Background: Platinum group metal (PGM) mining and refining is a large constituent of the mining sector of South Africa and contributes significantly to the gross domestic product. The PGMs include the rare metals platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir) and osmium (Os). During the refining process workers are potentially exposed to various chemical forms of the PGMs via the respiratory and dermal exposure routes. Historically, emphasis has been on respiratory exposure while the extent of skin exposure is still unknown. Among the different forms of PGMs, the salts are potential sensitisers, with platinum being a known respiratory sensitiser. Workers occupationally exposed to platinum and rhodium have reported respiratory as well as skin symptoms. However, it is unknown if these metals in the salt form are permeable through human skin, and whether dermal exposure could contribute to sensitisation. Evidence regarding differences between African and Caucasian skin anatomy and structure, as well as permeation through skin is contradictory, and no information is available on metal permeation through African skin. The in vitro diffusion method has been utilised successfully in occupational toxicology to demonstrate that metals such as chromium, cobalt and nickel, to name a few, permeate through human skin. The permeability of platinum and rhodium has not been investigated previously. Aims and objectives: The research aim was to obtain insight into the permeability of platinum and rhodium through intact human skin and to provide information needed to determine the potential health risk following dermal exposure to these metals. The specific objectives included: (i) to critically review the in vitro diffusion method that is used to determine the permeability of metals through human skin, (ii) to investigate the permeation of potassium tetrachloroplatinate (K2PtCl4) and rhodium chloride (RhCl3) as representative PGM salts through intact human skin over a 24-hour period, (iii) to evaluate the difference in permeability of platinum and rhodium through intact human skin, (iv) to evaluate the difference in permeability of platinum through intact African and Caucasian human skin. Methods: Abdominal skin obtained after cosmetic procedures was obtained from five female Caucasian and three female African donors between the ages of 28 and 52 with ethical approval from the North-West University. Full thickness skin tissue was mounted in a vertical Franz diffusion cell. Skin integrity was tested by measuring the electrical resistance across the skin before and after conclusion of the experiments, using a Tinsley LCR Data bridge Model 6401. The donor solution of 32.46 mg K2PtCl4 in 50 ml of synthetic sweat (pH 6.5), and 43.15 mg RhCl3 in 50 ml of synthetic sweat (pH 6.5) was prepared. The donor solution was applied to the stratum corneum side of the skin and physiological receptor solution (pH 7.35) was added to the receptor compartment. The concentration of the metals in the receptor solution was determined by high resolution inductively coupled plasma-mass spectrometry after extraction at various intervals during the 24 hours of the study. After completion of the study, the skin was rinsed four times to remove any platinum or rhodium remaining on the skin surface. The skin was digested using hydrogen peroxide, nitric acid and hydrochloric acid during different steps to determine the mass of the metals remaining in the skin by inductively coupled plasma-optical emission spectrometry. Results: The comparison of published in vitro skin permeation studies involving metals is impeded by the variations in the experimental design and dissimilarity in the reporting of results. Differences in experimental design included, most noticeably, the use of various donor and receptor solutions, different temperatures wherein the receptor compartment was placed, differences in skin thickness and variations in exposed skin surface areas. The metals considered in the review, namely chromium, cobalt, gold, lead, mercury, nickel, platinum, rhodium and silver, permeate through intact human skin under physiological conditions. Large variations in the permeability results were observed, with the notable differences in methodology as the probable reason. Results obtained from the in vitro experiments indicate that platinum and rhodium permeated through intact Caucasian skin with flux values of 0.12 and 0.05 ng/cm2/h, respectively. The cumulative mass of platinum (2.57 ng/cm2) that permeated after 24 hours of exposure was statistically significantly (p = 0.016) higher than rhodium permeation (1.11 ng/cm2). The mass of platinum (1 459.47 ng/cm2) retained in the skin after 24 hours of exposure was statistically significantly (p < 0.001) higher than rhodium retention (757.04 ng/cm2). The comparison of permeability between two different racial groups indicates that platinum permeated through the skin of both racial groups with the flux through African skin found as 1.93 ng/cm2/h and 0.27 ng/cm2/h through Caucasian skin. The cumulative mass of platinum permeated after 24 hours of exposure was statistically significantly (p = 0.044) higher through African skin (37.52 ng/cm2) than Caucasian skin (5.05 ng/cm2). The retention of platinum in African skin (3 064.13 ng/cm2) was more than twice the mass retained in Caucasian skin (1 486.32 ng/cm2). Conclusions: The in vitro diffusion method is an applicable method to determine skin permeability of metals. However, the experimental design and format of data reporting should be standardised to enable comparison of results from different studies. Platinum and rhodium permeated through intact human skin, with platinum permeation significantly higher. African skin was significantly more permeable by platinum than Caucasian skin. Both platinum and rhodium were retained inside the skin after 24 hours of exposure, possibly forming a reservoir which could contribute to continued permeation through the skin even after removal thereof from the skin. Platinum and rhodium permeated through full thickness skin and thereby could possibly contribute to local skin symptoms such as dermatitis and urticaria found in occupationally exposed workers. By permeating through the upper layers of the skin, these metals could potentially reach the viable epidermis and contribute to sensitisation. / PhD (Occupational Hygiene), North-West University, Potchefstroom Campus, 2015

Background: Platinum group metal (PGM) mining and refining is a large constituent of the mining sector of South Africa and contributes significantly to the gross domestic product. The PGMs include the rare metals platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), iridium (Ir) and osmium (Os). During the refining process workers are potentially exposed to various chemical forms of the PGMs via the respiratory and dermal exposure routes. Historically, emphasis has been on respiratory exposure while the extent of skin exposure is still unknown. Among the different forms of PGMs, the salts are potential sensitisers, with platinum being a known respiratory sensitiser. Workers occupationally exposed to platinum and rhodium have reported respiratory as well as skin symptoms. However, it is unknown if these metals in the salt form are permeable through human skin, and whether dermal exposure could contribute to sensitisation. Evidence regarding differences between African and Caucasian skin anatomy and structure, as well as permeation through skin is contradictory, and no information is available on metal permeation through African skin. The in vitro diffusion method has been utilised successfully in occupational toxicology to demonstrate that metals such as chromium, cobalt and nickel, to name a few, permeate through human skin. The permeability of platinum and rhodium has not been investigated previously. Aims and objectives: The research aim was to obtain insight into the permeability of platinum and rhodium through intact human skin and to provide information needed to determine the potential health risk following dermal exposure to these metals. The specific objectives included: (i) to critically review the in vitro diffusion method that is used to determine the permeability of metals through human skin, (ii) to investigate the permeation of potassium tetrachloroplatinate (K2PtCl4) and rhodium chloride (RhCl3) as representative PGM salts through intact human skin over a 24-hour period, (iii) to evaluate the difference in permeability of platinum and rhodium through intact human skin, (iv) to evaluate the difference in permeability of platinum through intact African and Caucasian human skin. Methods: Abdominal skin obtained after cosmetic procedures was obtained from five female Caucasian and three female African donors between the ages of 28 and 52 with ethical approval from the North-West University. Full thickness skin tissue was mounted in a vertical Franz diffusion cell. Skin integrity was tested by measuring the electrical resistance across the skin before and after conclusion of the experiments, using a Tinsley LCR Data bridge Model 6401. The donor solution of 32.46 mg K2PtCl4 in 50 ml of synthetic sweat (pH 6.5), and 43.15 mg RhCl3 in 50 ml of synthetic sweat (pH 6.5) was prepared. The donor solution was applied to the stratum corneum side of the skin and physiological receptor solution (pH 7.35) was added to the receptor compartment. The concentration of the metals in the receptor solution was determined by high resolution inductively coupled plasma-mass spectrometry after extraction at various intervals during the 24 hours of the study. After completion of the study, the skin was rinsed four times to remove any platinum or rhodium remaining on the skin surface. The skin was digested using hydrogen peroxide, nitric acid and hydrochloric acid during different steps to determine the mass of the metals remaining in the skin by inductively coupled plasma-optical emission spectrometry. Results: The comparison of published in vitro skin permeation studies involving metals is impeded by the variations in the experimental design and dissimilarity in the reporting of results. Differences in experimental design included, most noticeably, the use of various donor and receptor solutions, different temperatures wherein the receptor compartment was placed, differences in skin thickness and variations in exposed skin surface areas. The metals considered in the review, namely chromium, cobalt, gold, lead, mercury, nickel, platinum, rhodium and silver, permeate through intact human skin under physiological conditions. Large variations in the permeability results were observed, with the notable differences in methodology as the probable reason. Results obtained from the in vitro experiments indicate that platinum and rhodium permeated through intact Caucasian skin with flux values of 0.12 and 0.05 ng/cm2/h, respectively. The cumulative mass of platinum (2.57 ng/cm2) that permeated after 24 hours of exposure was statistically significantly (p = 0.016) higher than rhodium permeation (1.11 ng/cm2). The mass of platinum (1 459.47 ng/cm2) retained in the skin after 24 hours of exposure was statistically significantly (p < 0.001) higher than rhodium retention (757.04 ng/cm2). The comparison of permeability between two different racial groups indicates that platinum permeated through the skin of both racial groups with the flux through African skin found as 1.93 ng/cm2/h and 0.27 ng/cm2/h through Caucasian skin. The cumulative mass of platinum permeated after 24 hours of exposure was statistically significantly (p = 0.044) higher through African skin (37.52 ng/cm2) than Caucasian skin (5.05 ng/cm2). The retention of platinum in African skin (3 064.13 ng/cm2) was more than twice the mass retained in Caucasian skin (1 486.32 ng/cm2). Conclusions: The in vitro diffusion method is an applicable method to determine skin permeability of metals. However, the experimental design and format of data reporting should be standardised to enable comparison of results from different studies. Platinum and rhodium permeated through intact human skin, with platinum permeation significantly higher. African skin was significantly more permeable by platinum than Caucasian skin. Both platinum and rhodium were retained inside the skin after 24 hours of exposure, possibly forming a reservoir which could contribute to continued permeation through the skin even after removal thereof from the skin. Platinum and rhodium permeated through full thickness skin and thereby could possibly contribute to local skin symptoms such as dermatitis and urticaria found in occupationally exposed workers. By permeating through the upper layers of the skin, these metals could potentially reach the viable epidermis and contribute to sensitisation. / PhD (Occupational Hygiene), North-West University, Potchefstroom Campus, 2015

Die Hautsensibilisierung (Typ 4 Hautallergie) nimmt in der Toxizitätsbewertung einen wichtigen Stellenwert ein, was u.a. daran zu sehen ist, dass sie unter dem europäischen Chemikaliengesetz REACH schon sehr früh, d.h. ab einer Jahrestonne, abgeprüft werden soll. Die Dissertation untersucht, ob die derzeit im Tierversuch stattfindende Toxizitätsprüfung durch computerchemische Methoden ersetzt werden kann. Dazu wurde eine Datenbank aus über 2000 Stoffen erstellt, aus der wichtige Unterschiede zwischen den eingesetzten Tiermodellen herausgelesen werden konnten. In den Untersuchungen trat auch zu Tage, dass – entgegen vorheriger Annahmen – die Bioverfügbarkeit, d.h. die Aufnahme von Substanzen über die Haut, im Tierversuch nur eine untergeordnete Rolle spielt. Zudem ist eine Abschätzung des hautsensibilisierenden Effekts von Stoffen anhand eines Read-across (Interpolation aus strukturähnlichen Verbindungen) und von Strukturalarmen (Substruktur­elemente als Indikatoren für einen bestimmten Effekt) möglich. Wenn beide Ansätze im Rahmen einer Konsens­modellierung miteinander verschränkt werden, ergibt sich sogar eine gute Vorhersagestatistik. / Skin sensitisation (type 4 skin allergy) is an important parameter in the toxicity assessment of chemicals, which is underlined by the fact that it is evaluated even at the lowest tonnage (1 t/a), which can be registered under the european chemicals regulation (REACH). In this thesis it was investigated if the currently used animal models can be replace or refined with computational (in silico toxicological) methods. In this regard a data base consisting of about 2000 substances was build. With its data important differences between the currently applied animal tests could be derived. Furthermore, the investigation found that – in contrast to previous assumptions – the bioavailability of a chemical compound, i.e. the uptake via the skin, has only a minor impact on the test result of the evaluated animal models. Moreover, it was demonstated that the skin sensitising potential of chemicals can be predicted by read-across (interpolation with structurally similar substances) and with structural alerts (substructural elements which indicate a particular effect). Combining both prediction methods with consensus modelling lead to a good prediction regarding the question whether a particular chemical compound is a sensitiser or not.