The persistence of fibres in head hair

The persistence of fibres in head hair

ORIGINAL PAPERS The persistence of fibres in head hair CM ASHCROFT, S EVANS and IR TEBBETT* Forensic Science Unit, University of Strathclyde, 204 Ge...

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ORIGINAL PAPERS

The persistence of fibres in head hair CM ASHCROFT, S EVANS and IR TEBBETT*

Forensic Science Unit, University of Strathclyde, 204 George Street, Glasgow, United Kingdom G I 1XW

Abstract Woollen and acrylic ski masks were used to investigate the persistence of fibres in both Caucasian and negroid head hair. Fibre loss followed an exponential decay with both types of fibre persisting for up to six days if the hair was not washed or up to three days if the hair was washed. Key Words: Fibre persistence; Human hair. Journal of the Forensic Science Society 1988; 28: 289-293 Revision received 15 August 1988; accepted 6 October 1988 Introduction The incidence of violent crime such as armed robbery, rape and terrorist activities has increased dramatically over recent years. These crimes are often carried out with the perpetrator wearing some form of mask or disguise, making eye witness identification impossible. The masks used in this type of crime are usually ski masks or simply the sleeve of a sweater. If the mask can be recovered after the incident, it is routinely examined for the presence of saliva and hairs so that comparison can then be made with samples taken from any suspects. Probably of greater evidential value, however, is if fibres from the mask can be found in the hair of the accused. The method of removing fibres from head hair was described by McKenna and Sherwin in 1975 and is now employed by a number of laboratories [I]. A comb plugged with cotton wool is used to comb the suspect's hair. Any fibres present in the hair, together with loose hairs, cosmetics, debris and lice, adhere to the cotton wool which can then be examined under a stereomicroscope. In this study, an attempt was made to determine the persistence of fibres in the hair of an individual after wearing such a mask, and the variables, if any, which will affect this persistence. * Corresponding author. Present address: Department of Criminal Justice, The University of Illinois at Chicago, Box 4348, Chicago, Illinois 60680, USA

0Forensic Science Society

1988

289

Materials and methods Two ski masks, one consisting of wool and the other of acrylic, were worn by volunteers for periods of between 5 and 30 minutes. The hair was then immediately combed with a comb packed with cotton wool and the number of fibres recovered from the hair was counted under a stereomicroscope. This control reading was determined ten times for each individual and an average value calculated. This 100% reading was later checked at random throughout the study to determine the reproducibility of the results. The mask was then worn by an individual for a fixed time period, before removal, and the hair was combed 1 hour later. The number of fibres recovered was determined and this figure was expressed as a percentage of the original control reading. Readings were similarly taken after 2, 4, 6, 8, 12 and 24 hours and 2, 3, 4, 5 and 6 days. Each determination was repeated ten times for each individual and the mean percentage persistence was calculated for each time period. Volunteers were asked to carry out normal activities during the study with the exception that they were to refrain from washing their hair. In a second study to determine the effect of washing the hair on fibre persistence, individuals were asked to wear the mask for a fixed time period after which the hair was immediately washed and towel dried. The hair was then combed at various time intervals as previously described. All of the above experiments were carried out on three Caucasians and two negroid volunteers.

Results and discussion Results typical of those obtained during this study are shown in Tables 1 and 2 and represented graphically in Figure 1. Some of the factors which have an influence upon the initial transfer of fibres to the hair of a volunteer are obvious. The longer the hair of an individual the greater is the number of fibres transferred. Also, the longer the time the mask is in contact with the hair, the more fibres are transferred. For example, a mean value of 73 was found for fibres in the hair of a volunteer who had worn the mask for 30 minutes compared with only 25 when the mask was worn for 5 minutes. Initial fibre transfer is probably due to the friction or pressure of the mask being pulled over the head causing fibres to be pulled from the weave and others to be broken or fragmented. This is followed by fibres shedding from the mask as a result of contact with the hair during wear. Persistence studies carried out on the Caucasian and negroid subjects suggest that fibre loss approaches an exponential rate of decay. The majority of the transferred fibres, approximately two-thirds from Caucasian hair and approximately one half from negroid hair, were lost during the first hour after the mask was removed. The rate of fibre loss from the hair then slowed in both racial groups to leave a residual 8 to 10% of the original 290

TABLE 1. Persistence of woollen and acrylic fibres in Caucasian hair. The mean number of fibres recovered from each individual is shown with the corresponding range values, and the percentage fibres persisting in the hair after a given time period Woollen Time since removal of the mask

Acrylic

Recovery

Recovery

Mean Range

Persistence 9%

Persistence

Yo

Mean Range

0 hours 1 hour 2 hours 4 hours 6 hours 8 hours 12 hours 24 hours 2 days 3 days 4 days 5 days 6 days

TABLE 2. Persistence of woollen and acrylic fibres in negroid hair. The mean number of fibres recovered from each individual is shown with the corresponding range values, and the percentage fibres persisting in the hair after a given time period Woollen Time since removal of the mask

0 hours 1 hour 2 hours 4 hours 6 hours 8 hours 12 hours 24 hours 2 days 3 days 4 days 5 days 6 days

Acrylic

Recovery Mean Range

38 30 21 IS 13 11 8 6

6 5 4 4 4

25-50 20-44 16-30 10-23 8-20 8-15 5-10 4-8 4-8 3-8 3-6 3-5 3-5

Recovery Persistence %

100 80 55 40 35 30 22 15 15 13 10 10 10

Mean Range

38 34 27 17 12 11 8 5 4 4 3 3 3

28-49 22-43 19-36 12-23 8-19 8-17 5-11 4-8 3-6 3-5 2-4 2-4 2-4

Persistence %

100 90 70 45 30 28 20 12 10 10 9 9 9

number of fibres transferred still present in the hair after 24 hours. Whilst this percentage represented only 2 to 10 fibres, these then tended to persist in the hair for up to 6 days, providing that the hair was not washed during this period. Fibres were recovered from all the subjects after 5-6 days under these conditions.

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2

1

6

12

2

hours

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3

4

5 da$

FIGURE 1 Persistence of (a) woollen and (b) acrylic fibres in head hair Caucasian x-x Negroid 0- -0

It was thought that woollen fibres would persist in the hair for longer periods than acrylic fibres, the former having rougher surfaces which might interlock with the hair. However, this was not found to be the case, no differences being seen in persistence between woollen fibres and acrylic fibres. This agrees with previous suggestions that woollen and synthetic fibres are lost at the same rate from various garments [2,3]. Fibres were found to persist in tightly curled negroid hair, and in relatively straight Caucasian hair, for a similar length of time. It was found, however,

that fibres could be more efficiently recovered from negroid hair by using an Afro comb, plugged with cotton wool, rather than a standard comb.

1

1

2

days

3

FIGURE 2 The effect of washing the hair on fibre persistence. Hair was washed and towel dried immediately after removing the ski mask (i.e., time 0)

The effect of washing the hair immediately after removing the mask is shown in Figure 2. Washing the hair, followed by towel drying, removed only approximately 80% of the fibres initially present in the hair. The remaining 20% then fell off exponentially with time, fibres still being recovered up to 3 days after the hair was washed. This somewhat surprising observation was found to be reproducible in all five subjects, each having carried out the experiment ten times. It would appear that the hair must be washed more than once to remove all of the fibres.

References McKenna FJ and Sherwin JC. A simple and effective method for collecting contact evidence. Journal of the Forensic Science Society 1975; 15: 227-280. 2. Kidd CBM and Robertson J. The transfer of textile fibres during simulated contact. Journal of the Forensic Science Society 1982; 22: 301-308. 3. Robertson J , Kidd CBM and Parkinson HMP. The persistence of textile fibres transferred during simulated contacts. Journal of the Forensic Science Society 1982; 22: 353-360. 1.