Regulatory implications of airborne respirable free silica variability in underground coal mines.

The respirable dust standard for respirable free crystalline silica in underground coal mines is expressed as milligrams per cubic meter (mg/m3) of respirable dust and is determined by the silica content of the dust. The Mine Safety and Health Administration (MSHA) regulates silica exposure by determining and enforcing compliance with the respirable dust standard for each active mine section. The MSHA strategy for regulation is examined in the context of respirable free crystalline silica and dust data. Deficiencies of the strategy include the same enforcement efforts regardless of compliance history, inappropriate treatment of data, and emphasis on short-term variability of silica content. These deficiencies result in inadequate enforcement in chronically dusty mines, "game playing" with optional samples, and an overall approach that does not focus on the long-term impact of silica exposure on lung health. Alternative approaches include enforcement efforts proportional to compliance history, use of a moving average silica content, and more statistically sound approaches to data interpretation.     

Respiratory effects of exposure to low levels of concrete dust containing crystalline silica

BACKGROUND: Dusts containing crystalline silica are generated in mining, construction, glass, granite and concrete production industries. The association between exposure to low levels of concrete dust containing crystalline silica and reduction in lung function, was evaluated in a cross-sectional study. METHODS: The study was carried out among 144 concrete workers, from two factories, with exposure assessment of respirable dust and silica by personal samplers. Results of respiratory questionnaires and standardized measurements of lung function were compared with the results in a control population. Multiple linear regression analysis was used in selecting factors that predict (age and standing height standardized residual) lung function. RESULTS: The average concentration of respirable dust in both factories was 0.8 mg/m(3) and 0.06 mg/m(3) for respirable silica. The average silica content of the dust was 9%. The average cumulative dust exposure was 7.0 mg/m(3) year and cumulative silica exposure was 0.6 mg/m(3) year. Significant associations between exposure to concrete dust and a small lung function (FEV(1)/FVC ratio, MMEF) loss were found, independent of smoking habits and of a history of allergy. CONCLUSIONS: Our results indicate that, concrete workers with chronic obstructive pulmonary symptoms and/or work-related lower respiratory symptoms are at risk of having a reduction in lung function (FEV&(1)/FVC ratio) outside the 5th percentile of the external reference population, and therefore, of mild chronic obstructive pulmonary disease, at respirable concrete dust levels below 1 mg/m(3) with a respirable crystalline silica content of 10% (TWA, 8 hr).     

Airborne crystalline silica concentrations at coal-fired power plants associated with coal fly ash

This study presents measurements of airborne concentrations of respirable crystalline silica in the breathing zone of workers who were anticipated to encounter coal fly ash. Six plants were studied; two were fired with lignite coal, and the remaining four plants used bituminous and subbituminous coals. A total of 108 personal breathing zone respirable dust air samples were collected. Bulk samples were also collected from each plant site and subjected to crystalline silica analysis. Airborne dust particle size analysis was measured where fly ash was routinely encountered. The results from bituminous and subbituminous fired plants revealed that the highest airborne fly ash concentrations are encountered during maintenance activities: 0.008 mg/m3 to 96 mg/m3 (mean of 1.8 mg/m3). This group exceeded the threshold limit values (TLV) in 60% of the air samples. During normal production activities, airborne concentrations of crystalline silica ranged from nondetectable to 0.18 mg/m3 (mean value of 0.048 mg/m3). Air samples collected during these activities exceeded the current and proposed TLVs in approximately 54% and 65% of samples, respectively. Limited amounts of crystalline silica were detected in samples collected from lignite-fired plants, and approximately 20% of these air samples exceeded the current TLV. Particle size analysis in areas where breathing zone air samples were collected revealed mass median diameters typically between 3 microm and 8 microm. Bulk and air samples were analyzed for all of the common crystalline silica polymorphs, and only alpha quartz was detected. As compared with air samples, bulk samples from the same work areas consistently yielded lower relative amounts of quartz. Controls to limit coal fly ash exposures are indicated during some normal plant operations and during episodes of short term, but high concentrations of dust that may be encountered during maintenance activities, especially in areas where ash accumulations are present.     

高浓度矽尘接触对大鼠氧化应激的作用研究

目的探讨大鼠高浓度矽尘接触过程中是否存在氧化应激反应。方法选40只SPF级Wistar雄性大鼠随机分为4组,即高剂量染尘组（1 000mg/m^3）、中剂量染尘组（500mg/m^3）、低剂量染尘组（100mg/m^3）和对照组,选用自然动式染尘装置每天染尘2h。染尘49d后处死大鼠,测定肺组织匀浆超氧化物歧化酶（SOD）、总抗氧化能力（T-AOC）活性及丙二醛（MDA）、还原性谷胱甘肽（GSH）含量。结果长时间、高浓度矽尘接触降低大鼠肺组织的SOD（30.25±0.49）U/ml、T-AOC活性（7.93±0.74）kU/L和GSH（2.34±0.96）g/L含量,同时MDA（5.65±0.13）nmol/ml水平升高,与对照组比较,差异有统计学意义（P〈0.01）。结论大鼠高浓度矽尘接触过程早期炎症反应发生可能与机体氧化应急有关。     

Comparison of hemolytic activities of coal fly ash and its soluble and insoluble fractions

Coal fly ash of a particle diameter smaller than 10 micron was collected from the precipitator of a power plant in Hong Kong. Comparison of hemolytic activities between fly ash and free silica showed that fly ash had a lower biological effect than free silica. The hemolytic activities of the soluble and insoluble fractions of fly ash were further compared by two methods: total hemoglobin method and cyanmethemoglobin method. An analysis of results showed significant differences for fly ash and its soluble fraction between methods (P less than 0.05 and P less than 0.01, respectively). Fly ash, which contained a silicate level similar to its insoluble fraction, had a hemolytic activity higher than the summation of both its soluble and insoluble fractions. This indicates that the hemolytic activity was independent of the silicate content in the fly ash samples.     

Mortality among workers in the diatomaceous earth industry.

A cohort mortality study was conducted among workers from two plants in the diatomaceous earth mining and processing industry in California. Diatomaceous earth consists of the skeletal remains of diatoms. Exposure to amorphous (non-crystalline) and crystalline silica in the form of quartz results from open pit mining and exposure to crystalline silica (principally cristobalite) occurs in the processing of the material. Lung cancer and non-malignant respiratory diseases have been the health outcomes of greatest concern. The main study cohort included 2570 white men (533 Hispanic and 2017 non-Hispanic workers) who were employed for at least 12 months cumulative service in the industry and who had worked for at least one day during the follow up period, 1942-87. Vital status was ascertained for 91% of the cohort and death certificate information was retrieved for 591 of 628 (94%) identified deaths. The all causes combined standardised mortality ratio (SMR) was slightly increased (SMR = 1.12; 628 observed) compared with rates among US white males. The principal contributors to this excess were increased risks from lung cancer (SMR = 1.43; 59 observed) and non-malignant respiratory disease (NMRD) excluding infectious diseases and pneumonia (SMR = 2.59; 56 observed). The excess of lung cancer persisted when local county rates were used for comparison (SMR = 1.59). Internal rate comparisons by Poisson regression analysis were conducted to assess potential dose-response relations for lung cancer and NMRDs. Mortality trends were examined in relation to duration of employment in dust exposed jobs and with respect to an index of cumulative exposure to crystalline silica. The crystalline silica index was a semiquantitative measure that combined information on duration of exposure, differences in exposure intensity between jobs and calendar periods, the crystalline content of the various product mixes, and the use of respiratory protection devices. Increasing gradients of risk were detected for lung cancer and NMRD with both exposure indices. The relative risk trends for lung cancer and NMRD with crystalline silica exposure lagged 15 years were respectively: 1.00, 1.19, 1.37, and 2.74, and 1.00, 1.13, 1.58, and 2.71. Based on a review of available but limited data on cigarette smoking in the cohort and from application of indirect methods for assessing confounding variables, it seems unlikely that smoking habits could account for all of the association between exposure to dust and lung cancer. The intense and poorly controlled dust exposures encountered before the 1950s were probably the most aetiologically significant contributors to risks from lung cancer and NMRDs. The absence of an excess of lung cancer among workers hired since 1960, and the finding of no deaths attributed to pneumoconiosis as an underlying cause of death among workers hired since 1950 indicate that exposure reductions in the industry during the past 40 years have been successful in reducing excess risks to workers. Further mortality follow up of the cohort and the analysis of radiographic data will be needed to determine conclusively the long term patterns of disease risks in this industry.     

黄麻粉尘致肺纤维化的实验研究

以黄麻粉尘（游离ＳｉＯ２含量分别为９．３２％、４、４％、０．９８％）８０ｍｇ经气管注入大白鼠肺内，于染尘后１、３、６、１２个月时处死动物，进行病理学观察及肺胶原蛋白含量测定。染尘早期，肺内为细胞性反应和肺间质肺炎样改变。染尘晚期，含游离ＳｉＯ２９．３２％的黄麻粉尘可引起细胞纤维性结节，含游离ＳｉＯ２４．４０％的黄麻粉尘可引起细支气管旁轻度纤维增生，含游离ＳｉＯ２０．９８％的黄麻粉尘仅见细胞性结节。     

Respirable concrete dust--silicosis hazard in the construction industry

Concrete is an extremely important part of the infrastructure of modern life and must be replaced as it ages. Many of the methods of removing, repairing, or altering existing concrete structures have the potential for producing vast quantities of respirable dust. Since crystalline silica in the form of quartz is a major component of concrete, airborne respirable quartz dust may be produced during construction work involving the disturbance of concrete, thereby producing a silicosis hazard for exposed workers. Silicosis is a debilitating and sometimes fatal lung disease resulting from breathing microscopic particles of crystalline silica. Between 1992 and 1998, the National Institute for Occupational Safety and Health (NIOSH) made visits to construction projects where concrete was being mechanically disturbed in order to obtain data concerning respirable crystalline silica dust exposures. The construction activities studied included: abrasive blasting, concrete pavement sawing and drilling, and asphalt/concrete milling. Air samples of respirable dust were obtained using 10-mm nylon cyclone pre-separators, 37-mm polyvinyl chloride (PVC) filters, and constant-flow pumps calibrated at 1.7 L/min. In addition, high-volume respirable dust samples were obtained on 37-mm PVC filters using 1/2" metal cyclones (Sensidyne model 18) and constant-flow pumps calibrated at 9.0 L/min. Air sample analysis included total weight gain by gravimetric analysis according to NIOSH Analytical Method 600 and respirable crystalline silica (quartz and cristobalite) using x-ray diffraction, as per NIOSH Analytical Method 7500. For abrasive blasting of concrete structures, the respirable crystalline silica (quartz) concentration ranged up to 14.0 mg/m3 for a 96-minute sample resulting in an eight-hour time-weighted average (TWA) of 2.8 mg/m3. For drilling concrete highway pavement the respirable quartz concentrations ranged up to 4.4 mg/m3 for a 358-minute sample, resulting in an eight-hour TWA of 3.3 mg/m3. For concrete wall grinding during new building construction the respirable quartz measurements ranged up to 0.66 mg/m3 for a 191-minute sample, resulting in an eight-hour TWA of 0.26 mg/m3. The air sampling results for concrete sawing ranged up to 14.0 mg/m3 for a 350-minute sample resulting in an eight-hour TWA of 10.0 mg/m3. During the milling of asphalt from concrete highway pavement, the sampling indicated a respirable quartz concentration ranging up to 0.34 mg/m3 for a 504-minute sample, resulting in an eight-hour TWA of 0.36 mg/m3. The results of this work indicate the potential for respirable quartz concentrations involving disturbance of concrete to range up to 280 times the NIOSH Recommended Exposure Limit (REL) of 0.05 mg/m3 assuming exposure for an eight- to ten-hour workday. Considering the aging of the concrete infrastructure in the United States, these results pose a challenge to all who have an interest in preventing silica exposures and the associated disease silicosis.     

二氧化硅对大鼠血浆中微量元素含量的影响

给大鼠经气管分别注入４、１６、６４ｍｇ石英粉尘生理盐水混悬液。在染尘后第７，１５和３０天解剖动物，测定血浆中Ｃｕ、Ｚｎ，Ｍｎ、Ｍｏ、Ｓｉ、Ｒｂ和Ｓｒ等７种微量元素的含量。实验结果表明，随着染尘剂量的增加，血浆中铜的含量明显增高，呈正相关，相关系数为０．９９９。而血浆锌的含量随着染尘剂量的增加而明显的降低，呈负相关，相关系数为－０．９９８。血浆中Ｍｏ、Ｍｎ、Ｓｉ、Ｒｂ和Ｓｒ含量与对照组比无明显差异。     

褐煤尘细胞毒性和致纤维化作用

为了评价褐煤尘的细胞毒性及致纤维化作用，对２个褐煤矿的煤尘成分、体外细胞毒性、体内致肺损伤作用进行了研究。甲矿煤尘游离ＳｉＯ２含量为３．４％，乙矿煤尘为１．５％。培养液中加入褐煤尘后肺巨噬细胞存活率明显降低，乳酸脱氢酶（ＬＤＨ）和酸性磷酸酶（ＡＣＰ）活力明显增高。乙矿煤尘组细胞存活率明显低于甲矿煤尘组，ＬＤＨ明显高于甲矿煤尘组。染褐煤尘大鼠肺体积、湿重、干重和胶原蛋白含量增加，病理检查可见到煤尘灶、肺气肿、网状纤维和胶原纤维轻度增生。结果表明：褐煤尘具有细胞毒性和轻度致纤维化作用，乙矿煤尘细胞毒性和致纤维化作用均高于甲矿煤尘，用煤尘中元素含量解释煤尘毒性与以往的研究结果不尽相同。     

Dust exposure during small-scale mining in Tanzania: a pilot study

Small-scale mining in developing countries is generally labour-intensive and carried out with low levels of mechanization. In the Mererani area in the northern part of Tanzania, there are about 15000 underground miners who are constantly subjected to a poor working environment. Gemstones are found at depths down to 500 m. The objectives of this pilot study were to monitor the exposure to dust during work processes, which are typical of small-scale mining in developing countries, and to make a rough estimation of whether there is a risk of chronic pulmonary diseases for the workers. Personal sampling of respirable dust (n = 15) and 'total' dust (n = 5) was carried out during three consecutive days in one mine, which had a total of 50 workers in two shifts. Sampling started immediately before the miners entered the shaft, and lasted until they reappeared at the mine entrance after 5-8 h. The median crystalline silica content and the combustible content of the respirable dust samples were 14.2 and 5.5%, respectively. When drilling, blasting and shovelling were carried out, the exposure measurements showed high median levels of respirable dust (15.5 mg/m(3)), respirable crystalline silica (2.4 mg/m(3)), respirable combustible dust (1.5 mg/m(3)) and 'total' dust (28.4 mg/m(3)). When only shovelling and loading of sacks took place, the median exposures to respirable dust and respirable crystalline silica were 4.3 and 1.1 mg/m(3). This study shows that the exposure to respirable crystalline silica was high during underground small-scale mining. In the absence of personal protective equipment, the miners in the Mererani area are presumably at a high risk of developing chronic silicosis.     

Personal exposure to dust, endotoxin and crystalline silica in California agriculture

AIMS: The aim of this study was to measure personal exposure to dust, endotoxin and crystalline silica during various agricultural operations in California over a period of one year. METHODS: Ten farms were randomly selected in Yolo and Solano counties and workers were invited to wear personal sampling equipment to measure inhalable and respirable dust levels during various operations. The samples were analysed for endotoxin using the Limulus Amebocyte Lysate assay and crystalline silica content using X-ray diffraction. In total 142 inhalable samples and 144 respirable samples were collected. RESULTS: The measurements showed considerable difference in exposure levels between various operations, in particular for the inhalable fraction of the dust and the endotoxin. Machine harvesting of tree crops (Geometric mean (GM) = 45.1 mg/m3) and vegetables (GM = 7.9 mg/m3), and cleaning of poultry houses (GM = 6.7 mg/m3) showed the highest inhalable dust levels. Cleaning of poultry houses also showed the highest inhalable endotoxin levels (GM = 1861 EU/m3). Respirable dust levels were generally low, except for machine harvesting of tree crops (GM = 2.8 mg/m3) and vegetables (GM = 0.9 mg/m3). Respirable endotoxin levels were also low. For the inhalable dust fraction, levels were reduced considerably when an enclosed cabin was present. The percentage of crystalline silica was overall higher in the respirable dust samples than the inhalable dust samples. CONCLUSIONS: Considerable differences exist in personal exposure levels to dust, endotoxin and crystalline silica during various agricultural operations in California agriculture with some operations showing very high levels.     

陶瓷企业接尘工人肺功能研究

目的 探讨陶瓷粉尘作业工人肺功能损伤的特征。 方法 采用美能(日本) AS507型肺功能仪,测定某陶瓷厂"无尘肺"的967名接尘工人(接尘组)和197名非接尘健康工人(对照组)的肺通气功能。 结果 陶瓷粉尘总尘浓度为0.10~2.95 mg/m3,总尘浓度超标率为14.29%,呼吸性粉尘浓度超标率为3.57%;粉尘中游离SiO₂含量为21.3%~43.0%。接尘组VC、FVC、FEV 1、FEV 1%、FEF 25%和FEF 50%的数值均低于对照组,差异有统计学意义(P<0.05或P<0.01)。接尘组肺通气功能损伤和小气道损伤的发生率均高于对照组,差异有统计学意义(P<0.05)。接尘的吸烟者VC、FEV1和FEV1%数值与非吸烟者相比,差异有统计学意义(P<0.01)。接尘工人肺通气功能损伤发生率为7.45%(72/967),其中轻度损伤者占98.61%(71/72),限制性肺通气功能损伤占97.22%(70/72)。 结论 陶瓷粉尘可引起接尘工人肺功能的损伤,其损伤程度以轻度为多,损伤类型以限制型损伤为主。肺功能损伤主要与粉尘、接尘工龄、工种和吸烟等因素有关。     

Dust and Collagen Content of Lungs of Coal-Workers with Progressive Massive Fibrosis

In order to test the silica theory of the origin of progressive massive fibrosis (P.M.F.) in coal-miners' pneumoconiosis, separate dust analyses have been made of the massive lesions and of the rest of the lung from 18 coal-workers with P.M.F. who had been employed in several coalfields. The dry weight of the massive lesions ranged from 5 to over 100 g. and the dust concentration in the P.M.F. lesions was on an average twice as high as in the rest of the lung. It was found that the quartz percentage of the lung dust was almost identical in the two samples from each lung (Table 3). The quartz content of the average lung dust (P.M.F. and “rest of lung”) of 32 cases of P.M.F. was compared with that of 58 cases of simple pneumoconiosis. The quartz content of the P.M.F. dust was slightly higher but, allowing for variable dust composition in different coalfields, the difference was not significant (Table 8). The silica theory of P.M.F. cannot be supported by this study.

Comparisons of the collagen content of the P.M.F. lesions and of the rest of the lung in 17 lungs and of lesions of P.M.F. and of simple pneumoconiosis in 31 and 27 lungs, respectively, showed no clear difference between any of the groups and surprisingly low values for the massive lesions. A possible way in which this result could be reconciled with the histopathological observations is suggested. One gram of coal-mine dust produced, on an average, 0·4 g. of dust produced between 2 and 7 g. of extra collagen.

extra collagen. In five silicotic lungs, where the lung dust contained 15 to 50% of free silica, 1 g. of dust produced between 2 and 7 g. of extra collagen.

     

Occupational Exposure to Crystalline Silica at Alberta Work Sites

Although crystalline silica has been recognized as a health hazard for many years, it is still encountered in many work environments. Numerous studies have revealed an association between exposure to respirable crystalline silica and the development of silicosis and other lung diseases including lung cancer. Alberta Jobs, Skills, Training and Labour conducted a project to evaluate exposure to crystalline silica at a total of 40 work sites across 13 industries. Total airborne respirable dust and respirable crystalline silica concentrations were quite variable, but there was a potential to exceed the Alberta Occupational Exposure Limit (OEL) of 0.025 mg/m³ for respirable crystalline silica at many of the work sites evaluated. The industries with the highest potentials for overexposure occurred in sand and mineral processing (GM 0.090 mg/m³), followed by new commercial building construction (GM 0.055 mg/m³), aggregate mining and crushing (GM 0.048 mg/m³), abrasive blasting (GM 0.027 mg/m³), and demolition (GM 0.027 mg/m³). For worker occupations, geometric mean exposure ranged from 0.105 mg/m³ (brick layer/mason/concrete cutting) to 0.008 mg/m³ (dispatcher/shipping, administration). Potential for GM exposure exceeding the OEL was identified in a number of occupations where it was not expected, such as electricians, carpenters and painters. These exposures were generally related to the specific task the worker was doing, or arose from incidental exposure from other activities at the work site. The results indicate that where there is a potential for activities producing airborne respirable crystalline silica, it is critical that the employer include all worker occupations at the work site in their hazard assessment. There appears to be a relationship between airborne total respirable dust concentration and total respirable dust concentrations, but further study is require to fully characterize this relationship. If this relationship holds true, it may provide a useful hazard assessment tool for employers by which the potential for exposure to airborne respirable silica at the work site can be more easily estimated.     

地质勘探工人历年接触矽尘的评价

目的对接触矽尘的地质勘探工人的接尘水平进行定量评价.方法收集9个省地质矿产勘查局所属队(厂、矿)50年代以来各工种历年的矽尘监测资料30000个,还收集历史的和近期的其他矽尘监测资料,以及1627个研究对象的工作史.利用上述资料按队(厂、矿)、工种和年段建立了矽尘接触水平,并用呼吸性矽尘量、全肺的矽尘量和总粉尘中矽尘含量估算矽尘的接触量.结果呼吸性粉尘接触浓度平均值是3mg/m3;总粉尘浓度平均值是14mg/m3,早年是29mg/m3,近年是3mg/m3;粉尘中游离二氧化硅含量平均值是(28.0±8.2)%.结合每个研究对象的工作史和矽尘接触水平分别计算出不同的矽尘接触指数.结论这种大规模矽尘接触评价方法对历年接触矽尘的估算可为剂量-反应关系评价提供依据.     

蔺草加工业劳动卫生学调查

目的 了解蔺草加工业粉尘对工人健康的影响程度及其原因,制订相应的预防措施。方法 对蔺草作业工人劳动卫生学调查。生产环境粉尘浓度监测和实验室检测相结合。结果 作业环境呼吸性粉尘（0.5-33.0)mg/m^3;总粉尘在（3.5-314.8)mg/m^3;游离二氧化硅含量22.5%-35.6%。调查164人,肺功能损害率13.4%（22/164）。结论 蔺草加工业存在较严重的粉尘污染,应采取多种措施加以控制。     

2014年上海市矽尘接尘工人呼吸系统健康监测

目的 评估2014年上海市矽尘接尘工人的呼吸系统健康状况。 方法 选择2014年上海市所有涉及矽尘作业的181家企业中从事矽尘作业1年以上的2 196名劳动者为研究对象,进行肺功能、X线胸片等医学检查。运用问卷调查获得研究对象的人口学特征、吸烟状态、职业史等信息。采用分层随机抽样,对其中169家企业的619名矽尘接尘工人进行矽尘总尘个体采样,计算矽尘总尘的8 h时间加权平均浓度并进行评价。 结果 矽尘总尘的几何均数和几何标准差分别为1.4 mg/m3和3.3 mg/m3。矽尘总尘的总体超标率为66.4%。矽尘接尘工人X线胸片中可疑尘肺病或观察对象检出率为0.4%;FVC、FEV₁和FEV₁/FVC的异常检出率分别为21.3%、13.4%和0.1%。限制性通气功能障碍总体检出率为19.6%,随接触矽尘接尘工龄的增加而增加(趋势χ2=9.2143,P<0.05)。不同行业、不同工种间限制性通气功能障碍、X线胸片异常检出率差异均有统计学意义(P均<0.01),其中通用设备制造业、黑色金属冶炼和压延加工业等行业以及造型、型砂、清理、熔化、铸造、抛(磨)光等工种的异常检出率较高。 结论 上海市目前作业场所矽尘危害基本保持在历史的低水平,但整体普遍超标,矽尘接尘工人肺功能损伤明显,尤其表现为限制性通气功能障碍,建议加强对高危行业和工种的监管。     

Respirable crystalline silica dust exposure during concrete finishing (grinding) using hand-held grinders in the construction industry

Studies reporting the findings of exposure to crystalline silica dust during concrete finishing in construction settings are scarce due to the dynamic nature of the activity and the existence of many confounding factors. This study was initiated to explore the issue. A total of 49 personal respirable dust samples were collected during concrete finishing while workers used hand-held grinders. Only 15 (31%) of the grinders were equipped with local exhaust ventilation (LEV) systems. The confounding factors (e.g. wind velocity, wind direction, relative humidity and ambient temperature) were determined. To make the sampling task-specific, air sampling was activated only during actual grinding. Task-specific sampling times during each work shift ranged from 10 to 200 min. The concentration of total respirable particulate ranged from 0.34 to 81 mg/m3, with a mean +/- SD of 18.6 +/- 20.4 mg/m3, and the concentration of crystalline silica in the samples ranged from 0.02 to 7.1 mg/m3, with a mean +/- SD of 1.16 +/- 1.36 mg/m3. LEV on the grinders reduced the silica dust level significantly (P < 0.01) compared to grinders without LEV. Increased wind velocity also reduced the silica dust concentration significantly (P < 0.03). Working upwind reduced the exposure to silica dust compared to working downwind, but the difference was not statistically significant. The time-weighted average concentration of silica dust in 69% of the samples exceeded the current recommended threshold limit value of 0.05 mg/m3, indicating a strong need to devise methods for controlling workers' exposure to crystalline silica dust during concrete finishing activities.     

Crystalline silica exposure and lung cancer mortality in diatomaceous earth industry workers: a quantitative risk assessment

OBJECTIVE—To use various exposure-response models to estimate the risk of mortality from lung cancer due to occupational exposure to respirable crystalline silica dust.
METHODS—Data from a cohort mortality study of 2342 white male California diatomaceous earth mining and processing workers exposed to crystalline silica dust (mainly cristobalite) were reanalyzed with Poisson regression and Cox's proportional hazards models. Internal and external adjustments were used to control for potential confounding from the effects of time since first observation, calendar time, age, and Hispanic ethnicity. Cubic smoothing spline models were used to assess the fit of the models. Exposures were lagged by 10 years. Evaluations of the fit of the models were performed by comparing their deviances. Lifetime risks of lung cancer were estimated up to age 85 with an actuarial approach that accounted for competing causes of death.
RESULTS—Exposure to respirable crystalline silica dust was a significant predictor (p<0.05) in nearly all of the models evaluated and the linear relative rate model with a 10 year exposure lag seemed to give the best fit in the Poisson regression analysis. For those who died of lung cancer the linear relative rate model predicted rate ratios for mortality from lung cancer of about 1.6 for the mean cumulative exposure to respirable silica compared with no exposure. The excess lifetime risk (to age 85) of mortality from lung cancer for white men exposed for 45 years and with a 10 year lag period at the current Occupational Safety and Health Administration (OSHA) standard of about 0.05 mg/m³ for respirable cristobalite dust is 19/1000 (95% confidence interval (95% CI) 5/1000 to 46/1000).
CONCLUSIONS—There was a significant risk of mortality from lung cancer that increased with cumulative exposure to respirable crystalline silica dust. The predicted number of deaths from lung cancer suggests that current occupational health standards may not be adequately protecting workers from the risk of lung cancer.


Keywords: crystalline silica; cristobalite; lung cancer