Forensic Science Lesson 09: Hair Evidence Part 2

• Distinguish between different types of hair

• Understand the effects of chemical treatment on the ability to use hair as evidence

Types of Hair

• Between people, hair can vary in many ways

  • Shape

  • Cross Section Shape

    • Affects the curl of the hair

  • Length

  • Texture

  • Color: Both from natural pigments and dyes

Hair Variation

• Not all hair on a single person is the same

  • Different parts of the body have different types of hair

    • Generally distinguished by the shape of the hair's cross-section

  • Even the same part of the body can have variations in hair color

6 Types of Human Hair and Their Properties-Head Hair

• Head Hair:

  • The longest hair on the body

  • Tends to have split tips

  • Circular/elliptical Cross section

  • Easiest to transfer

  • Most likely to show artificial coloring and chemical treatments

6 Types of Human Hair and Their Properties-Eyebrows & Eyelashes

• Eyebrows/Eyelashes:

  • Short Stubby Hair

  • Circular with tapering ends (egg-like shaped) cross-section

6 Types of Human Hair and Their Properties

• Beard/Mustache:

  • Triangular cross-section

  • Coarse Diameter

  • Can have a double Medulla

6 Types of Human Hair and Their Properties-Underarm & Body Hair

• Underarm

• Auxiliary/Body Hair:

  • Oval or triangular cross-section depending on shaving habits

  • It has a blunt tip

  • Maybe frayed from abrasion damage

  • Stiffer texture than body hair

6 Types of Human Hair and Their Properties-Pubic Hair

• Pubic Hair:

  • Oval/Triangular cross-section

  • Tips tend to be rounded

  • The root is often attached

  • Shows "Buckling"

6 Types of Human Body Hair and Their Properties

The Life Cycle of Hair

• Hair goes through three stages in its life

  • Anagen Stage

    • ~1,000 days

    • 80-90% of your hair is in this stage

    • Cells along the follicle develop and divide rapidly

      • Full length if allowed can be around 100 cm Long

    • Hair Follicle is firmly secured in place

The Life Cycle of Hair

• Hair goes through three stages in its life

  • Catagen Stage

    • lasts 7-14 days

    • hair growth slows down

    • The base of the Follicle begins to degrade and break down the root of the hair

The Life Cycle of Hair

• Hair goes through three stages in its life

  • Telogen Stage

    • lasts 3-4 months

    • 10-18% of hair is in this stage

    • The hair follicle rests and the hair ceases all growth

      • A new root may begin to form, but not grow

    • The hair continues to break down until it falls out

      • Each hair falls out by chance

    • The hair may contain fragments of mitochondrial DNA

      • Hair follicle is completely decayed

Treating Hair

• Hair can be treated in several ways

  • Bleaching to remove Pigments

    • Makes hair lighter/more yellow

    • Disturbs the cuticle scales and makes the hair more brittle

    • Can be done naturally with sunlight or artificially with chemicals

      • Artificial bleaching results in sharp damage to the cuticle

      • Natural causes more gradual changes

Treating Hair-Artificial Bleaching

Treating Hair-Sun Bleaching

Treating Hair

• Hair can be treated in several ways

  • Dyeing hair changes the pigment of the shaft

    • both the cuticle and the cortex take the color of the dye

  • Practice makes it easy to identify when hair has been dyed

    • Able to see inconsistencies in how pigment is applied

Calculating Time Since Last Dye Session

• Treating hair does not color 100% of the hair, and the end closest to the root tends to maintain its natural color

• Hair grows at a rate of 1.3 cm per month (0.44 mm per day)

• By measuring the length of uncolored hair and dividing by the growth rate, you can approximate how much time has passed since the hair was last dyed

  • Can help determine when someone was last alive, or previous activities

Calculating Time Since Last Dye Session Sample Problem 1

• A body with treated hair is found, and a time frame for death needs to be established. The untreated portion of hair measures 2.5 cm long. How many days has it been since this person last dyed their hair?

  • Step 1: Establish your conversion factors

Calculating Time Since Last Dye Session Sample Problem 1

• A body with treated hair is found, and a time frame for death needs to be established. The untreated portion of hair measures 2.5 cm long. How many days has it been since this person last dyed their hair?

  • Step 1: Establish your conversion factors

  • Step 2: Set up a conversion equation

Calculating Time Since Last Dye Session Sample Problem 2

• A piece of hair is found at the scene of a robbery that has been dyed. The untreated portion of hair measures 6.5 cm long. How many months has it been since this person last dyed their hair?

  • Step 1: Establish your conversion factors

  • Step 2: Set up a conversion equation