Occupational exposure to platinum at South African precious metals refineries

Abstract

Background: South Africa is the largest producer of platinum in the world. During the refining of platinum, complex intermediary compounds are formed, many of which are potent sensitisers. Occupational exposure to soluble platinum has been associated with the development of soluble platinum sensitisation, which is characterised by adverse effects of the respiratory system and the skin. Urinary platinum excretion has been shown to be an effective biomarker for occupational exposure to platinum and has been positively correlated with respiratory exposure. Inhalation is seen as the primary route of exposure to soluble platinum. However, the possible role of dermal exposure in the development of respiratory sensitisation has received an increasing amount of attention recently. Dermal exposure to soluble platinum and its possible correlation with platinum body burden have not previously been investigated. Aims and objectives: The research aim of this thesis was to evaluate occupational exposure to soluble platinum of South African precious metals refinery workers and to examine the contribution of the dermal and respiratory exposure routes to the platinum body burden of workers. The specific objectives for the thesis were: (i) to conduct a critical review of the available published scientific literature on respiratory exposure to platinum group metals (PGMs) in occupational settings; (ii) to assess the platinum body burden of precious metals refinery workers through analysis of their urinary platinum excretion; (iii) to assess the respiratory exposure of precious metals refinery workers to soluble platinum using established methodology; (iv) to assess the dermal exposure of precious metals refinery workers to soluble platinum by making use of a commercially available wipe; (v) to examine the relationship between respiratory and dermal exposure to soluble platinum, and urinary platinum excretion in order to establish the contribution of each route of exposure to the platinum body burden; and (vi) to assess the effectiveness of disposable coveralls in reducing dermal exposure to soluble platinum. Methods: Forty workers from two South African precious metals refineries participated in this study. Dermal and respiratory exposure to soluble platinum as well as the urinary platinum excretion of workers was measured concurrently over two consecutive working days. Dermal exposure was assessed using Ghostwipes™ on four anatomical areas (palm of hand, wrist, neck and forehead) and respiratory exposure was assessed using the Methods for the Determination of Hazardous Substances (MHDS) 46/2 method. For biological monitoring, three spot urine samples were collected from each worker. The first was collected prior to the start of the first day of exposure monitoring, the second prior to the second day of exposure monitoring and the third prior to the start of the following day's shift. Additionally surface wipe samples were also collected to examine soluble platinum surface contamination. All samples were analysed for soluble platinum according to a method based on MDHS 46/2 that uses Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). Ethics approval for the study was obtained from the Health Research Ethics Committee of the North-West University (NWU-00128-14-A1). Results: A number of published research articles have reported occupational respiratory exposure to platinum compounds. However, the manner in which results are reported vary, which makes the comparison of results between different studies challenging. Authors often only report the number of measurements that exceeded the occupational exposure limit (OEL) of 2 μg/m3 and do not report more detailed descriptive statistics or whether the soluble or total fraction was analysed. Analysis of the available data showed that the highest concentrations of airborne soluble platinum were reported in precious metals refineries. The degree of exposure is the greatest risk factor for the development of soluble platinum sensitisation and is influenced by a worker's area of work, the tasks performed and the fraction of soluble platinum in the workplace air. The OEL of 2 μg/m3 has been in use since 1970. A number of studies have questioned its relevance, since sensitisation has been shown to occur at exposure below 2 μg/m3. Furthermore, very few research articles have reported respiratory exposure to PGMs other than platinum (palladium, rhodium, iridium, ruthenium and osmium). The results obtained from the biological and exposure monitoring studies indicated that quantifiable concentrations of soluble platinum were present in the urine of precious metals refinery workers and that workers were exposed to soluble platinum via the dermal and respiratory exposure routes. The geometric mean of the urinary platinum excretion was 0.212 μg/g creatinine [95% confidence interval (CI): 0.169-0.265 μg/g creatinine] and ranged from < 0.1 to 3.0 μg/g creatinine. The results from the three spot urine samples did not differ significantly. Significantly higher urinary platinum excretion was found for workers directly exposed to platinum compounds during production activities compared to that of non-production workers who were indirectly exposed (p = 0.007). The geometric mean of the average dermal exposure experienced on all four anatomical areas was 0.008 μg/cm2 (95% CI: 0.005-0.013 μg/cm2). The geometric mean of the respiratory exposure was 0.301 μg/m3 (95%CI: 0.151-0.601 μg/m3). Directly exposed workers experienced significantly higher dermal (p = 0.002) and respiratory (p = 0.002) exposure to soluble platinum. The urinary platinum excretion of workers correlated positively and significantly with their dermal exposure (r = 0.754) and respiratory exposure (r = 0.580) to soluble platinum. Detectable concentrations of soluble platinum were found on a variety of surfaces in production and non-production areas. The use of disposable coveralls and the adherence to usage procedures by workers who were directly exposed to platinum compounds significantly reduced their dermal exposure to soluble platinum (p = 0.018). Conclusions: According to the literature, the highest concentrations of airborne soluble platinum are reported in precious metals refineries. Limitations in the published body of literature investigating occupational exposure to PGMs were identified. It was clear that no standardised approach is followed for reporting respiratory exposure results which makes the comparison of studies difficult. Recommendations are made for the standardisation of the reporting methods in order to facilitate the comparison of occupational respiratory PGM exposure results from different studies in future. The urinary platinum excretion of South African precious metals refinery workers reported in this study is comparable to that of other studies conducted in precious metals refineries in the United Kingdom, Europe and the United States of America. The urinary platinum excretion of workers showed low variability and spot urine tests can, therefore, be used to evaluate the platinum body burden of precious metals refinery workers. South African precious metals refinery workers are exposed to soluble platinum via the dermal and respiratory exposure routes and both these routes are positively correlated with the platinum body burden, as determined by urinary platinum excretion. The dermal and respiratory exposure routes should therefore be considered when investigating occupational exposure to platinum. Disposable coveralls and strict usage procedures are effective in reducing the dermal exposure of workers to soluble platinum. Finally, 19 recommendations are made to the specific precious metals refineries included in this study as well as precious metals refineries in general to reduce dermal and respiratory exposure to soluble platinum. Some limitations experienced during the study are also identified along with recommendations for future studies.