Annals of Occupational Hygiene Advance Access originally published online on September 26, 2005
Annals of Occupational Hygiene 2006 50(1):85-94; doi:10.1093/annhyg/mei037
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© 2005 British Occupational Hygiene Society Published by Oxford University Press
Original Article |
Development of a Biologically Relevant Dermal Sampler
1 Institute of Occupational Medicine, Edinburgh, EH14 4AP, Scotland, UK; 2 Department of Environmental and Occupational Medicine, University of Aberdeen, Aberdeen AB25 2ZP, UK
* Author to whom correspondence should be addressed. Tel: +44-131-449-8032; fax: +44-870-850-5132; e-mail: john.cherrie{at}iomhq.org.uk
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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There are currently no appropriate methods for measuring dermal exposure to volatile agents. To address this we have produced a prototype Institute of Occupational Medicine (IOM) dermal sampler consisting of an adsorbent sandwiched between a permeable membrane and an impervious backing. The concentration of solvent on the membrane surface may be estimated from the mass collected on the adsorbent and the known permeation rate through the membrane. We have developed the prototype IOM dermal sampler for measurement of toluene exposures. Evaluation of the prototype sampler was undertaken in two stages: laboratory performance in controlled exposure situations and two short-field evaluations, which included simultaneous measurement of inhalation exposure. In all cases we compared the prototype IOM dermal sampler with activated charcoal cloth (ACC). Laboratory trials were split into spray, pour and immersion tests. The data from these suggest that the sampler responds to concentration rather than the mass on the surface of the sampler. The field study showed that the prototype sampler was suitable for measuring dermal exposure. However, the mean permeation rate of the best membrane was 78 000 µg cm–2 h–1, which is higher than the permeation rate through skin. This high permeation rate created difficulties throughout the study, particularly as it allowed the adsorbent to become rapidly saturated. The prototype IOM dermal sampler is the first practical dermal exposure sampler to mimic uptake through the skin. The sampler gave reproducible results in the laboratory and field trials. Future work is required to identify a less permeable membrane, which has characteristics closer to that of human skin. Additionally, a higher capacity adsorbent would be desirable. We have demonstrated a major difference when calculating the total contribution to body burden via the dermal exposure pathway using the prototype IOM dermal sampler and ACC patches, 1.5% of the total body burden compared with 95%. The prototype IOM dermal sampler provides a more biologically relevant exposure metric than the alternatives.
Keywords: dermal sampler • exposure assessment • solvents
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Occupational dermal exposure to hazardous substances is known to cause a variety of diseases, including skin cancer and dermatitis. In addition, many chemicals, such as organic solvents and polycyclic aromatic hydrocarbons, can pass through unbroken skin and contribute to the systemic dose.
Practical sampling methods available to assess dermal exposure have been summarized by Schneider et al. (2000)
. These fall into three broad categories: in situ techniques such as those using fluorescent tracers (Cherrie et al., 2000), removal techniques (Brouwer et al., 2000) and interception methods (Soutar et al., 2000). However, there is little standardization in the approaches used by different researchers using these techniques.
In situ techniques generally rely on the natural fluorescence of the hazardous substance being investigated or of an added tracer compound. These techniques use an ultraviolet light source to illuminate the skin and a sensitive camera to capture the resultant fluorescent image. Computer analysis of the image can be used to provide a quantitative estimate of the mass of fluorescent compound on the skin and the exposed area. Removal techniques use wiping or washing to remove residues present on the skin surface for analysis. Fenske et al. (1999) have demonstrated substantial differences in recovery between wiping and washing methods.
Finally, there are interception methods using patches or suits for collection, the aim of which is to provide a covering surrogate ‘skin’ for the body that will collect all the contaminant that would otherwise have landed on the skin. Interception samplers are not generally designed to take account of the run-off and other losses of contaminants that occur in the work place. Patches are often constructed from a 25 cm2 of cotton or other fabric with an impervious backing. Patches can be attached to various parts of the body to provide a measure of the mass of contaminant that has landed at that location but require complex extrapolation procedures to provide a measure of whole body exposure. Suit samples are advantageous in that they can potentially sample the whole body, although the amount of sampling material used presents practical difficulties during sampling and analysis.
Sampling volatile materials presents particular problems in terms of evaporation and retention of substances in the collecting media. Cohen and Popendorf (1989) used an interception sampler comprising a patch of activated charcoal cloth (ACC) to absorb volatile chemicals in the same way that the cotton pads collect non-volatile agents. ACC used in this manner is likely to absorb all of the material that comes into contact with the patch and will not reflect the processes of evaporation or run-off that would take place in most normal exposure scenarios involving volatile substances. Cherrie and Robertson (1995) suggested using a patch sampler that would collect a contaminant by diffusion, comprising an adsorbent and a semi-permeable membrane. Uptake of chemicals through the skin is dependent on the concentration on the skin, rather than the mass (Fiserova-Bergerova, et al., 1990; Fiserova-Bergerova, 1993). Cherrie and Robertson (1995) suggested that the sampler should mimic the process of uptake through the skin and provide estimates of exposure that are ‘biologically relevant’. The aim of the present study was to develop and test a prototype sampler capable of reflecting the uptake of toluene through the skin in a typical work environment, in addition to simulating potential contaminant loss from the dermal surface.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Component selection
The proposed design of the sampler was based on existing inhalation diffusion samplers and included a diffusive membrane, a separator, an adsorbent and a backing. A literature review was carried out to identify suitable candidate materials for the sampler.
One of the primary aims of the sampler development phase was to identify an adsorbent with a high capacity for volatile materials. It also had to be relatively inexpensive, flexible and permit easy desorption of chemicals. The most widely used, cheapest and most effective adsorbent for organic substances is activated carbon. ACC from Chemviron Carbon Limited (2001) was selected as the adsorbent for the prototype sampler because it provided the highest capacity of the cloth investigated.
The membrane materials were required to be resistant to degradation by organic solvents, be flexible, robust and more importantly have solvent flux rates similar to those reported for human skin. We identified a small number of studies that have attempted to measure the permeation rate of toluene through human skin in human volunteers. Interpretation of these studies was not straightforward as exposure circumstances and experimental design varied greatly between studies. For example, Boman and Maibach (2000) do not quote the permeation rate of toluene through the skin, only the percentage of the absorbed dose recorded under steady state diffusion conditions. From their information we estimated the flux to be between 144 and 219 µg cm–2 h–1. Ke
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et al. (2001) exposed the forearms of volunteers to toluene for very short durations. The permeation rate quoted was 1200 µg cm–2 h–1, which is not representative of steady state diffusion conditions but is perhaps more representative of realistic workplace exposure. We believe the permeation rate of toluene through the skin for the purpose of membrane selection is between 144 and 1200 µg cm–2 h–1. We aimed to produce a sampler with uptake characteristics as close to this as possible.
A total of 29 candidate membranes were tested, each with varying physical and chemical properties. The permeation rate of each material was measured using a modification of EN 374-3 (European Standard, 1994). A Thermo Electron Instruments TVA 1000B was used to measure the concentration of toluene in the system. The arrangement of this test apparatus is shown schematically in Fig. 1.
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Clean air was pushed into the system at one end while simultaneously being drawn through the other end by the TVA 1000B. This equalized the pressure across the test material. Air and toluene vapour met in the glass flask and were encouraged to mix through turbulence via an in-line chamber, before passing into the TVA 1000B for measurement.
For each test the candidate membrane was clamped in between the upper and lower section of the flow cell. A quantity of 200 µl of toluene was applied to cover the top surface of the test material. The flow cell was enclosed to reduce evaporation of toluene from the surface of the material. Each material was tested at least three times and a mean permeation rate was calculated.
A number of the membranes that were tested had very low permeation rates and were essentially impervious to toluene. Many others provided only a limited barrier to the diffusion of toluene. Two membranes were unstable in toluene showing visible damage to the membrane structure and four produced an unstable diffusion process, reflected in rapidly varying toluene concentrations in the test cell. Similar variability in permeation data was observed across the membranes that permitted diffusion. The relative standard deviation (SD) varied from 1 to 9% of the mean.
The membrane Pallflex AO1603 permitted permeation of toluene and overall had the second lowest permeation rate. However, it had much more favourable physical characteristics than the membrane with the lowest permeation rate. Pallflex AO1603 is resistant to degradation by organic solvents and flexible, robust and contains a binder suitable for heat-sealing, which could be used to bond to other components. It was, as a result, selected for use in the prototype IOM dermal sampler The membrane gives mean toluene permeation rate of 78 000 µg cm–2 h–1, which is 
400 times the flux rate through the skin derived by Boman and Maibach (2000) and 65 times that of Kei et al. (2001).
The backing material for the sampler also had to be chemically stable, impervious, strong, flexible and suitable for sealing to other materials. It was also required to provide good thermal contact with the skin; allowing simulation of the evaporative effect caused by surface skin temperature. A coated aluminium foil from GTS Flexible Materials was selected for this purpose. This aluminium foil (26 µm thick) had a clear laminate which was sufficient to form a strong thermal weld with the Pallflex AO1603 membrane.
The sampler was constructed in layers (Fig. 2); the laminated aluminium foil backing, a layer of ACC, a polytetrafluoro-ethylene (PTFE) mesh separator and the Pallflex AO1603 membrane. The PTFE mesh simply acts as a physical spacer between the ACC and the membrane, it does not form a physical barrier to the diffusion of absorbed material.
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To allow enough space in the sampler for an adequate layer of ACC cloth, the dimensions of the sampler were arbitrarily set at 6 cm x 9 cm (the absorption capacity was 300 mg of toluene). The inner components (ACC and PTFE mesh) were cut slightly smaller,
5 cm x 8 cm, to allow space for the thermal weld at all four edges of the sampler. The top surface of the membrane was therefore 54 cm2, although the actual effective sampling area of the membrane was 34 cm2 (4.5 x 7.5 cm2). All edges of the sampler were thermally welded using a commercial heat sealer (Hulme Martin 240 V Impulse). Finished samplers were stored in a desiccator to minimize water adsorption on the ACC prior to use. The vapour uptake rate for the prototype IOM sampler was 340 cm3 min–1 and that of the ACC was 3600 cm3 min–1.
Laboratory trials
The laboratory evaluations were undertaken to investigate the basic performance of the sampler in a range of controlled situations. These consisted of spray tests, pour tests and immersion tests. In all laboratory trials prototype IOM dermal samplers and simple samplers consisting only of ACC were exposed to toluene over a range of different concentrations, but with the same total mass of toluene used in each case. For each test the performance of the prototype IOM dermal samplers was compared to the ACC samplers. In order to challenge the samplers with different concentrations, but the same total mass of toluene for each test, it was necessary to adjust the time of each test relative to the concentration of the challenge solution, with the exception of the immersion test, where we felt that long immersion times would be of no benefit as it was likely that one immersion, even for very short duration, would saturate the ACC. This study was designed to determine if concentration was the driving factor for uptake, rather than mass. Therefore the total mass for each test was kept constant. Table 1 illustrates the parameters for each test.
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In the spray tests a range of concentrations of toluene were sprayed onto the surface of the prototype IOM dermal samplers. Four samplers (two ACC samplers and two prototype IOM dermal samplers) were mounted on a vertically rotating cylinder. Toluene was diluted with ethanol to a predetermined concentration and each solution was sprayed at 90° to the cylinder. In the pour tests, a sampler was fixed onto an angled board. A mixture of toluene and ethanol was then poured over the surface of the sampler for a predetermined period of time. In addition, IOM prototype diffusive samplers and ACC samplers were immersed in different concentrations of toluene solution. They were placed into a glass container, along with the relevant solution, and fully immersed.
These data from the laboratory trials were used to estimate the average concentration over the surface of the sampler during the tests. We have assumed that there was steady state diffusion through the membrane and that the permeation rate for the membrane that we measured in the development work was applicable.
Therefore the magnitude of the concentration on the sampler (Csk) is given by:
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Csk is a dimensionless unit that represents the ratio of mass of toluene measured by the sampler to the mass of the toluene that would have been measured if a pure solution of toluene completely covered the sampler surface. Csk therefore indirectly enables calculation of the average concentration of toluene that had been present on the skin surface.
Field trials
Two short-field tests of the sampler were carried out. Company A manufactured a range of specialist printing blankets and rubber coated fabrics, including specialist blankets for use in the security printing industry. Toluene was added to drums of rubber compound by a mixing employee. The mixed rubber compound was then spread onto a material backing and pressed onto the backing by rollers on the spreading machine. The sheet then passed through a series of drying ovens to cure the rubber and the finished product was rolled. Four employees were selected for sampling, the mixing employee and three ‘spreaders’. They were each sampled for 2 h in the morning and then this was repeated again in the afternoon with the same employees.
Company B used toluene in both a pigment mixing process and in a coating process. Toluene was added to the pigment by direct feed, but it was often necessary for an employee to transfer small volumes by jug. The print rollers were cleaned with solvent rags. Three pigment mixing staff and three coating line staff were sampled. Each employee was sampled for 2 h during each shift.
In both factories, each employee wore a prototype IOM dermal sampler and ACC sampler side-by-side on their forearm and chest and they also wore a 3M 3500 passive diffusion badge on their chest. The 3M passive diffusion badge was also included to measure each individual's inhalation exposure during the same sampling period as the ACC and prototype IOM dermal sampler. The prototype IOM dermal sampler operates on similar principles as the 3M passive diffusion badge and hence absorbed toluene will come from both vapour and liquid splashes. A measure of the toluene vapour concentration during the sampling permitted the calculation of a correction factor for the vapour uptake of the prototype IOM dermal sampler.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Laboratory trials
Three ACC patches were spiked with 10 µl of neat toluene (8650 µg) using a microsyringe. They were then removed and desorbed in 10 ml of carbon disulphide. An aliquot of each sample was analysed by gas chromatography to determine the mass of toluene present on each patch. The recovery efficiency was calculated as a percentage of the mass of toluene recovered from the total mass originally added. In each case 8650 µg of toluene was spiked, the mass recovered for each test was 7970, 7970 and 7860 or 98.5, 98.5 and 97% recovery efficiencies.
The stability and recovery efficiency of the selected activated carbon cloth were satisfactory. The mean recovery efficiency from this grade of ACC was 98% of the spiked mass of toluene.
Table 2 shows the mass per unit area range and mean SD for each respective laboratory test. There were clear differences between the mass uptake of the prototype IOM dermal sampler and ACC samplers, with the IOM sampler lower than the ACC samplers, and the lower concentration toluene mixtures producing lower mass uptake for both types of sampler when compared to pure toluene.
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A summary of the data obtained with the prototype IOM dermal sampler is presented as calculated uptake rates (mg cm–2 h–1) in Fig. 3. Each series of three box plots represents the data from spraying, pouring and immersion, respectively, for the prototype IOM dermal sampler only. The dashed line on each box plot represents the mean uptake rate, the middle of each box represents the median uptake rate, the grey shaded box represents the 5th/95th percentiles and the whiskers represent the range and any outliers. The first block of results are for pure toluene, then for 50 and 25% mixtures. Note, the 75 and 5% spray tests have been excluded, as also the 60 min immersion test. There were no immersion measurements for the 50% toluene mixture.
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The mean uptake rates for the prototype IOM dermal sampler during the spray test were 7.6 (SD 2.3, n = 7), 3.7 (SD 0.33, n = 4) and 0.80 (SD 0.20, n = 16) mg cm–2 h–1 for the 100, 50 and 25% toluene mixtures, respectively. The uptake rates for pouring simulation were 48 (±7.7, n = 8), 24 (SD 1.3, n = 8) and 8.9 (SD 1.5, n = 8) mg cm–2 h–1, for the 100, 50 and 25% toluene mixtures, respectively. For the immersion tests they were 450 (SD 120, n = 2) and 130 (SD 8.5, n = 2) mg cm–2 h–1 for the 100 and 25% toluene mixtures, both latter immersion tests were for 10 s only.
The mean uptake rates for the ACC patches during the spray test were 77 (SD 11, n = 7), 30.9 (SD 2.3, n = 8) and 11 (SD 1.6, n = 24) mg cm–2 h–1 for the 100, 50 and 25% toluene mixtures, respectively. The uptake rates for pouring simulation were 110 (SD 8.4, n = 2), 47 (SD 4.8, n = 2) and 16 (SD 2.4, n = 2) mg cm–2 h–1, for the 100, 50 and 25% toluene mixtures, respectively. For the immersion tests they were 1400 (SD 240, n = 2) and 1100 (SD 55, n = 2) mg cm–2 h–1 for the 100 and 25% toluene mixtures, both latter immersion tests were for 10 s only.
Estimated toluene concentrations were obtained using the expression described above, and these were compared to the concentrations of the bulk liquid used in the laboratory tests. These data are illustrated in Fig. 4. The y-axis on this chart represents the ratio of the concentration of the estimated concentration to the bulk test solution, calculated using the relationship described above.
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The differences between test conditions are apparent. We believe the spray test did not uniformly cover the sampler membrane, and it is also possible that not all of the sprayed solution reached the membrane. Clearly the pouring test provides a greater coverage of the sampler surface with the test liquid. Here the ratio of the measured concentration to the toluene concentration in the bulk liquid is closer to expected, although still somewhat lower. On the short duration immersion tests the estimated toluene concentration was much higher than expected from the bulk liquid concentration, and for the long-term sample it was much lower. In the latter case the adsorbent was clearly saturated and this is the explanation for this anomalous result. In the short duration samples we believe that it was unlikely that the actual duration of the test would properly reflect the time that toluene was permeating through the membrane; there would still be some toluene on the sampler continuing to diffuse towards the adsorbent after removal. However, this would probably also occur with dermal exposure on the skin, suggesting that short-term exposure may lead to relatively more uptake than longer term exposure.
Field trials
Company A
The airborne personal toluene exposure measurements from the operators ranged from 78 to 230 mg m–3, or from 21 to 60 p.p.m., the exposure of the mixer was much greater than the spreader operators. Using data from the personal inhalation exposure measurements we corrected the mass of toluene on the prototype IOM dermal sampler by subtracting the likely contribution from vapour uptake. The resulting figures ranged from 0.3 to 17 mg of toluene for the prototype IOM dermal sampler, and from 3.1 to 180 mg for the ACC samplers; the sample with the greatest mass was for the employee mixing. In this case it is theoretically possible that the ACC patch could have been saturated by vapour uptake alone. This assumes that vapour uptake is somehow preferential to liquid uptake, which we believe is implausible and it is likely that the ratio of vapour to liquid toluene deposited on a given saturated sampler will vary depending on the time course of the presentation of the splashes and any changes in the airborne concentration.
On average, vapour was estimated to have contributed 9.6% of the toluene mass on the prototype IOM dermal sampler and 23% of the mass on the ACC samplers.
Table 3 shows the dermal exposure measurements for toluene in Company A, adjusted for vapour uptake. The measurements for the samplers on the workers' chest were generally lower than corresponding data from the forearm. The measurements made with the ACC were higher than the corresponding IOM prototype dermal sampler data.
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Company B
The airborne toluene concentrations as measured by the 3M badges for the ‘mixer’ workers ranged from 27 to 140 mg m–3 (7.2–37 p.p.m.), while the personal exposure of the coating operators ranged from 2.6 to 32 mg m–3 (0.7–8.4 p.p.m.).
Table 4 shows the dermal exposure measurements for toluene in Company B, adjusted for vapour uptake. These lower measurements for coating operators are explicable by the lower level of contact with toluene for these workers compared with mixing operators. As before the data from the ACC samplers were higher, although there were still differences between the chest and forearm.
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Comparison of the data from both surveys
Figure 5 shows the toluene dermal exposure measurements from both factories, with the diagonal showing the line of equality. The black, filled points are from Company A and the grey points are from Company B. The measurements at the second plant were generally lower than those data from the first factory.
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Figure 5 confirms that the ACC measurements were generally higher than the corresponding prototype IOM dermal sampler. There is also a reasonable correlation between the two types of measurement (correlation coefficient = 0.80), suggesting that both samplers are responding to the contact the operators made with toluene.
Figure 6 shows a scatter plot of the inhalation exposure level against the corresponding dermal exposures measured using both the prototype IOM dermal sampler and the ACC patches. For each sampler dermal exposure data points are given for both the forearm and chest locations and these are linked with either a dotted or solid line. The regression lines for the data from the IOM prototype sampler and ACC, for both chest and forearm against the inhalation exposure level are also shown. Data from both factories are shown on this figure.
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From the graph there appears to be an association between the dermal and inhalation data, with r2 values ranging from 0.30 to 0.69. The measurements from the prototype IOM sampler from the forearms were always higher than the chest as would be expected, but the ACC sampler sometimes produced higher values from the chest compared with the forearm.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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The purpose of this study was to develop a sampling device that would measure the mass of toluene that was likely to be absorbed through the skin. At the end of the laboratory tests we concluded that both the prototype IOM dermal sampler and ACC patches became saturated in some extreme exposure scenarios, particularly the pouring tests. Any future developments of this work should aim to have greater adsorbent capacity and a less permeable membrane to minimize the potential for saturation. In general, the results from the laboratory trials of the prototype IOM dermal sampler were reproducible and reflected changes in the exposure event and the concentration of the toluene solution applied. The data for the prototype IOM dermal sampler and the ACC correlated fairly well. However, we acknowledge that one of the weaknesses of the field studies is that the findings are based upon a small number of samples. We recommend that further development of this sampler undertakes a much larger number of samples in a wider variety of exposure scenarios.
Clearly, it would also be desirable to use a less permeable membrane in further developments of this dermal sampling method to minimize problems associated with saturation but also to be more representative of actual skin. The possibility of using multiple layers of membrane has been examined by Said (2004) but double or triple layers provided little reduction in permeation levels. The use of water or some aqueous solution between a double membrane layer may more closely mimic the conditions found in human skin and should be considered for future samplers.
We have acknowledged that samplers based on an adsorbent also uptake vapour, and we have corrected the dermal measurements for vapour uptake by collecting data on vapour concentrations using conventional diffusive samplers. We believe that if we were able to identify a more suitable membrane for the sampler, i.e. with uptake properties for vapour and liquid similar to those of skin, it would not be necessary to correct for vapour uptake since the sampler would take up toluene vapour similar to skin. Work by Brooke et al. (1998) and Jones et al. (2003) indicated that the contribution from dermal uptake during combined inhalation and dermal exposure to toluene vapour was in the region between 1 and 11% of the total body burden.
In the field study we found that there was an association between the inhalation exposure level and dermal exposure to toluene measured with the prototype IOM dermal sampler. This, in part, reflected the difference in control measures at each plant; Company A had much poorer control measures than Company B for inhalation and dermal exposure. It is also possible that we underestimated the vapour contribution or alternatively that there was direct splashing onto the conventional diffusive samplers.
Cohen and Popendorf (1989) and more recently van Wendel de Joode et al. (2004) have investigated the use of activated charcoal pads for measuring dermal exposure and also note that ACC responds to exposures from the vapour phase and direct contact. van Wendel de Joode et al. (2004) reported that ACC acts like a sink, and evaporation from the skin to the atmosphere is not taken into account. They suggest that the design of the charcoal pad sampler might be improved by utilizing a layer to protect the sampler against direct contact and splashes; the prototype IOM dermal sampler includes such a layer. While not ideal, our selected membrane which covers the activated charcoal adsorbent provides a barrier, which allows deposition to and evaporation from the surface in a similar manner to the skin and also has a measured flux through the layer.
In order to demonstrate the concept of a biologically relevant exposure metric we have included a worked example. The employee with the highest measured dermal exposure, as measured by the prototype IOM dermal sampler, was exposed to 0.91 mg toluene per cm2 of skin. His corresponding inhalation exposure, measured using 3M passive diffusion badge, was 98 mg m–3, averaged over the 120 min sampling duration. Assuming that his hands and forearms were uniformly exposed during the sampling period [area = 2000 cm2, OECD (1997)] and if the prototype IOM dermal sampler is between 65 and 400 times more permeable than skin, then it is possible to estimate the dermal uptake of toluene, e.g. 0.91 mg cm–2 x 2000 cm2/400 = 4.5 mg of toluene for the lowest skin permeation rate. The estimated dermal toluene uptake ranges from 4.5 to 30 mg. The corresponding inhalation uptake equates to the following: 98 mg m–3 x 120 min x 0.025 m3 = 296 mg of toluene. Therefore, the total body burden would range from 300 to 326 mg. The dermal component in this scenario therefore only represents between 1.5 and 9% of the total body burden.
Following this same process through, but with data for the ACC sampler, gives a radically different measure of potential dermal exposure. 1.8 mg cm–2 x 2000 cm2 = 3600 mg. Using these data the total body burden would therefore equate to 3896 mg. In this case the dermal exposure route contributes 92% of the total body burden, which we believe is unlikely. However, the use of ACC is a currently accepted method of assessing dermal exposure. This example illustrates the need for a biologically relevant exposure metric for volatile agents.
The prototype IOM dermal sampler is the first practical dermal exposure sampler to allow both deposition and transfer to and from its surface. The sampler gives sensible, reproducible results in the laboratory and field trials. These results indicate that the principle does work; further development will improve on this and provide additional evidence on the concentration effects for adsorption.
Any future research will require a membrane material that is less permeable, with closer characteristics to skin. Potential routes forward may include development of custom-made synthetic or biological membranes. In addition to an improved membrane, a higher capacity ACC may be required.
Received December 29, 2004; in final form June 27, 2005
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REFERENCES |
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