HAIR CRIMINALISTICS

• Jurisdiction: Australia

Occurrence of hair - transfer and persistence ............................................. [88B.200]

Recovery of hairs .......................................................................................... [88B.220]

Sampling of individuals for exemplar or known hairs ................................... [88B.240]

[88B.200] Occurrence of hair - transfer and persistence

As has been discussed, on average, over 100 scalp hairs can be expected to be lost each day for every individual. It might be fair to form a visual image of a world covered in hair! Those of you who live in close proximity to a dog or a cat will readily appreciate this point. It is also empirically known that hairs, and animal hairs in particular, can be very difficult (impossible!) to remove from clothing. However, there have been only a very small number of specific studies of the transfer and persistence of human hairs, perhaps because these types of experiments are difficult to set up and counting hairs can be tedious. Hence, it has been often assumed, and stated, that the transfer and persistence of hairs will be analogous to textile fibres; after all aren't wool fibres just animal hairs? When reference is made to the persistence of hairs, it is usually stated that it shows the same classic decay curve seen in fibre persistence studies, that is, an initial very rapid loss followed by a slower gradual loss - see Figure 5.

Figure 5. Persistence of wool fibres on three articles of clothing; O, wool jacket; ?, wool/nylon sweater; X, wool jacket. (From: Robertson, 1999.)

Recent studies of human hair and animal hair have shown a more complex picture. The "classic" decay curve still applies for recipient surfaces which comprise relatively smooth construction and fibres. Typically these include fabrics made of cotton, polyester or nylon fibres. For recipient surfaces comprising fabrics constructed of acrylic or wool, persistence does not follow this classic fibre decay pattern. In some instances, there is still an initial more rapid decay but in others the loss is almost linear with hairs retained in much higher percentage terms than with fibres. These studies also looked at the underlying reasons for these different patterns of retention. Put simply, and perhaps not surprising, it would appear those hairs which are retained become physically "held" within the fabrics - see Figures 6 and 7.

See Dachs et al (2003) and Boehme et al (2008) for greater detail on hair persistence.

Figure 6. The...