Trace Evidence: Hair

by: Sarah VanderPol & Reagan Micheli

Physical Properties of Hair


Because of its elasticity, hair can resist forces that could change its shape, its volume or its length. Its elasticity lets it spring back to its original form without damage. The elasticity of hair depends on the long keratin fibres in the cortex. Chemical treatments of hair (perming, bleaching) can alter the cortex after repeated damage, and change the hair's elasticity. Hair with poor elasticity will stretch only to a limited extent. It will not curl, it will break easily when it is groomed and it cannot be permed satisfactorily.

static electricity:

When dry hair is rubbed, static electric charge builds up on the hairs. This is especially noticeable in hot, dry weather. The charges tend to push each other apart, and as a result charged hairs can never lie smoothly against each other.

moisture content:

The moisture content of hair is greater when the atmosphere is moist and humid, and less when the air is dry.

heat and humidity:

-more moisture

-less static electricity


heat and dryness:

-less moisture

-more static electricity

-more volume (body)


In a normal, undamaged hair shaft, very little water can get either into or out of the cortex. This is because the cuticle covering the cortex is intact, and is then almost waterproof. When hair is permed or tinted, the chemicals have to penetrate the cortex in order to react with the keratin inside it. After the processing is finished the scales gradually close up again. If hair is processed too many times the cuticle scales may never return to their original tightness and strength. The cuticle can also be damaged in the same way by too much blow drying, curling irons that are too hot, and the effects of wind and sun. The hair becomes increasingly porous, and water can then pass in and out of the cortex. Over-porous hair is dry, and tends to develop split ends.


The texture of hair depends on several things. The first is the average diameter of the individual hairs. The larger the hair diameter, the coarser it will feel. Secondly, different people's hair naturally feels different: some hard and others soft, some silky and others wiry. Thirdly, the texture is affected by the degree of weathering of the hair (drying, straightening, dying, curling). Finally, hair texture is affected by what has been put on it (hair spray, conditioners, gel).

Chemical Properties of Hair

Hair is made mostly of keratin, a protein. Proteins are composed of amino acids. Hair has a high content of a particular amino acid called cystine. Cycstine forms strong bonds within each protein strand. What's more, if the pH is low , cystine on one keratin strand bonds strongly to keratin strands nearby. When hair is wet, it can be stretched to 1 1/2 times its dry length because water weakens the strand-strand bonds and causes the strands to swell as well. This is key; it makes it so that hair can be rolled and then set with chemicals.

Hair reacts differently to the two groups of chemical substances known as acid and alkaline, and their identification in hairdressing preparation is therefore important. The presence of acids and alkalis may be detected by the change in colour of an indicator paper impregnated with litmus dye. Red litmus paper turns blue in alkaline liquids and blue litmus paper turns red in acid liquids. The term PH means the power of hydrogen ions.

Action of salt on the hair:

The chemical reaction between an acid and an alkaline is called neuralization and result in formation of salt and water. Salt always can consist of either metal or ammonium(alkali)and an acid radical(acid).

The scalp and hair growth:

Scalp hair grows by the division of cells at the base of the hair follicle in the skin of the scalp to enable that growth to take palce, nutrients from food we eat and oxygen from the air we breathe in must be carried though the blood vessels to the dividing cells

There are two main parts to the scalp:

- the skin with the scalp hair

- the epicranial aponeurosis, which is blood strong sheets of fibrous tissue or tendon lying under the skin.

-The epicranial aponeurosis is firmly attached to the skin by a fatty layer of connective tissue and is also held by frontails muscle of the fore head and the occiptal muscle at the back of the head. Very loose connective tissue attachesthe epicranial aponeurosis to the bone of the skull, enabling the whole scalp to be move slightly over the surface of the skull.

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How Forensic Scientists Analyze Hair

Forensic scientists assess a number of different characteristics of hairs, usually hairs from "known" sources and those recovered during forensic examinations and or "questioned" hairs in order to determine whether they come from a common source. For example, comparing hair found at the scene of the crime with hair samples taken from a suspect. Typically, examinations of this type are conducted using a stereo-microscope for initial preparation, and a comparison microscope for detailed, in depth analysis.

"Caught by a Hair" Case

In 1990 in Telluride, Colorado, Eva Shoen's young daughter found her dead from a single gunshot to her head. Her husband, Sam, became the first person of interest, but ended up to truly be the grieving, shocked husband of a victim of random violence. At first, the police were confident they would solve the case due to the fact that the bullet taken from Eva's skull had the distinct markings of a particular type of pistol. However, the case made its way into the cold cases file. There were no leads.

Three years later, the Telluride police received a call from a man in Arizona who believed that his own brother, Frank Marquis, had been the perpetrator. Marquis had confessed this crime, but an attempt to trap him during a phone conversation failed. Nevertheless, when the gun was recovered, an arrest seemed a sure thing. Unfortunately, Marquis had covered his tracks all too well. He even tamperied with the barrel of the gun so that the bullet fired from it could no longer be matched. All they had on him was a hearsay conversation.

However, tracing Marquis's movements indicated that he had indeed been in Telluride during that weekend for a festival, and that he had a police record for rape. This was the man, the detectives felt, and they had to find a way to get him. Putting pressure on Marquis' travel companion, they learned that at some point along the road back, Marquis had tossed two bundles out the window of the car. They suspected that this was the clothing he had worn to commit the crime. Still, it was a long and winding road between Telluride and the point where Marquis had ended his journey some four hundred miles away.

Detectives searched the roadway until they narrowed the possibilities down to four places. As luck had it, a construction crew had recently moved a pile of dirt, exposing a preserved pile of clothing. On the shirt was a single strand of hair, which was examined in the lab against a sample taken from Eva Shoen. Forensic trace expert Joseph Snyder analyzed the color and structure, and pronounced them a close match.

When the investigators told Marquis of their findings, he confessed. It was a "burglary gone wrong", he said, indicating his knowledge of the plea-bargaining system. Although the officers in charge of the case believed that he had in fact planned to rape Eva Shoen and had killed her in the process, they knew that this would be impossible to prove. Marquis got a sentence of twenty-four years for manslaughter.


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