong odor when burned.
Synthetic fiber - A manmade fiber such as nylon, rayon, plastics, or processed fiberglass. Many synthetics melt before burning. Burning releases free radicals and toxic fumes from many of these materials which can be extremely dangerous.
Dye – Either a natural or chemical colorant that attaches to the fiber. Dyes can be produced by combining chemicals containing particular elements or from extracts from plants or animals.
Tensile Strength – Tensile strength is the fibers ability to hold a certain amount of weight. The tensile strength is the maximum amount of weight that is supported before the strand fails. Twisting multiple strands together increases load carrying ability over the same number of strands individually.