Work-related mesothelioma in Québec, 1967-1990

Fiber analysis vignettes: Electron microscopy to the rescue!

There has been considerable interest in the exposure doses that contribute to the various asbestos-associated diseases. Epidemiological studies have shown important differences in the contributions of the various fiber types to asbestos-related diseases, with the amphiboles showing a greater degree of potency as compared to chrysotile. However, epidemiological studies have occasionally provided misleading results. Over the past several decades, there have been several examples where fiber analysis using electron microscopy produced unexpected results which were important to our understanding of disease-exposure relationships. It is the purpose of this article to summarize these fiber analysis vignettes.

Toxicological and epidemiological studies on effects of airborne fibers: coherence and public [corrected] health implications

Airborne fibers, when sufficiently biopersistent, can cause chronic pleural diseases, as well as excess pulmonary fibrosis and lung cancers. Mesothelioma and pleural plaques are caused by biopersistent fibers thinner than ∼0.1 μm and longer than ∼5 μm. Excess lung cancer and pulmonary fibrosis are caused by biopersistent fibers that are longer than ∼20 μm. While biopersistence varies with fiber type, all amphibole and erionite fibers are sufficiently biopersistent to cause pathogenic effects, while the greater in vivo solubility of chrysotile fibers makes them somewhat less causal for the lung diseases, and much less causal for the pleural diseases. Most synthetic vitreous fibers are more soluble in vivo than chrysotile, and pose little, if any, health pulmonary or pleural health risk, but some specialty SVFs were sufficiently biopersistent to cause pathogenic effects in animal studies. My conclusions are based on the following: 1) epidemiologic studies that specified the origin of the fibers by type, and especially those that identified their fiber length and diameter distributions; 2) laboratory-based toxicologic studies involving fiber size characterization and/or dissolution rates and long-term observation of biological responses; and 3) the largely coherent findings of the epidemiology and the toxicology. The strong dependence of effects on fiber diameter, length, and biopersistence makes reliable routine quantitative exposure and risk assessment impractical in some cases, since it would require transmission electronic microscopic examination, of representative membrane filter samples, for determining statistically sufficient numbers of fibers longer than 5 and 20 μm, and those thinner than 0.1 μm, based on the fiber types.

Evaluation of tremolite asbestos exposures associated with the use of commercial products

Tremolite is a noncommercial form of amphibole mineral that is present in some chrysotile, talc, and vermiculite deposits. Inhalation of asbestiform tremolite is suspected to have caused or contributed to an increased incidence of mesothelioma in certain mining settings; however, very little is known about the magnitude of tremolite exposure that occurred at these locations, and even less is known regarding tremolite exposures that might have occurred during consumer use of chrysotile, talc, and vermiculite containing products. The purpose of this analysis is to evaluate the exposure-response relationship for tremolite asbestos and mesothelioma in high exposure settings (mining) and to develop estimates of tremolite asbestos exposure for various product use scenarios. Our interpretation of the tremolite asbestos exposure metrics reported for the Thetford chrysotile mines and the Libby vermiculite deposits suggests a lowest-observed-adverse-effect level (LOAEL) for mesothelioma of 35-73 f/cc-year. Using measured and estimated airborne tremolite asbestos concentrations for simulated and actual product use, we conservatively estimated the following cumulative tremolite asbestos exposures: career auto mechanic: 0.028 f/cc-year; non-occupational use of joint compound: 0.0006 f/cc-year; non-occupational use of vermiculite-containing gardening products: 0.034 f/cc-year; home-owner removal of Zonolite insulation: 0.0002 f/cc-year. While the estimated consumer tremolite exposures are far below the tremolite LOAELs derived herein, this analysis examines only a few of the hundreds of chrysotile- and talc-containing products.

Malignant mesothelioma in Hong Kong

Malignant mesothelioma (mesothelioma) is rare. We conducted the first systematic study of the epidemiology of mesothelioma in Hong Kong from 1988 to May 2002 by reviewing medical records. Mesothelioma patients were identified from the database of 12 out of 20 hospitals that would have admitted mesothelioma patients territory-wide. These 12 hospitals served 73% of the total hospital bed-years of the 20 hospitals. We identified 67 mesothelioma patients. The estimated annual incidence was one per million, which was similar to the background incidence of one to two per million among Caucasians. Occupational history was available in 43 subjects. Three quarters of mesothelioma patients with available occupational history had occupational asbestos exposure. Restricting analysis to 48 patients with accessible medical records and using 67 occupational asbestosis patients for comparison, the epidemiology of mesothelioma in Hong Kong shares similarities with the literature: mean age of 63 years upon diagnosis, mean latency of 46 years, median survival of 9.5 months, male predominance, selective presentation among women, high prevalence among workers in ships and dockyards, predominantly epithelioid type, lower prevalence of asbestos bodies, and negative association with pleural plaques. Asbestos consumption in Hong Kong rose in the 1970s and peaked in early 1980s and late 1990s. Hong Kong may encounter an epidemic of mesothelioma in the 2010s if effective occupational asbestos control measures are not in place.

Exposing the "myth" of ABC, "anything but chrysotile": a critique of the Canadian asbestos mining industry and McGill University chrysotile studies

BACKGROUND

Beginning in the 1930s, the Canadian asbestos industry created and advanced the idea that chrysotile asbestos is safer than asbestos of other fiber types.

METHODS

We critically evaluate published and unpublished studies funded by the Quebec Asbestos Mining Association (QAMA) and performed by researchers at McGill University.

RESULTS

QAMA-funded researchers put forth several myths purporting that Quebec-mined chrysotile was harmless, and contended that the contamination of chrysotile with oils, tremolite, or crocidolite was the source of occupational health risk. In addition, QAMA-funded researchers manipulated data and used unsound sampling and analysis techniques to back up their contention that chrysotile was "essentially innocuous."

CONCLUSIONS

These studies were used to promote the marketing and sales of asbestos, and have had a substantial effect on policy and occupational health litigation. Asbestos manufacturing companies and the Canadian government continue to use them to promote the use of asbestos in Europe and in developing countries. Am. J. Ind. Med. 44:540-557, 2003.

Asbestos fibers contributing to the induction of human malignant mesothelioma

To elucidate the features of the asbestos fibers contributing to the induction of human malignant mesothelioma, we used high-resolution analytical electron microscopy to determine the type, number, and dimensions of asbestos fibers in lung and mesothelial tissues in 168 cases of mesothelioma.

RESULTS

1. Asbestos fibers were present in almost all of the lung and mesothelial tissues from the mesothelioma cases. 2. The most common types of asbestos fibers in lung were either an admixture of chrysotile with amphiboles, amphibole alone, and occasionally chrysotile alone. In mesothelial tissues, most asbestos fibers were chrysotile. 3. In lung, amosite fibers were greatest in number followed by chrysotile, crocidolite, tremolite/actinolite, and anthophyllite. In mesothelial tissues, chrysotile fibers were 30.3 times more common than amphiboles. 4. In some mesothelioma cases, the only asbestos fibers detected in either lung or mesothelial tissue were chrysotile fibers. 5. The average number of asbestos fibers in both lung and mesothelial tissues was two orders of magnitude greater than the number found in the general population. 6. The majority of asbestos fibers in lung and mesothelial tissues were shorter than 5 micro m in length.

CONCLUSIONS

1) Fiber analysis of both lung and mesothelial tissues must be done to determine the types of asbestos fibers associated with the induction of human malignant mesothelioma; 2) short, thin asbestos fibers should be included in the list of fiber types contributing to the induction of human malignant mesothelioma; 3) RESULTS support the induction of human malignant mesothelioma by chrysotile.

Asbesto, asbestose e câncer: critérios diagnósticos

As doenças asbesto-induzidas constituem um grave problema de saúde em decorrência de grande número de trabalhadores expostos ao asbesto ao longo dos últimos 50 anos. Processos judiciais contra indústrias que lidam com asbesto somam centenas, com crescente adição de novos casos. O assunto relativo à asbestose é complexo e, muito embora a história natural das doenças induzidas esteja bem estabelecida, muitas áreas importantes, como a patologia, permanecem ainda pouco compreendidas. No Brasil, desde 1940, o asbesto é explorado comercialmente e nos últimos anos sua produção foi da ordem de 200.000 toneladas por ano, estimando-se que na atividade de mineração cerca de 10.000 trabalhadores foram expostos a essa fibra, desconhecendo-se a estimativa do número de pessoas expostas na produção de fibrocimento, especialmente telhas e caixas d'água. Um estudo, com metodologia de investigação científica apropriada, para avaliar as repercussões sobre a saúde dos trabalhadores nas minas de asbesto em nosso país, foi elaborado e intitulado "Morbidade e mortalidade entre trabalhadores expostos ao asbesto na atividade de mineração 1940-1996", de cunho interinstitucional. O objetivo deste trabalho foi fornecer uma visão ampla das doenças asbesto-induzidas, com ênfase nas dificuldades no diagnóstico histopatológico, através da experiência adquirida com o desenrolar desse projeto.

[Asbestos and disease: review of the scientific knowledge and a rationale for urgent change in the current Brazilian policy about this question]

This paper is a state-of-the-art review of scientific knowledge on both the health effects of asbestos fiber inhalation and possibilities for safe and sustainable prevention, from an ethical, political, and technological point of view. The author provides scientific background and arguments from the ongoing discussion in Brazil concerning the need to redefine current asbestos policy, in order to establish a more advanced and appropriate policy whose priority is the protection of life, human health, and the environment. The first part deals with several technological and economic aspects of asbestos-chrysotile. In the second part, the author presents and discusses a bibliographic review of the construction of scientific knowledge on the health effects of asbestos fibers, first within an international perspective, and then (in the third part), from a Brazilian view. The fourth part analyzes the current debate on the fibrogenicity and carcinogenicity of asbestos-chrysotile. Some current responses from the international community towards the asbestos-chrysotile ban are also discussed. Finally, the author discusses the historical inadequacy of Brazilian asbestos policy and the urgent need to revise it to include a ban on asbestos-chrysotile in this country.

Estimation of the past and future burden of mortality from mesothelioma in France

OBJECTIVES

Firstly to evaluate future mortality from mesothelioma in France with an age-period-cohort approach and evaluate different hypotheses on risk of mesothelioma for the most recent birth cohort. Secondly to compare the results with a British and an American study. Thirdly to study if any trends were detectable on data for women which would be consistent with the consequences of increasing environmental exposure to asbestos.

METHODS

Estimates of mortality from mesothelioma among men and women in France from 1950 to 1995 were based on the analysis of the pleural cancer mortality data coded 163 in the ninth revision of the international classification of diseases (ICD-9). Correction factors were used to derive the mortality from mesothelioma from these data, based on two regional registries. The analysis of the past mortality data has been performed by an age-cohort model (with a maximum likelihood technique). Predictions of deaths from mesothelioma over the next 50 years were based on four different assumptions on the risk of death from mesothelioma in future birth cohorts.

RESULTS

The predicted lifetime probability of dying from mesothelioma increases until the last birth cohort 1964-8 among men whereas it decreases strongly from the 1954-8 birth cohort among women. The projected numbers of deaths from mesothelioma in France until 2020 are similar, whichever hypothesis is considered: around 20,000 deaths from mesothelioma might occur among men and 2900 among women from 1996 to 2020.

CONCLUSIONS

French data show an increasing lifetime probability of death from mesothelioma in the more recent male cohorts. Although the mortality burden can be predicted until 2020, and is intermediate between the United Kingdom and United States estimates, there is still high uncertainty on the figures after 2020. No increase is found in women, and this does not support the hypothesis that current environmental exposure to asbestos could be associated with a detectable risk of death. Specific surveillance should be set up to monitor future trends or their absence.

Metabolic imaging of malignant pleural mesothelioma with fluorodeoxyglucose positron emission tomography

BACKGROUND

The diagnosis of malignant mesothelioma is a challenging medical problem. CT often cannot differentiate between benign diffuse pleural thickening and malignant mesothelioma, while thoracentesis and CT-guided biopsies are insensitive. We have assessed the value of positron emission tomography (PET) with 2-fluoro-2-deoxy-D-glucose (FDG) in the evaluation of malignant mesothelioma.

METHODS

Twenty-eight consecutive patients referred for the evaluation of suspected malignant mesothelioma were evaluated by FDG-PET imaging. Measured attenuation correction was performed in 26 of 28 cases for quantitation with the standardized uptake value (SUV) method. The results of PET imaging were compared with those of video-assisted thoracoscopy or surgical biopsies.

RESULTS

Surgical biopsy specimens confirmed the presence of malignant disease in 24 patients and demonstrated benign processes in the remaining four. The uptake of FDG was significantly higher in malignant than in benign lesions (SUV=4.9+/-2.9 and SUV=1.4+/-0.6, respectively; p<0.0001). With a SUV cutoff of 2.0 to differentiate between malignant and benign disease, a sensitivity of 91% and a specificity of 100% could be achieved, although the activity in some epithelial mesotheliomas tended to be close to this threshold. FDG-PET images provided excellent delineation of the active tumor sites. Hypermetabolic lymph node involvement was noted on FDG-PET images in 12 patients, 9 of which appeared normal on CT scans. Histologic examination in six patients confirmed malignant nodal disease in five cases and indicated granulomatous lymphadenitis in one.

CONCLUSION

In this highly selected population, FDG-PET imaging was a sensitive method to identify malignant mesothelioma and determine the extent of the disease process.

Chrysotile asbestos fibers mediate homologous recombination in Rat2 lambda fibroblasts: implications for carcinogenesis

Asbestos fibers are widespread environmental carcinogens whose mutagenicity is now established. Nonetheless, the molecular nature of these mutations and the mechanisms by which they accelerate carcinogenesis remain poorly understood. We have assessed the ability of asbestos fibers to promote homologous recombination, a potent mechanism for generating intrachromosomal rearrangements, such as deletions, and mitotic recombination. For this, we have developed a new assay which determines the extent to which a marker gene present in DNA introduced by asbestos can recombine with homologous genes residing in a transfected cell. We have demonstrated that Calidria chrysotile fibers are mutagenic and are able to mediate transfection of molecularly marked mutant lacI genes in a manner that results in their preferential recombination with homologous wild-type genes in the transfected cell. Asbestos induced recombination events may play a significant role in asbestos mutagenesis and carcinogenesis, and promotion of recombination may underlie the well-recognized synergy of asbestos with other carcinogens.

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.

Fibers in lung tissues of mesothelioma cases among miners and millers of the township of Asbestos, Quebec

Twenty cases of mesothelioma among miners of the township of Asbestos, Quebec, Canada, have been reported. To further explore the mineral characteristics of various fibrous material, we studied the fibrous inorganic content of postmortem lung tissues of 12 of 20 available cases. In each case, we measured concentrations of chrysotile, amosite, crocidolite, tremolite, talc-anthophyllite, and other fibrous minerals. The average diameter, length, and length-to-diameter ratio of each type of fiber were also calculated. For total fibers > 5 microns, we found > 1,000 asbestos fibers per mg tissue (f/mg) in all cases; tremolite was above 1,000 f/mg in 8 cases, chrysotile in 6 cases, crocidolite in 4 cases, and talc anthophyllite in 5 cases. Among cases with asbestos fibers, the tremolite count was highest in 7 cases, chrysotile in 3 cases, and crocidolite in 2 cases. The geometric mean concentrations of fibers > or = 5 microns were in the following decreasing order: tremolite > crocidolite > chrysotile > other fibers > talc-anthophyllite > amosite. For total fibers < 5 microns, we found > 1,000 fibers per mg tissue (f/mg) in all cases; tremolite was above 1,000 f/mg in 12 cases, chrysotile in 8 cases, crocidolite in 7 cases, and talc-anthophyllite in 6 cases. Tremolite was highest in 8 cases, chrysotile in 2 cases, and crocidolite and amosite in 2 cases. The geometric mean concentrations of fibers < 5 microns were in the following decreasing order: tremolite > other fibers > chrysotile > crocidolite > talc-anthophyllite > amosite. We conclude, on the basis of the lung burden analyses of 12 mesothelioma cases from the Asbestos township of Quebec, that the imported amphibole (crocidolite and amosite) were the dominant fibers retained in the lung tissue in 2/12 cases. In 10/12 cases, fibers from the mine site (chrysotile and tremolite) were found at highest counts; tremolite was clearly the highest in 6, chrysotile in 2, and 2 cases had about the same counts for tremolite and chrysotile. If a relation of fiber burden-causality of mesothelioma is accepted, mesothelioma would be likely caused by amphibole contamination of the plant in 2/12 cases and by the mineral fibers (tremolite and chrysotile) from the mine site in the 10 other cases.

Asbestos-related disease associated with exposure to asbestiform tremolite

Tremolite is nearly ubiquitous and represents the most common amphibole fiber in the lungs of urbanites. Tremolite asbestos is not mined or used commercially but is a frequent contaminant of chrysotile asbestos, vermiculite, and talc. Therefore, individuals exposed to these materials or to end-products containing these materials may be exposed to tremolite. We have had the opportunity to do asbestos body counts and mineral fiber analysis on pulmonary tissue from five mesothelioma cases and two asbestosis cases with pulmonary tremolite burdens greater than background levels. There were no uncoated amosite or crocidolite fibers detected in any of these cases. Three patients were occupationally exposed to chrysotile asbestos; two patients had environmental exposures (one to vermiculite and one to chrysotile and talc) and one was a household contact of a shipyard worker. The tremolite burdens for the asbestosis cases were one to two orders of magnitude greater than those for the mesothelioma cases. Our study confirms the relationship between tremolite exposure and the development of asbestos-associated diseases. Furthermore, the finding of relatively modest elevations of tremolite content in some of our mesothelioma cases suggests that, at least for some susceptible individuals, moderate exposures to tremolite-contaminated dust can produce malignant pleural mesothelioma.

Asbestos-related mesothelioma in Monfalcone, Italy

The Monfalcone area, in northeastern Italy, is a small industrial territory (population about 60,000), with a large shipyard. Between October 1979 and April 1992, ninety-two malignant mesotheliomas were diagnosed at the Monfalcone Hospital. The series included 84 men and 8 women, aged 42 to 89 years (median age 68 years). There were 89 pleural and 3 peritoneal tumors. Seventy patients (69 men and 1 woman) had worked in the shipyards; six were seamen, and four insulators. Five men had been exposed to asbestos in various industries; six women had histories of domestic exposure, and one woman had a history of possible environmental exposure. The latency periods (intervals between first exposure to asbestos and diagnosis of the tumor) ranged from 20 to 65 years (median 52 years). Latency periods among insulators were significantly lower than among shipyard workers, as well as lower than among the other categories (p < 0.01). Lung asbestos bodies were isolated after chemical digestion in 73 cases at necropsy, and in two cases at surgery. In necropsy cases, asbestos body burdens ranged between 100 and 10,000,000 bodies per gram of dried tissue (median 11,000). Pleural plaques were observed at necropsy in 62 out of 73 cases, and in two cases at surgery. In the time period we considered, the annual incidence rates for pleural mesothelioma were very high among male Monfalcone residents, being 189 per 100,000 among people aged 75 years or more. On the basis of occupational data and of objective signs (lung asbestos bodies, pleural plaques), all the cases of the present series but one (that with possible environmental exposure) were considered as asbestos-related. The role of co-factors in the development of asbestos-related mesothelioma is suggested.