According to Locard’s exchange principal, ‘every contact leaves a trace’. This means that physical contact between individuals or between individuals and a location or object will ultimately result in the transfer of materials which may be used as physical evidence. This type of evidence is known as trace evidence. Trace evidence is a form of physical evidence which may be found at a scene in very small amounts known as trace quantities. Examples include hairs, fibres, glass, soil, vegetable matter, gunshot residues and bodily fluids. Trace evidence is a form of associative evidence, meaning that persons are associated with a crime by the type of trace materials left behind. Trace evidence is not necessarily solid evidence that a person can be associated with a crime, as traces may be left at a scene before any crime was committed.
Introduction to hair
Hair is a filamentous outgrowth of protein (keratin), and is found in all mammals. Hair grows everywhere on the body with the exception of lips, eyelids, palms and the soles of the feet. The shaft of the hair is the visible protrusion from the epidermis of the skin, and is formed from the hair follicles located within the dermis. The root of the hair is located in the dermis; at the end of the root is the bulb of the hair. The bulb receives nutrients from the blood stream, enabling the hair to grow. There is a sebaceous gland within the dermis which lubricates the hair. The erector muscle is anchored to the follicle, this muscle reacts to stimuli (cold and fear) causing the muscle to contract, the hair then stands up straight.
The bulb receives nutrients from the bloodstream at the capillaries; from these nutrients new hair cells are formed. As the new cells move up through the root of the hair they mature through a process called keratinisation, they then fill with a fibrous protein and lose their nucleus.
The shaft of the hair is comprised of keratin scales known as the cuticle. The major function of the cuticle is to maintain the structural integrity of hair. It also prevents the transfer of soluble materials from the environment to the hair. The cuticle also gives the hair strength, preventing any fraying and breaking or fragmentation.
A middle layer of keratinized protein fibres makes up the cortex. The centre of the hair is a series of round cells known as the medulla.
There are three main stages in the cycle of hair growth; these are the anagen phase, the catagen phase and the telogen phase.
At the anagen phase, the hair is actively growing; new cells are being produced and are given nutrients at the follicle. At the catagen phase, active growth ceases, an upward movement of the hair root occurs, the root becomes bulb shaped and is surrounded by a capsule of keratinized cells. After this, the telogen phase begins, where the hair rests and no noticeable changes to the structure occur.
The anagen phase will then commence, this allows for the growth of new hairs in the same follicle. The original hair in the telogen phase is then shed allowing the new hair to form and grow.
There are four main types of human hairs, terminal are presented in the form of scalp, pubic, beard, axillary, eye brow, eye lash and nasal hairs. Vellus hairs are very fine, short and have no present medulla. They cover the entire body. Lanugo hairs are fine, soft, and non-pigmented and again have no medulla present. They cover the entire body in the prenatal period of foetuses.
The main functions of human hairs include insulation, protection against injury, the can increase the perception of the touch sense, they draw perspiration away from the body and act as a filter mechanism.
Drug testing in human hair
Hair samples can provide proof on whether a subject has been abusing drugs like cocaine, heroin and cannabis or using drugs for therapeutic reasons. Hair samples can show the trend of drug use over a relatively vast period of time (compared to blood, urine and saliva samples) and accurately determine what drugs have been used. Hair can act as a drug usage diary, showing how much substance was used at which particular times. 4cms of hair cut close to the scalp can provide months of information about drug ingestion.
Drugs which have been ingested into the body will circulate in the bloodstream. The blood stream nourishes developing hair follicles; consequently trace amounts of the drug are deposited in the follicle. The drug traces are then entrapped in the hair shaft as it grows out of the follicle. It takes approximately 5 to 7 days for drug traces in the follicle to grow above the scalp and will be detectable for months later. In testing hair samples for drug usage, scalp hair is generally used, if no scalp hair is available however body hair may be used as an alternative as the same principles apply.
Drug traces that enter the follicles are structurally and chemically stable for a long period of time, this means that they cannot be washed, flushed or bleached out of the hair’s structure.
When cocaine, opiates, methamphetamine and PCP are ingested into the body, they are rapidly excreted and virtually undetectable in urine within 72 hours of use. The drug testing using a hair sample covers a period of months ensuring that the user cannot evade the test by abstaining drug usage for a short period. Similarly to this, certain drugs taken for therapeutic reasons can be viewed. If the subject stops taking the medication, it will show up in hair fibres.
Hair testing can also differentiate between first time drug users, recreational users and addicts, first time users will have a relatively small amount of trace substances in their hair close to the root. Frequent users will have areas in their hair which correlate to periods where no drugs have been ingested; addicts will have high levels of trace substances consistent through out their hair.
Hair testing does have some negative associations, as false positives may occur. This means that a drug free sample may be falsely reported as showing positive for certain drugs. Poppy seeds which may have been ingested on bread contain trace amounts of morphine; this can lead to a false positive for various opiates. Codeine, which is found in many pain relievers, may produce a false positive for morphine and heroin. Newly produced antibiotics including amoxicillin and ampicillin can produce a false negative for cocaine.
The forensic value of hair samples
Hair is an example of trace evidence which may be discovered at a crime scene. Hair samples are generally investigated to assist in the identification of persons present at a crime scene; hair samples are a form of associative evidence. Hair examinations using light microscopy are used to differentiate between animal and human hairs. All hair samples are different, ethnic origin can be determined from a sample, pigmentation granules, cross sectional shape, and texture are viewed to distinguish between ethnic origins. It is also possible to decipher the somatic origin of a hair sample, eyelash, pubic, auxiliary, and limb hairs show different qualities and characteristics from each other.
If the hair sample was forcibly removed from a subject, the bulb of the hair may still be present; this occurs commonly in murder and rape cases where either the victim or the attacker may have left trace samples. From the bulb, individual cells may be present allowing for the extraction of DNA, this is a unique identifier as each individuals DNA is genetically dissimilar, and this can determine if a subject was present at a particular scene.
It is sometimes possible to determine if a hair came from a child, children’s hair is usually finer and fairer than adult hair. The average human loses approximately 100 hairs per day without being consciously aware of it; this is why a hair sample is a good form of trace evidence.
Hair samples are not concrete evidence that a subject was at a crime scene, sometimes it is difficult to determine the origin of a hair sample. Hair can show great morphological differences, even when taken from the same individual. A hair sample may have been shed at a scene for a relatively long time before an offence is committed. This then places an individual at a crime scene before it is one.
Trace evidence in the form of hair samples are only presented in court when other examples of evidence are available, a conviction cannot be made on the sole evidence of a hair sample. Hair samples can place an individual at a scene but not prove when they were present.