Association between respiratory exposure and urinary excretion of both soluble platinum and rhodium at a precious metals refinery

Abstract

Background: In 2025, South Africa supplied more than 56% of the total worldwide platinum (Pt) and rhodium (Rh). During the refining of Pt and Rh, soluble Pt and Rh salts are formed as by-products of the extraction process. Metallic Pt and Rh have low toxicity; however, soluble Pt and Rh salts are especially sensitising. Soluble Rh is only a dermal sensitiser, whereas soluble Pt is both a dermal and a respiratory sensitiser. The primary route of exposure to soluble Pt and Rh is inhalation, with less exposure through the dermal and ingestion routes. The excretion of Pt and Rh via the urine has been reported to occur in refinery workers following exposure via various routes. Aims and objectives:This retrospective study aimed to evaluate the association between respiratory exposure and urinary excretion of workers exposed to soluble Pt and Rh at a precious metals refinery (PMR) in South Africa and to determine the optimal time for performing urinary biological monitoring. This was done by using retrospective data received from a PMR. The objectives of the study were to correlate workers' respiratory exposure to soluble Pt/Rh in the PMR to the concentration of urinary Pt/Rh measured in urine collected before and after their shifts. Additionally, to describe how workers' urinary Pt and Rh excretion fluctuated over the sampling period and to determine when the optimal time would be to collect urine samples for Pt or Rh exposure monitoring. Method: The retrospective data received from the PMR was collected in 2024 and included data for respiratory exposure to soluble Pt and Rh, urinary Pt and Rh concentrations, as well as creatinine concentrations for six refinery workers. Respiratory exposure was monitored during five consecutive shifts (Monday to Friday) and urine samples were collected pre- and post-shift during five consecutive shifts (Monday to Friday) with an additional pre-shift sample collected on Saturday morning. Three workers were anticipated to be primarily exposed to soluble Pt, and the other three were anticipated to be primarily exposed to soluble Rh. The respiratory exposure was monitored for both soluble Pt and Rh, and the urine samples were analysed for Pt, Rh, and creatinine for all six participants. All the data for the project were analysed and graphically represented using GraphPad Prism 10 (GraphPad® software). β-substitution was used to substitute urine samples and respiratory results with results below the detection limit. The D'Agostino & Pearson test was used to test the distribution of the data. Pearson correlations were used to investigate the relationship between respiratory exposure and urinary Pt and Rh excretion, as well as the relationship between creatinine concentration in urine and urinary Pt and Rh concentrations. Paired t-tests were performed on the data to compare pre- and post-shift urinary Pt and Rh excretion results. Results: The exposure to soluble Pt ranged between 0.015 and 99.224 µg/m3, and Rh exposure ranged between below detection limit (BDL) and 6.782 µg/m3 via inhalation. However, only the participants who had relatively high respiratory exposure when compared to other participants who had lower respiratory Pt or Rh exposure, or performed high-exposure activities, had detectable levels of urinary Pt (range: BDL to 7.116 µg/g creatinine) or Rh (range: BDL to 4.995µg/g creatinine). Three participants had detectable levels of urinary Pt, and two participants had detectable levels of urinary Rh. There was no significant correlation between respiratory exposure and excretion of the metal on an individual level; however, there was a significant relationship for the results on a group level, including all six participants, for Pt (r = 0.487; p = 0.006) and for Rh(r = 0.691; p < 0.0001). Additionally, there was no significant difference between pre- and post-shift urinary metal excretion and no clear indication if pre- or post-shift samples best represented exposure via inhalation. Discussion and conclusions: Even though the method used to analyse urinary Pt and Rh had a high limit of detection (LOD) compared to other studies in literature, participants with higher Pt or Rh body burden could be identified. Three participants had detectable levels of urinary Pt, and two participants had detectable levels of urinary Rh. The urinary Pt and Rh excretion of workers could be linked to their respiratory exposure, work activities and years of employment in the specific area (accumulation in body burden of Pt and Rh). One participant anticipated to be mainly exposed to soluble Rh, was one of the three participants with detectable levels of urinary Pt excretion; however, none of the participants mainly exposed to Pt had detectable levels of urinary Rh excretion. Although there was a significant positive correlation for all six participants between both soluble Pt and Rh exposure and both pre- and post-shift Pt and Rh concentrations for the group-level analysis, the individual-level comparisons did not show a significant correlation on the day-to-day results. Therefore, individually, there was no significant relationship between respiratory exposure and urinary excretion of Pt and Rh. Furthermore, there was no consistent pattern of excretion for Pt or Rh and therefore, the specific days of peak exposure or the optimal time of sampling could not be determined using the results for urinary Pt or Rh. Recommendations: This analytical method used to analyse urinary Pt and Rh excretion is not a commercial method used by a pathology laboratory, but a method reported in literature by Crespo-Alonso et al. (2015) ("A simple, sensitive analytical method for platinum trace determination in human urine") using ICP-MS. This method required dilution of urine samples which resulted in the high LOD for Pt and Rh. Therefore, the method for analysing urinary Pt and Rh needs to be improved for possible future use by the refinery, or another laboratory reporting lower LOD should be used. The use of a personal direct reading optical particle counter could assist future studies in the identification of the exact time of peak respiratory exposure, which could be used to explain the variation in pre-and post-shift urinary excretion concentrations. The optimal time to conduct biological monitoring of refinery workers for Pt and Rh could be more accurately identified if the exact time of peak exposure can be identified.