Hair is a filamentous biomaterial consisting primarily of tightly packed, keratin-filled cells (see Figure 10.5.5). It is surrounded by a protective layer called the cuticle. The inner core is known as the cortex and contains a hierarchical structure consisting of coiled-coil proteins and structural lipids. Hair is also a significant part of the human body’s thermal insulation. Hair helps trap warm air next to the skin, preventing heat loss from the head in cold environments.
In the case of forensic examinations, hair provides important clues in determining a person’s identity, as well as his or her association with a crime scene. The location, number, and condition of a person’s hairs are key to establishing these associations.
There are many different types of hair, and each type has distinct microscopical characteristics. These microscopic differences are useful for forensic investigations, as they can be used to identify the source of the hairs. Hair can be found throughout the human body, but forensic investigations typically focus on the head and pubic areas. Hairs in these regions have distinctive microscopic features that can be compared with a reference hair collection of a particular species to determine the probable animal origin of the questioned hair.
Hairs can be classified as fine, medium, or coarse based on their circumference and their state of development. Fine hairs have the smallest circumference, and coarse hairs have the largest. Hairs are also characterized by their natural curl pattern, whether they are straight or wavy, and by the amount of pigment granules present in the hair shaft.
Forensic investigators can use hair to identify a person’s sex, gender, age, and race. Sex can be determined by examining the color and thickness of hairs for sex chromatin, a chemical indicator. However, this method is time-consuming and requires a large sample of hairs.
During the cellular development of a hair, a pigment called melanin is produced, which gives hair its characteristic colors. The hair shaft subsequently matures into a hair follicle, which is fed by a blood supply from the surrounding skin. Hairs are then able to emerge from the hair follicle and grow toward the surface of the skin.
There are several ways to style a person’s hair, and each has a unique effect on the way the hair looks. Some styles, such as braiding and the use of rubber bands with metal clasps, can pull at the hair roots and cause traction alopecia, a condition in which the hair breaks off at the scalp. Other styles, such as a ponytail or an updo, are more gentle on the hair and are less likely to cause traction alopecia.