The Mine Safety and Health Administration (MSHA) analyzes respirable coal mine dust samples for quartz content using infrared spectrometry. Samples are low-temperature ashed in an oxygen plasma, suspended in isopropanol, and redeposited onto a vinyl/acrylic copolymer filter for analysis using a Fourier transform infrared spectrometer. The on-filter infrared method was developed by the United States Bureau of Mines and collaboratively tested by Stanford Research Institute (SRI) under contract to the National Institute for Occupational Safety and Health and the Bureau of Mines. The results of the collaborative study were published in 1983. Although much work has been performed since then to improve the precision of the method, details of those improvements have not always been published. Standard methods often do not discuss analytical theory and the preliminary steps necessary to achieve precise results. This paper gives a brief background of the changes that have been made in the procedures used for the analysis of respirable coal mine dust samples for quartz and discusses the current procedures used by MSHA to analyze such samples. Factors affecting the analysis such as optimization of the deposit size of the ashed sample, the importance of centering the sample in the infrared beam, baseline selection points, and peak measurement techniques, as well as the quality assurance procedure and the precision of the analysis, are discussed.
The American Conference of Governmental Industrial Hygienists (ACGIH) accepted a lower threshold limit value (TLV) for respirable crystalline silica (RCS) exposure of 25 μg/m3, half of the previous TLV. This change is problematic because the current standard sampling and measurement practices used by NIOSH, OSHA, and MSHA are not sensitive enough to allow an analyst to confidently determine samples acquired near the TLV. In response to this need for a more sensitive method to analyze respirable dust filter samples for crystalline silica, a modification of current NIOSH infrared spectrometric methods is being developed. The additional sensitivity is gained by performing the infrared absorbance measurements at 77 K where absorbance peaks are more intense by virtue of being narrower. A quick-change cryostat has been fabricated such that a sample can be introduced to the spectrometer and cooled to 77 K in 5 min, interrogated for 1 min, and removed and the cryostat readied for another sample in 2 min, for a turnaround time of 8 min per sample, which is brief compared to the time required to prepare and redeposit a sample. Therefore, samples can be acquired and interrogated with legacy samplers, filters, pumps, spectrometers, and sample preparation, the only modification being the addition of a cryostat to the spectrometer. Preliminary experiments demonstrate that the peak-to-background ratio of the quartz signature band near 800 cm-1 increases by approximately 50 % on cooling from room temperature to 77 K. The slopes of the calibration curve derived from standards interrogated at both room temperature and 77 K indicate that the low-temperature method is approximately 25 % more sensitive.
Air generated quartz samples have been used for many years in the American Industrial Hygiene Association (AIHA) Proficiency Analytical Testing (PAT) program. It has been observed by the Wisconsin Occupational Health Laboratory (WOHL) that there is material adhering to the cassette interior of several PAT samples. WOHL has collected data on this problem for a number of AIHA PAT rounds and found that the cassette tops can contain a significant amount of quartz. Using data collected from PAT Rounds 146–155, it was found that half of the samples contained quartz on the cassette interior above the WOHL reporting level (RL) of 10 µg. The quartz on the cassette interior is one possible explanation for some of the performance issues found in laboratories participating in the AIHA PAT proficiency program. This problem is not unique to PAT samples. WOHL has noticed the same issue with field samples, which can lead to lower levels of quartz being reported.
Restoration stone work regularly involves work with high-silica-content materials (e.g., sandstone), but low-silica-content materials (<2 % quartz) such as limestone and lime mortar are also used. A combination of short sample duration and low silica content makes the quantification of worker exposure to respirable crystalline silica (RCS) difficult. This problem will be further compounded by the introduction of lower occupational exposure standards for RCS. The objective of this work was to determine whether higher-flow samplers might be an effective tool in characterizing lower RCS concentrations. A short study was performed to evaluate the performance of three high-flow samplers (FSP10, CIP10-R, and GK2.69) using side-by-side sampling with low-flow samplers (SIMPEDS and 10-mm nylon cyclones) for RCS exposure measurement at a restoration stonemasonry field site. A total of 19 side-by-side sample replicates for each high-flow and low-flow sampler pair were collected from work tasks involving limestone and sandstone. RESULTS. Most of the RCS (quartz) masses collected with the high-flow-rate samplers were above the limit of detection (62 % to 84 %) relative to the low-flow-rate samplers (58 % to 78 %). The average of the respirable mass concentration ratios for CIP10-R/SIMPEDS, GK2.69/10-mm nylon, FSP10/SIMPEDS, and FSP10/10-mm nylon pairs and the range of the quartz concentration ratios for the CIP10-R/SIMPEDS, CIP10-R/10-mm nylon, GK2.69/10-mm nylon, FSP10/SIMPEDS, and FSP10/10-mm nylon pairs included unity with an average close to unity, indicating no likely difference between the reported values for each sampler. Workers reported problems related to the weight of the sampling pumps for the high-flow-rate samplers. Respirable mass concentration data suggest that the high-flow-rate samplers evaluated would be appropriate for sampling respirable dust concentrations during restoration stone work. Results from the comparison of average quartz concentration ratios between high- and low-flow samplers suggest that the higher mass collected by the high-flow-rate samplers did not interfere with the quartz measurement. A significant portion of the data collected with the high-flow-rate samplers (>82 %) were greater than the limit of detection, which indicates that these samplers are suitable for quantifying exposures, even with low-quartz materials.
Respirable crystalline silica (RCS) concentrations are related to the crystalline silica content of the rock being worked, which means stone workers working with high-silica-content materials can be exposed to excessive levels of respirable dust containing crystalline silica. Little information exists on the RCS exposure concentrations, work practices, and worker knowledge of the hazards associated with RCS exposure among stone workers in Ireland. The objective of this study is to collect information on health and safety practices and worker knowledge of the health risks associated with RCS among stone workers using a questionnaire survey. To design and execute a pilot study to collect personal RCS exposure measurements among a group of restoration stone workers. A self-report quantitative questionnaire was designed based on previously published work and administered to a convenience sample of 130 stone workers engaged in various stonework trades throughout the Republic of Ireland. The questionnaire was designed to collect information on worker demographics, work practices, health and safety practices, knowledge of the risks associated with RCS, and diagnosed respiratory illnesses. Personal exposure measurements of respirable dust were collected using direct reading and gravimetric sampling methods from a group of 14 restoration stone workers working at historic sites throughout Ireland. Respirable dust samples were further analysed for RCS. A questionnaire response rate of 48 % (n = 63) was achieved. Sixty-six percent of respondents reported regularly using power tools with high-silica-containing materials. The most frequently employed control measure used was respiratory protective equipment (RPE) (85 %), followed by general ventilation (60 %), and more than half of the respondents (58 %) used sweeping as their primary method of cleaning up their work area. Geometric mean 8-h, time-weighted average (TWA) concentrations of RCS ranged from 0.002 to 1.38 mg/m(3), higher concentrations were reported for work involving sandstone, with 78 % of exposure samples exceeding the Irish occupational exposure limit value (OELV) of 0.1 mg/m(3). A training program focusing on inhalation exposure risks associated with RCS and effective engineering controls is needed for this occupational group. The results from this pilot study will be used to design a larger study, involving more exposure measurements, to investigate determinants of RCS exposure within restoration stone masonry. This will aid in the design of a technical intervention for high-risk exposure tasks, involving work with power tools and high-silica-content materials.
Members of an international standards working group for silica measurement (ISO/TC146/SC2/WG7 Silica) collaborated to assess the differences between sample preparation approaches for the analysis of respirable crystalline silica (RCS) by X-ray diffraction (XRD). They also assessed the relative collection efficiencies of 13 respirable samplers. The evaluation involved nine laboratories from eight countries. Samplers were exposed to airborne concentrations of ultrafine and medium Arizona road dust (ARD) in a calm air chamber. Each participating laboratory analysed samples following their own method and the Health and Safety Laboratory (HSL) retained a third of the samples for verification. All methods and analytical approaches applied in this study obtained comparable results (most were within 12 %). An exception was a method used with the CIP10 R sampler, which reported lower values. Correcting for the crystallinity of the calibration quartz dust using a verified value tested against a certified reference material has one of the largest impacts on the comparability of results. When following good analytical practice, the main factors affecting the comparability of results for RCS are significant differences in sampler efficiencies. In particular, the conductive sampler from SKC obtained a higher concentration of respirable dust (1.3-1.4x) when compared with the average air concentration. The Dorr Oliver, SKC aluminium, CIP10 R, and IOM head (with polyurethane foam separator) samplers all reported lower respirable dust air concentrations than average with the ultrafine ARD. Their lower collection efficiency compared with other samplers is explainable from published sampler information. The Dorr Oliver sampler also had a tendency to collect a lower proportion of RCS in the respirable dust than others. The working group propose that more stringent particle size selection and mass collection criteria are used to improve consistency and crossutilisation of exposure data between countries.
Standard Reference Material (SRM®) series 2951 to 2958 (5 μg to 1000 μg) Respirable Alpha Quartz on Filter and SRM series 2961 to 2967 (5 μg to 1000 μg) Respirable Cristobalite on Filter were prepared gravimetrically by depositing SRM 1878a Respirable Alpha Quartz and SRM 1879a Respirable Cristobalite on filters, respectively. These new SRMs are developed to assure the quality of respirable crystalline silica measurements using x-ray diffraction and infrared spectrometry around the regulatory limits that are enforced by the Occupational Safety and Health Administration (OSHA). The mass of silica determined by highly sensitive, but non-polymorph-specific, inductively coupled plasma optical emission spectrometry (ICP-OES) was successfully used to calculate the mass of polymorphspecific crystalline silica on filter after all sources of Si on the new SRMs were carefully analyzed. The certified values of alpha quartz in SRMs 2951 to 2958 and cristobalite in SRMs 2961 to 2967, respectively, were established from the gravimetric preparation values and the spectrometric measurement values.
SRI International has prepared dynamically generated silica samples since 1980 for the National Institute of Occupational Safety and Health (NIOSH) and the American Industrial Hygiene Association (AIHA) Proficiency Analytical Testing (PAT) programs. Aerosol-generated samples were developed in 1980 to more closely approximate real world samples and to improve intrabatch precision. Liquid-generated samples may provide tighter control limits, and this method has been reexamined as the generation procedure of choice. Sample preparation procedures have also been investigated to minimize analytical uncertainty and, hence, obtain a true evaluation of the sampling error. Samples were analyzed by SRI, NIOSH, and the Wisconsin Occupational Health Laboratory, using X-ray diffraction or Fourier transform infrared spectrometry (FTIR). Results were plotted and statistically evaluated, then compared to the existing PAT interlaboratory database.
An increasing number of cases of silicosis have been diagnosed in Spain in the last few years, reversing the trend of previous decades. Most cases have been found in marble masons because of the exponential growth in the use of quartz agglomerates as substitutes of marble. Quartz agglomerates are synthetic materials containing mainly quartz, but also other silica polymorphs (cristobalite and tridymite), which are considered more pernicious than quartz. The presence of different polymorphs of silica implies the need for x-ray diffraction (XRD) analysis for these to be distinguished, not only in bulk materials but also in respirable fractions from workplace atmospheres, as different threshold-limit values (TLVs) are established by Spanish legislation. For analysis of manufactured products, the Rietveld method allows the quantification of the mineral phases present in bulk materials from good-quality data obtained by XRD without the use of standards. Although it is known that high silica content in a bulk material does not necessarily mean high silica content in its respirable fraction, the aim of this work was to estimate the potential risk of quartz agglomerates by analyzing bulk samples and to evaluate the real risk for workers handling these materials by measuring their exposure to respirable crystalline silica. The Spanish National Silicosis Institute has analyzed in its laboratory a great amount of quartz agglomerate samples, corresponding both to workplace atmospheres and bulk material of manufactured products. A summary of the results obtained is presented here.
There is an association between exposure to silica dust and the development of silicosis. Studies have shown that the weight of silica dust retained in the lung is related to the presence and severity of silicosis. To study the dust retained in the lung a method of isolating it is required. Some studies suggest that lung tissue digestion methods, using acids, alkalis, or heat, alter the composition of silica dust retained in the lung. This study aimed to investigate whether methods used to digest lung tissue and isolate the retained dust are suitable for midsagittal sections of lung from deceased South African gold miners with silicosis. Ten methods were identified to digest lung tissue in the literature. Seven of these methods were carried out on midsagittal lung sections from cases with either moderate or marked sili- cosis. Of these, four methods digested a substantial portion of the lung
There is an international debate about the possibility of lowering the current occupational exposure limits (OELs) for respirable crystalline silica (RCS). Part of the discussion centers on knowing whether the OEL adequately protects workers. Exposure sampling using low-flow-rate pumps forms the basis of epidemiological studies, and thus the Technical Department of the National Silicosis Institute has carried out a study into the precision of workplace measurements with personal respirable dust samplers. The study consisted of taking two samples (one from either side of the worker's breathing zone) during different types of activities (quarries, marble workshops, slate workshops) and in semi-controlled conditions (dust tunnel). A total of 366 sample pairs were obtained, of which 186 were acquired in the dust tunnel and 180 came from the workplace. The statistical analysis of the difference between the pairs of results, expressed as the percentage error, showed the following results for RCS in workplace measurements: the median was 27.4 % (range: 0 % to 181 %), the 75th percentile was 51.7 %, and the 25th percentile was 12.7 %. The workplace measurements were divided between two industry sectors (marble and slate workshops), which were examined separately. Results obtained in a dust tunnel showed that for RCS, the percentage error of the difference between the pairs of results had a median of 23.9 % (range: 0 % to 90 %) with a 75th percentile at 40.1 % and a 25th percentile at 11.1 %. The results from the dust tunnel were not found to be statistically significantly different from the field results. Several important conclusions were obtained: firstly, a single sample is not enough to adequately evaluate exposure in order to avoid errors stemming from differences in "valid" measurements as shown in the statistical analysis, and secondly, the main source of error is the variability of the dust concentration during real measurements.
Since the International Agency for Research on Cancer (IARC) has classified crystalline silica (quartz and cristobalite) dust as a group 1 carcinogen in 2009, new studies and reviews on respirable crystalline silica (RCS) have been published. Evidence has been obtained in scientific literature that RCS toxicity is variable; in particular, the carcinogenic potency does not express in all industrial circumstances and in any case is secondary to fibrosis. In light of the implementation of the Globally Harmonized System (GHS) in Europe, the European Industrial Minerals Association (IMA-Europe) submitted a group notification for RCS with the classification Specific Target Organ Toxicity Repeated Exposure Category 1 (STOT RE 1) for silicosis. Consequently, generic cut-off values for hazard classification and labelling apply for products containing RCS in amounts >1 % (STOT RE 2) and >10 % (STOT RE 1), respectively. This triggers the need for a method to quantify the amount of RCS in a bulk material for classification and labelling purposes. The Metrology Working group of IMA-Europe has therefore developed the so-called SWeRF method (size-weighted relevant fine fraction), which is currently being dealt with in the European Committee for Standardization (CEN). In light of the possible setting of an EU limit value for respirable crystalline silica, the latest toxicological and epidemiological findings on exposure to RCS, as well as socio-economic assessments, provide important new information.
Exposure to crystalline silica in mining can lead to silicosis, a potentially fatal lung disease, and it may be contributing to the increase of coal workers' pneumoconiosis (CWP) seen in Appalachian miners. Exposure to silica in mines is controlled indirectly by reducing the respirable dust exposure limit through a formula that employs the % of silica in the dust. To reduce this exposure, control technologies and specific monitoring techniques need to be developed and implemented and the knowledge of the % of silica in mine dusts can help this process. This manuscript analyzes the % of silica in dust samples for the U.S. mining industry collected from 1997 to 2011. In the metal/nonmetal (M/NM) industry, metal and sand and gravel mines showed the highest silica % (8.2 %, 9.8 %) along with the highest variability. The silica % was found to be lower for samples collected in underground by comparison to surface and mill. In the coal industry, the samples collected in surface locations showed high silica % in the dust. For both the coal and M/NM industries, the % of silica and the respirable dust concentration were inversely related-i.e., the lower the dust concentration, the higher and more variable silica percentages were observed. The respirable dust limit formula suggests the first explanation: a mine with a high silica % in the dust is required to keep the dust concentration low under the reduced standard. Additional explanations are also proposed: the variability of the % of silica in the dust, the selective efficiency of control technologies, and different transport properties for dust with variable silica content. The findings improve the understanding of exposure to silica in mining environments and the data presented will be helpful in developing monitoring strategies for the measurement of silica and for the design of control technologies.
India is endowed with huge reserves of various minerals (as many as 89) and, therefore, the mining sector is very large employing millions of workers who are regularly exposed, in various degrees, to quartz dust in workplaces. Although the mining sector is quite old and well established, the reported data related to dust and its quartz content are few and far between. The work presented herein is a study of the quartz content in respirable airborne dust generated in coal and metal (zinc and manganese) mines, with a view to evaluating the health risk of miners as per mines regulations. The direct-on-filter method using an Fourier transform infrared (FTIR) spectrometer has been adopted for the determination of quartz. Personal air samplers were used to collect dust from different locations in mines on GLA-5000 PVC membrane filters. The air samplers were either attached with different workers engaged in a shift or placed in a position near to the dust-generation source in the mines to collect dust for direct-on-filter analysis. The proportion of quartz in the dust was determined from an estimation of the intensity of the doublet at 800cm(-1) calibrated against that of on-filter standard quartz dust. It has been found that the percentage of quartz in the dust in coal mines, especially in coking coal mines situated in the Jharia coalfield, is less than 1 % in almost all the workings, barring a few cases where it has exceeded this value. In contrast, the proportion of quartz in dust in metal mines exceeds 5 % in many workings. It has been found that wet drilling and good ventilation systems help to control dust problems at some locations, whereas rotation of workers may be needed in some places where it is difficult to suppress dust to a safe level.
Crystalline silica collected on membrane filters is commonly analyzed by X-ray diffraction (XRD). If the dust layer on the filter is very thin, the linearity of X-ray response is not affected. As loading increases, the X-ray beam is attenuated and the linear relationship no longer holds. The basic method for correction is to employ a silver filter to assess and compensate for the reduced X-ray response. No comprehensive study has yet been published that defines the dust loading beyond which the linear relationship no longer holds. As a consequence, a variety of dust loading limits are currently taken for linearity, ranging from 0.2 up to 2 mg. Members of the ISO working group for silica measurement collaborated in an experimental study on absorption for defining the critical sample mass, m(cr), above which the measured intensity of a reflection of the analyte deviates from the theoretically correct intensity by more than a set threshold. Quartz, calcite, hematite in the respirable size range, as pure minerals and mixtures, were used to test both the indirect and the direct-on-filter XRD methods. Analyses were carried out in three laboratories using CuK alpha radiation. The critical sample mass for a weight fraction of quartz, f(Qtz), depends on the area of deposition on the filter, A, and the mass absorption coefficient of the sample, mu(sample). For a typical f(Qtz) = 5% value, if a 10 % deviation is accepted to represent the limit of linearity of XRD response, the critical sample mass derived from theoretical calculations is given by the equation: m(cr10) = 35.711 . A . mu(-1.065)(sample), where m(cr10) is given in mg, A in cm(2), and mu(sample) in cm(2)/g. Equations were also found for deviation values of 15 % and 20 %. A standard "average" sample of dust was defined, characterized by f(Qtz) = 5% and mu(sample) = 100cm(2)/g, allowing the setting of a critical sample mass valid for the general case.
