Hair and fibre are two of the most important resources in Forensic Science and are often responsible for providing valuable clues as to the identity of an assailant or attacker.
Hairs, which are composed primarily of the protein keratin, can be defined as slender outgrowths of the skin of mammals. Each species of animal possesses hair with characteristic length, color, shape, root appearance, and internal microscopic features that distinguish one animal from another. Considerable variability also exists in the types of hairs that are found on the body of an animal. In humans, hairs found on the head, pubic region, arms, legs, and other body areas have characteristics that can determine their origin. On animals, hair types include coarse outer hairs or guard hairs, the finer fur hairs, tactile hairs such as whiskers, and other hairs that originate from the tail and mane of an animal.
The discovery of hair on the body of a victim or on the clothes of someone who has been the victim of an assault can often be used to determine race and sex. It can also be used to extrapolate DNA for comparison. As hairs can be transferred during physical contact, their presence can associate a suspect to a victim or a suspect/victim to a crime scene. The types of hair recovered and the condition and number of hairs found all impact on their value as evidence in a criminal investigation. Comparison of the microscopic characteristics of questioned hairs to known hair samples helps determine whether a transfer may have occurred.
The examination of human hairs in the forensic laboratory is typically conducted through the use of light microscopy. This examination routinely involves a two-step process—the identification of questioned hairs and the comparison of questioned and known hairs. The purpose for conducting this examination is to ascertain whether two or more individuals could have come into contact or whether one or more individuals could have come into contact with an object. This associative evidence is particularly useful in crimes of violence, such as homicide, sexual assault, and aggravated assault, where physical contact may have occurred. Crimes such as burglary and armed robbery typically involve the recovery of debris and articles of clothing which may contain hairs useful for the identification of suspects.
The value of hair evidence is related to the variability of hair characteristics between individuals in the population, which can be visualized through the use of comparison microscopy. There are many factors that impact on the reliability of a hair association, including experience, training, suitability of known hair standards, and adequacy of equipment. Although hair evidence is a valuable tool in human identification, it is difficult to establish a statistical probability for a particular association due in part to the lack of reliable quantitative assessments of the microscopic characteristics present in hairs.