Hairs and fibres
by Dave Barclay
Hairs and fibres are the most commonly transferred materials during physical contact between two people. The closer and more long lasting the contact the greater the chance of a two way transfer, which provides the strongest evidence. This means they are likely to be particularly useful in sex offences and assaults, but they can also be found at points of entry, on masks and in vehicles.
HAIRS: Human hairs can be head, eye brow, eyelash, body or pubic – all of which are different in appearance, and of course animal hairs may also be present. They may be found in rape cases, for example in pubic hair combings or on underclothing where their position may help negate a suspect’s explanation. Hairs are also found on weapons used in assaults or on shoes in kicking offences, and in burglaries and robberies, either at the scene or as head hair in a mask.
In the UK we consider hairs may yield little evidential value because they can vary considerably on any one person: my own, apparently mousey-brown hair, contains red/blonde, black, brown and (increasingly!) white hairs. Hairs found in a crime scene will be similar to those from many other people. However, they can sometimes provide strong evidence because, as with all forensic science, the context is key:
- The type can confirm the story of a witness
- The race of the donor can sometimes be established
- Bleached, dyed or treated hair can be very characteristic
- Hairs inside masks can provide evidence to show a suspect could have used them
- Hairs in significant positions may enable suspects to be eliminated
- Nuclear DNA analysis may possible on hair roots, particularly if they have been pulled out
Drug analysis is also possible, and will give a chronology of drug use whether licit or illicit, but requires a substantial number of hairs.
Hair evidence should normally be regarded as supporting other evidence or confirming circumstances. At present, only in the most exceptional circumstances will it provide strong evidence on its own. For comparison with possible transferred hairs, control samples of plucked head hair from the suspect should include at least 25, taken from different parts of the head.
In the interests of science I did this just now on myself: I had 21 various shades of lightish brown, 7 grey or white, 6 apparently black and 4 noticeably reddish blonde. Yes, I know that is more than 25 – I obviously used too big a pair of forceps!
FIBRES: Retrieval and subsequent laboratory examination of fibres, is very skilled and time consuming, as is the assessment of significance. Many types of fibre, either man-made or natural, exist in countless colours, shades and chemical dye combinations. Transfer of fibres, particularly from multiple-fibre type garments or items such as rugs or carpets, can provide excellent evidence of contact, but of course many garments are mass produced, and so there may be other possible sources.
So, the more unusual the fibre type or combination, the stronger the evidence, and as usual a two way transfer greatly increases the strength of the evidence. On the other hand, suspects wearing white tee shirts and denim jeans provide little scope for useful evidence. The nature of the garment is crucial in their ability to shed fibres and thus transfer them – woolly jumpers are good, shell suits are not! And even a black wool glove is not necessarily really black under the microscope.
As a rule, the greater the time interval between contact and the clothing being seized, the smaller the chance of fibres being retained. CSI often use adhesive tape to lift possible fibres from points of entry, car seats etc. These fibres are often so small that they cannot be seen without magnification and hence must be analysed at the laboratory. The tapes are stuck to acetate sheeting and examined under a low power microscope. Fibres of interest are marked and individually cut out through the back of the tape for further analysis. Told you it was time consuming!
Torn materials, e.g. in rape cases, can sometimes provide fibre or thread matches which would otherwise be very unlikely, and which is therefore particularly strong evidence. In a case from Australia a young girl was attacked and murdered at an unknown location. Her body was later found on waste ground, and it was noted that her panties had been torn through. They were trimmed with tiny stitched lilac forget-me-nots, with a very small amount of green silk thread stitching for the stalk of the flower. Analysis of suspect’s sofa revealed an area of saliva which gave the girl’s DNA and a large number of tiny green silk fibres which matched the panties.
Fibres of apparently similar colour may be very different when analysed for their chemical dye composition. I think the best example was a balaclava and ‘matching’ black gloves bought as a set from the same shop, and used in a series of terrorist offences in Northern Ireland. Sure, they were all superficially black, but between them they included four different fibre types and seven different dye combinations, and not even the two gloves, sold as a pair, matched! The various items recovered from bomb scenes included all four fibres and all seven dyes – which was really quite useful.