Comparative epidemiology of men exposed to asbestos and man-made mineral fibers

Comparative analyses are presented of selected studies of long-term reactions to occupational exposures to asbestos and man-made mineral fibers (MMMF), with emphasis on studies with dose-response information and long enough period of follow-up to observe lung cancer excess, if it occurred. Uniform dose estimates based on average number of fibers per milliliter were derived and tabulated with the corresponding standard mortality (or morbidity) ratio (SMR), crude probability for each unfavorable outcome, and the likelihood that at least as many deaths would have occurred as a result of the expected numbers under Poisson assumptions. A dose-response relationship was said to have been indicated when the crude probability increased monotonically with dose and/or the Poisson probability decreased and reached a value of less than 0.05. Some arbitrary assumptions had to be made in estimation of the dose, and they may need to be corrected. Gravimetric dose estimates may have given different results. Studies selected for analysis included Quebec asbestos miners and asbestos cement workers exposed to asbestos, and pooled U.S. and European studies of MMMF workers, as well as a sample of cigarette-smoking fiberglass workers whose X-ray films were evaluated for fine nodular or irregular opacities. The lowest dose capable of showing either a statistically significant excess (single point criterion--SP) or the median dose in an apparent dose-response relationship with cause of death or radiological results is tabulated. Radiological changes show a dose-response relationship for all types, with a median dose for asbestos of 2.8 fibers/ml. For fiberglass workers, the median dose of electron-microscopically detected fibers was two orders of magnitude less. For asbestos SP, exposures of 1.4 to 22 fibers/ml were associated with increased lung cancer, while for mineral wool, the minimal level with significant SP increase in lung cancer was an order of magnitude less. Based on fiber or particle counts, man-made mineral fibers appear to be more potent than asbestos with regard to chronic pulmonary disease.     

An inter-laboratory study to determine the effectiveness of procedures for discriminating amphibole asbestos fibers from amphibole cleavage fragments in fiber counting by phase-contrast microscopy

The US Occupational Safety and Health Administration (OSHA) and Mine Safety and Health Administration do not regulate cleavage fragments of amphibole and serpentine minerals as asbestos, even when particles meet the dimensional criteria for counting under standard phase-contrast microscopy methods. The OSHA ID-160 method cautions that discriminatory counting is difficult and should not be attempted unless necessary and no procedure is provided for differentiation. A standard published by the American Society for Testing and Materials (ASTM International D7200-06) includes an attempt to codify a procedure but recognizes that the procedure should be validated in an inter-laboratory study. The US National Institute for Occupational Safety and Health has carried out such a study with multiple laboratories using slides made from riebeckite and crocidolite, grunerite and amosite, tremolite and tremolite asbestos, and actinolite and actinolite asbestos using two different measurement aids (graticules). The asbestos fibers had dimensions consistent with those reported for air samples from actual amphibole asbestos operations, and the cleavage fragments were also dimensionally consistent with those found in non-asbestos mining and milling operations. The procedure for discriminating asbestos fibers from other mineral particles in the ASTM Standard calls for the recognition of characteristics supposedly common to asbestos. For the asbestos fibers created in this study, these characteristics were found not to be common and generally a function of length. More importantly, different laboratories did not recognize these features consistently. Laboratories were much more consistent in measuring dimensions, but excessive overlap in the lengths of asbestos fibers and cleavage fragments rendered length a poor criterion for discrimination. The ASTM discrimination procedure as written could not be supported on the basis of this study. Width was a much more consistent parameter for distinguishing the asbestos and non-asbestos fibers in this study and inclusion of aspect ratio, while considered important by some researchers, did not refine the discrimination further. The ability of the majority of microscopists in this study to discriminate fibers and cleavage fragments through measurement of particle widths was determined and found to be within limits of uncertainty typical for air sampling measurements. A width criterion might be a very simple and useful aid where discrimination between asbestos and non-asbestos fibers in fiber counting by phase-contrast microscopy is required for further investigation. Recognition of asbestos features can also be retained as excessive recognition by some laboratories will lead to a conservative decision for additional investigation.     

Development of pleural hyalinosis in long term studies of persons exposed to asbestos dust

Radiological signs of pleural hyalinosis were found in 50 former and present employees previously exposed to asbestos dust; in 25 of them asbestosis was present sumultaneously. Periods of exposure, periods elapsed since onset of exposure and mortality rate, were ascertained until August 1, 1972, in long term studies. The mean values in the group with asbestosis amounted to 22.4 years, 33.7 years, and 40%, and in the group without asbestosis: 21.0 years, 27.6 years, and 12%. The difference in mortality rate was statistically significant. Bronchogenic carcinoma took first place as the cause of death. The regular follow-up allowed differentiation of two stages in the chronologic course of pleural hyalinosis: hyalinosis simplex and hyalinosis complicata. The characteristic pattern of hyalinosis simplex lies in its long term course with progressive calcification of the pleura without any striking response to the clinical state and respiratory function. In hyalinosis complicata acute exudative inflammation of the pleura occurs without any proof of Tbc etiology, and in the subsequent period this stage develops into extensive pachypleuritic reaction, which differs from the usual adhesive postpleuritic processes. Development in some of our patients disclosed general disease of the pleura affecting both sides. The restrictive disturbance of pulmonary ventilation was stressed simultaneously. Both our cases of pleural mesothelioma, one verified by autopsy the other clinically diagnosed (without autopsy) were from the group hyalinosis complicata. We did not find complicated hyalinosis in any of the patients without exposure to asbestes with radiological signs of pleural calcification.     

Occupational morbidity and mortality in malignant neoplasms among persons professionally exposed to asbestos dust

Asbestos fibres discharged as a result of machine processing of brakes, are characterized by a lowered biological activity due to the fixation layer which covers them. This has been confirmed through occupational diseases' analysis and new data on malignant neoplasms epidemiology among workers engaged in machine processing of asbestos-moulded machine parts. General mortality caused by malignant neoplasms in these groups higher than in the control group, although statistically is not significant. This necessitates further epidemiologic studies to be performed at similar plants.     

Determinants of respiratory symptoms in insulation workers exposed to asbestos and synthetic mineral fibres

The determinants of respiratory symptoms were studied in an active workforce of insulation workers exposed to asbestos and synthetic mineral fibres. Responses to a mailed respiratory symptom questionnaire from 537 insulation workers without diagnosed asbestosis were analysed using logistic regression. Wheezing complaints and breathlessness were related primarily to current cigarette smoking and to symptoms suggesting an asthmatic predisposition antedating work in the trade. There was also evidence that these complaints were related to occupational exposure (estimated by number of hours worked in the trade) in subjects with prior airways hyperreactivity. An asthmatic predisposition antedating work in the trade was the major determinant of acute respiratory symptoms in the workplace. The effects of workplace exposures on respiratory symptoms may have been underestimated due to selective withdrawal from the active workforce and due to inaccuracies in the measure of exposure used.     

Determinants of respiratory symptoms in insulation workers exposed to asbestos and synthetic mineral fibres.

The determinants of respiratory symptoms were studied in an active workforce of insulation workers exposed to asbestos and synthetic mineral fibres. Responses to a mailed respiratory symptom questionnaire from 537 insulation workers without diagnosed asbestosis were analysed using logistic regression. Wheezing complaints and breathlessness were related primarily to current cigarette smoking and to symptoms suggesting an asthmatic predisposition antedating work in the trade. There was also evidence that these complaints were related to occupational exposure (estimated by number of hours worked in the trade) in subjects with prior airways hyperreactivity. An asthmatic predisposition antedating work in the trade was the major determinant of acute respiratory symptoms in the workplace. The effects of workplace exposures on respiratory symptoms may have been underestimated due to selective withdrawal from the active workforce and due to inaccuracies in the measure of exposure used.     

Occupational exposure to chrysotile asbestos and cancer risk: a review of the amphibole hypothesis.

OBJECTIVES. This article examines the credibility and policy implications of the "amphibole hypothesis," which postulates that (1) the mesotheliomas observed among workers exposed to chrysotile asbestos may be explained by confounding exposures to amphiboles, and (2) chrysotile may have lower carcinogenic potency than amphiboles. METHODS. A critical review was conducted of the lung burden, epidemiologic, toxicologic, and mechanistic studies that provide the basis for the amphibole hypothesis. RESULTS. Mechanistic and lung burden studies do not provide convincing evidence for the amphibole hypothesis. Toxicologic and epidemiologic studies provide strong evidence that chrysotile is associated with an increased risk of lung cancer and mesothelioma. Chrysotile may be less potent than some amphiboles for inducing mesotheliomas, but there is little evidence to indicate lower lung cancer risk. CONCLUSIONS. Given the evidence of a significant lung cancer risk, the lack of conclusive evidence for the amphibole hypothesis, and the fact that workers are generally exposed to a mixture of fibers, we conclude that it is prudent to treat chrysotile with virtually the same level of concern as the amphibole forms of asbestos.     

Explanation for the insensitivity to tilts of the electron diffraction patterns of amphibole asbestos fibers

The previously reported observation that the electron diffraction patterns of single amphibole UICC asbestos fibers are insensitive to tilts of ± 20° along and perpendicular to the fiber axis (Skikne, M. I., et al. (1971). Electron diffraction patterns of UICC asbestos sample Environ. Res.4, 141–145) are experimentally confirmed. An explanation for this effect is proposed based on thin sample effects for small tilts and on the pseudohexagonal symmetry of these crystals for larger tilts. Because of this effect it is difficult to distinguish the three monoclinic forms of single amphibole asbestos fibers without previous study of standard (naturally occurring) single crystals.     

Mortality in a cohort of vermiculite miners exposed to fibrous amphibole in Libby, Montana

Background: Fibrous tremolite is a widespread amphibole asbestiform mineral, airborne fibres of which constitute an environmental hazard in Libby, Montana, northern California, and elsewhere.

Aims: To determine excess risk from lung cancer, mesothelioma, and all-cause mortality in a cohort of men exposed to tremolite, but no other form of asbestos.

Methods: Mortality by certified cause and various measures of exposure to tremolite and related amphibole fibres was assessed in a cohort of 406 vermiculite mineworkers in Libby, Montana, employed before 1963 and followed until 1999.

Results: Total deaths were: lung cancer 44 (SMR 2.40), non-malignant respiratory disease (NMRD) 51 (SMR 3.09), all causes 285 (SMR 1.27); included among the total were 12 deaths ascribed to mesothelioma (4.21% of all deaths). Adjusted linear increments in relative risks (per 100 f/ml.y), estimated by Poisson regression, were: lung cancer (0.36, 95% CI 0.03 to 1.20), NMRD (0.38, 95% CI 0.12 to 0.96), and all deaths (0.14, 95% CI 0.05 to 0.26).

Conclusions: The all-cause linear model would imply a 14% increase in mortality for mine workers exposed occupationally to 100 f/ml.y or about 3.2% for a general population exposed for 50 years to an ambient concentration of 0.1 f/ml. Amphibole fibres, tremolite in particular, are likely to be disproportionately responsible for cancer mortality in persons exposed to commercial chrysotile, but to what extent cannot be readily assessed.

     

石棉和人造矿物纤维诱导细胞凋亡机制的研究进展

石棉是一种天然无机结晶状矿物纤维,耐热、耐火、耐酸碱,接触石棉可导致石棉肺、支气管肺癌和恶性间皮瘤.     

Tyler asbestos workers: mortality experience in a cohort exposed to amosite

OBJECTIVES: To examine the causes of death among 1130 former workers of a plant in Tyler, Texas dedicated to the manufacture of asbestos pipe insulation materials. This cohort is important and unusual because it used amosite as the only asbestiform mineral in the production process. High level exposure of such a specific type was documented through industrial hygiene surveys in the plant. METHODS: Deaths were ascertained through various sources including data tapes from the Texas Department of Health and the national death index files. As many death certificates as possible were secured (304/315) and cause of death assigned. After select exclusions, 222 death certificates were used in the analysis. Causes of death were compared with age, race, and sex specific mortalities for the United States population with a commercial software package (OCMAP Version 2.0). RESULTS: There was an excess of deaths from respiratory cancer including the bronchus, trachea, and lung (standardised mortality ratio (SMR) 277 with 95% confidence interval (95% CI) 193 to 385). Four pleural mesotheliomas and two peritoneal mesotheliomas were identified. The analysis also showed an increasing risk of respiratory malignancy with increased duration of exposure including a significant excess of total deaths from respiratory cancer with less than six months of work at the plant (SMR 268 with 95% CI 172 to 399). CONCLUSIONS: The importance of the cohort lies with the pure amosite exposure which took place in the plant and the extended period of latency which has followed. The death certificate analysis indicates the pathogenicity of amosite, the predominant commercial amphibole used in the United States. These data confirm a link between amosite asbestos and respiratory malignancy as well as mesothelioma.