Assessment and control of respirable crystalline silica in quarries and dimension stone mines

Abstract

The health risk of breathing very fine particles of respirable crystalline silica (RCS) dust, resulting in poor lung health is not well defined in Australia, even though many hundreds of thousands of workers are exposed to uncontrolled dusty activities daily. It is now clear from international studies that lung health of workers continues to be affected at relatively low exposures to RCS, even at occupational exposure limits (OEL) including the current Safe Work Australia Exposure Standard (SWA-ES). This has prompted the United States to reduce their permissible exposure limit (PEL) to one half of the Australian SWA-ES, to 0.05 mg/m3, and challenges the adequacy of the current Australian SWA-ES, which is 0.1 mg/m3. In terms of numbers of workers affected, the mining industry employs approximately 267,000 workers, which is 2.3 percent of the total workforce, and accounts for about 10.2 % of Australia's Gross Domestic Product (GDP). Approximately 55,000 workers are employed in Queensland mining, many of whom work in quarries and dimension stone mines where exposure to RCS is known to be elevated. A recent assessment estimated that about 440,000 deaths from cancer attributable to RCS exposure will occur throughout Europe from 2010 to 2069. Unless exposure to RCS is well controlled, many hundreds of thousands of lives will also be cut short from silicosis and chronic obstructive pulmonary disease (COPD). As the weight of evidence grows in Australia, the disease risks attributable to RCS will increasingly become more obvious. This study evaluated the health risk from exposure to RCS for 47 quarry and dimension stone mine workers throughout Queensland, Australia. Personal exposures to RCS were measured across a range of exposures, and lung function testing was carried out in parallel. Findings revealed that about one in four workers were exposed to RCS above the SWA-ES, and more than one in ten were being exposed at a concentration of more than twice this limit. A major finding for those workers exposed to RCS at the SWA-ES was loss of lung function greater than 20%. The increased loss of lung function was positively correlated with jobs associated with increased RCS exposure. When similar exposure groups were combined into three RCS exposure ranges categorised as high (≥ 0.09 - ≤ 0.20 mg/m3), medium (≥ 0.04 - ≤ 0.08 mg/m3) and low (< 0.04 mg/m3), analysis of variance (ANOVA) confirmed that the loss of lung function below the lower limit of normal (LLN) at the current SWA-ES, is significant (p < 0.05). Abnormal lung function patterns were also more pronounced for smokers who were exposed to RCS ≥ 0.04 mg/m3 and not as obvious for smokers exposed to RCS < 0.04 mg/m3. This demonstrated that both smoking and RCS had a combined impact resulting in poor lung health. In this study, vehicles fitted with standard heating, ventilation and air-conditioning systems (HVAC) were tested for ingress of respirable dust into the operator's cabin, and compared with more recent technology. Evaluation of the effectiveness of newer technology, a RESPA® pre-cleaner, filter and pressurization (PFP) unit, demonstrated up to a four-fold reduction in RCS entering the cabin, when compared with standard air-conditioning systems. Electron microscopy (EM) was used to describe the physical characteristics of respirable silica and dust particles collected on respirable sample filters previously analysed for silica by infrared spectroscopy. Data revealed that silica particles were generally less than 5 I¼m in physical diameter and many particles were elongated. These smaller particles are known to be most hazardous to lung health. Findings also demonstrated that larger length elongated particles were collected by the cyclone sampler, which influenced the particle size distribution curve. There was a good fit between the physical cumulative silica particle size distribution, representing particle counts for selected workplace samples, when plotted against the theoretical AS2985-2009 (density dependent) Equivalent Aerodynamic Diameter (EAD) sampling efficiency curve. This means that density is not the only factor for particle capture in the cyclone sampler. These silica particles do not behave as perfect spheres, which is the premise underpinning the International Organization for Standardization (ISO) 7708-1995, sampling efficiency curve and AS2985-2009, adopted by the Australian Standard for sampling respirable dust. The science for these standards is based on information that is at least twenty years old, and results from the current study confirmed that particle shape must be considered in the sampling efficiency curve and lung health risk assessment. An unexpected finding from analysis of RCS dust by EM was the identification of fibrous mineral particles in several samples, with both morphology and elemental composition similar to erionite. Erionite is known to cause mesothelioma, which is typically associated with asbestos when inhaled at high enough concentrations. Further investigation and characterization of respirable dusts at mining sites where erionite is a potential contaminant is recommended. Confirmation of the presence and extent of erionite, and further characterization of exposure will assist in determining the extent of health risk to quarry and dimension stone mine workers in Queensland and elsewhere. Overall, the findings from this study challenge the adequacy of RCS health risk assessment standards in Australia. In addition, typical operator cabin air-conditioning technology will not reduce exposure to RCS where silica is present in dusty workplaces. The study also demonstrates the importance of health surveillance, to identify gaps, raise awareness about primary prevention, and drive timely intervention.

Date of Award	2016
Original language	English