In vitro toxicity assessment of dust emissions from six South African gold mine tailings sites

Abstract

Decades of mining in South Africa have produced waste material, which was deposited in numerous gold mine tailings storage facilities (TSFs). These TSFs have contributed to air pollution due to the lack of proper rehabilitation measures needed to mitigate erosion. Indeed, human populations residing in close proximity to these TSFs have raised concern about high dust levels and the onset of respiratory-related symptoms possibly caused by tailings emissions. Currently, South Africa does not have an efficient routine air monitoring system in residential areas of surrounding communities to determine the extent of air pollution. Moreover, it is not as yet known whether these tailings dusts are toxic and could be the cause of such ill effects. In addition, the physicochemical properties that may govern their toxicity have not as yet been identified. The present study has therefore conducted a risk assessment to:  Determine the toxicity of the dust emitted from five TSFs situated in the Gauteng and North West Provinces of South Africa (Hazard Identification): Bulk dust samples were collected from the TSFs and sieved to enrich the airborne particle fraction. Thereafter, physicochemical analysis on these particles were conducted i.e. size distribution analysis, analysis of specific surface area, shape and surface elemental composition, mineral composition, total elemental composition and surface activity. In addition, the potential toxicity of the particles was assessed in vitro using two human lung-derived cell lines i.e. BEAS-2B epithelial and U937 monocytic-macrophage cells. Toxicity assessment included label-free toxicity analysis as well as assessment of the degree of cellular internalisation.  Assess the levels of PM10 and PM4 dust as well as crystalline silica levels in environmental and personal filter samples (Exposure Assessment): Continuous real-time ambient PM10 at residential areas close to three TSFs were recorded and ambient PM10 were also collected on filters for silica content analysis. Furthermore, PM4 was collected from personal samplers attached to school children at schools located at various distances from all five TSFs for silica content analysis i.e. respirable crystalline silica (RCS).  Calculate the potential risk for the surrounding communities (Risk Characterisation): Cancer and non-cancer endpoints were used to calculate the potential risk of surrounding communities from lifetime exposure to inhalation of RCS. The results from Hazard Identification show that all tailings dusts exhibited in vitro toxicity, more so in the BEAS-2B cell line compared to the U937 cell line. This toxicity could have been governed either by their elemental composition or a combination of other physicochemical properties. The results from Exposure Assessment showed that the ambient PM10 levels in surrounding populations exceeded the current South African limit of 75 μg/m3 by several fold (i.e. three to four times). Furthermore, the RCS levels collected from PM4 from surrounding communities exceeded the current international interim RCS limit of 3 μg/m3. Finally, Risk Characterisation revealed surrounding communities to possibly develop non-cancer adverse health effects as hazard quotients as high as higher than 1 was calculated. Moreover, calculations of cancer adverse health effects also showed possible risk of regulatory concern as more than 1 individual per 1000 could potentially develop cancer when exposed to RCS levels recorded in this study. One site in particular showed an HQ of 11.6 (non-cancer) and the possibility of more than 2 individuals per 1000 to possibly develop cancer. This study could therefore provide mechanistic evidence to support future epidemiological studies attempting to link tailings dust exposure to adverse health effects. In addition, this study could serve as a starting point for future ambient and personal sampling campaigns and suggests that a RCS interim exposure limit, particularly for South Africa, be established.