Indoor/outdoor PM4 (respirable dust) and respirable crystalline silica source tracking in households located in close proximity to gold mine tailing dumps

Abstract

Background: Particulate matter (PM) is a major contributor to air pollution in indoor and outdoor environmental spaces. Exposure to respirable dust (PM4) and respirable crystalline silica (RCS) indoor and outdoor in communities located in close proximity to gold mine tailings dumps in South Africa has not yet been determined. Aim: The aim of this study was to investigate the concentration of RCS and PM4 mass in samples measured indoor and outdoor of the nine (9) selected households located in close proximity to a gold mine tailings dumps. Methodology: Sampling locations were separated according to grids, based on the distance from the mine tailings dumps. Three different grids were determined as follows: A (<500m from the dump), B (>500m<1km) and C (1km – 3 km). Three households were selected from each grid zone to measure indoor and outdoor PM4 samples continuously over a 24-hour period using GilAir constant sampling pumps calibrated at the flowrate of 2.2 L/min in both the dry and wet seasons. PM4 samples were collected on a 37mm polyvinyl chloride (PVC) filter with a pore size of 0.8, which was assembled on the Higgin Dewell cyclones fitted with a filter pad of the same pore size. PM4 sample filters were gravimetrically weighed before and after sampling to determine the mass concentration of PM4. The respirable crystalline silica in PM4 samples were analysed by an X-ray diffraction method by South African National Accreditation System (SANAS) accredited laboratory of the National Institute for Occupational Health (NIOH). Samples were collected during the dry and wet seasons in the Riverlea community, Johannesburg. Results: During the wet and dry seasons, the mean indoor and outdoor PM4 mass concentration ranged from 0.02±0.01 µg/m3 to 2.26±0.02 µg/m3, respectively. The dry season mean PM4 mass concentrations were higher than the wet season PM4 mass concentrations in all zones. The pairwise comparison of PM4 mass concentration for dry and wet season revealed no statistically significance difference (p<0.05) at 95% confidence interval. Results presented in Figure 5 depicts the mean indoor PM4 mass concentration distribution for the dry season. The zone with the highest mean indoor PM4 mass concentration was zone A, followed by zone B. Since the mean outdoor PM4 concentration in zone C was the lowest, this suggests that the mine tailings dumps were the primary source of PM. The dry season mean indoor/outdoor ratio was greater than one across all zones; indicating that indoor activities were the primary source of PM. In both seasons, the mean indoor and outdoor percentages of crystalline silica ranged from 0.08±0.01% to 0.08±0.01%. The mean indoor and outdoor 24hr RCS concentrations in both seasons were below the California Office of Environmental Health Hazard Assessment (OEHHA) defined 24hr ambient exposure threshold of 3µg/m3. Recommendations: The results of this study suggest that nearby mine tailings dumps may be the primary source of PM in the indoor and outdoor environments; however the strength of this source in comparison to other sources remains unknown. Therefore, it is recommended that further studies focusing on source apportionment be carried out to determine the relative contribution of the mine tailings dust to the overall PM load in the environment. Although the difference was not statistically significant, indoor and outdoor PM4 concentrations were greater in Zones A&B, with the lowest PM4 concentrations in Zone C. The I/O ratio indicated that there was contribution of PM from outdoor. It is also recommended that further studies be conducted, with focus on monitoring PM4 over a 30 days period, to determine the level of free crystalline silica that may be present in PM4 mass concentrations. Conclusion: In the South African context, studies that focus on the investigation of indoor and outdoor PM4 concentrations in households located in close proximity to gold mine tailings are limited. The findings of this study can be used to provide valuable information on the indoor and outdoor PM4 concentrations, which can be used in modelling exposure and conducting probabilistic health risk assessment. High dust levels are related with dry season weather conditions due to strong wind conditions. Therefore, the PM4 mass concentrations in all zones were higher during the dry season than during wet season. Since the mean outdoor PM4 concentration in zone C was the lowest, this suggests that the mine tailings dumps were the primary source of PM.