Risk of mesothelioma from exposure to crocidolite asbestos: a 1995 update of a South African mortality study

OBJECTIVE—To find the risk of developing mesothelioma in a cohort born in 1916-36 in Prieska, Northern Cape Province, South Africa.
METHODS—A birth cohort mortality study was carried out in a small town in the Northern Cape Province, South Africa, with a history of crocidolite asbestos mining and milling. The cohort comprised all white births registered in the magisterial district of Prieska from 1916 to 1936, inclusive (2390). Causes of death due to mesothelioma and other cancers as recorded on medical certificates of cause of death were investigated. Person-years analysis was used to calculate mortalities due to mesothelioma, other respiratory cancers, and other non-respiratory cancers. Proportional cancer mortalities were also calculated for mesothelioma and other specific neoplasms.
RESULTS—The follow up rate for the cohort was 74.3% in 1995, and 683 traced members (38.6%) had died. Cause of death was unknown for 6.4% of deaths. There were 118 cases of cancer, 28 of them from mesothelioma, giving a cause specific mortality for mesothelioma of 277 (170-384) per 10⁶ person-years. The rates for men and women were 366 and 172 per 10⁶ person-years, respectively. The mortality for lung cancer (29 deaths) was 287 (135-436) per 10⁶ person-years, and that for other non-respiratory cancers (60 deaths) was 593 (442-745). Two cases of laryngeal and four of colon cancer were observed. All cancer mortality, mesothelioma, and lung cancer proportional cancer mortality ratios were increased.
CONCLUSION—The mortality for mesothelioma in men was twice that in women, probably because men were more likely to have had both occupational and environmental exposure to asbestos. Nevertheless, the mortality in women was still high and is probably indicative of the environmental exposure as white women were rarely employed in the asbestos industry in the Prieska area. Due to the long latency from first exposure to diagnosis of the neoplasm, the cause specific mortality in this cohort could be expected to increase rapidly over the next 10 years.


Keywords: South Africa; asbestos; environmental exposure; occupational exposure; cohort; mesothelioma; lung cancer     

Epidemiology of occupational asbestos-related diseases in China

In 1950s and 60s, asbestosis had been a major health hazard for asbestos exposed workers. In the late 1970s, lung cancers with or without asbestosis were found among asbestos workers. All cohort studies on asbestos workers and on chrysotile miners in China showed excess deaths from lung cancer. In a large scale of cohort study on asbestos workers, a synergistic effect was found between cigarette smoking and asbestos exposure in the production of lung cancer. There have been not so many cases of malignant mesotheliomas reported, so far. In the cohort of chrysotile miners, 4 cases of pleural mesothelioma were observed. In the large scale of cohort study on asbestos workers in 9 factories using only chrysotile only one case of pleural mesothelioma was detected for 10 years' observation. In another 2 cohort studies, 2 cases of peritoneal mesotheliomas were found, one in Shanghai asbestos factory where a small amount of crocidolite had been used in 1960s, and one in Anqing asbestos factory that was located near tremolite mine. Further study is needed especially for the relationship between exposure to Chinese chrysotile and malignant mesotheliomas.     

Malignant pleural mesothelioma in parts of Japan in relationship to asbestos exposure

Malignant pleural mesothelioma is induced by asbestos exposure. Many reports have described this situation in America and European countries, but a few have been published in Japan. In this study malignant pleural mesothelioma cases in hospitals located in an area facing the Seto Inland Sea were evaluated. A total of 106 patients were examined with 100 patients having had occupational exposure to asbestos and 6 patients without such histories of asbestos exposure. Ninety seven were male and 9 were female. Ages ranged from 41 to 87 yr with mean of 64.8+/-5.3 yr. Thirty seven cases showed epithelial type of tumor, 25 biphasic type and 15 showed sarcomatous. The remaining 23 cases had insufficient evidence for typing the tumor. The mean survival rate for all cases was 9.2+/-11.6 months. Fifty-one patients had occupational histories of shipyard work, 16 patients worked in asbestos cement piping, and the remainder were employed in miscellaneous jobs related asbestos exposure. The duration of asbestos exposure ranged up to 20 yr or longer with the mean of 17.2+/-8.9 yr and the average latent period for the occurrence of malignant pleural mesothelioma was more than 31 yr with the mean of 37.0+/-13.3 yr. Quantification of asbestos bodies in the lungs indicated a high concentration in most patients and the major types of asbestos fibers were crocidolite and amosite. Six cases appeared after exposure to chrysotile. These results indicated that ninety four percent of malignant pleural mesothelioma appeared due to the exposure to asbestos including crocidolite and amosite. The remainder may be blamed on exposure to chrysotile.     

Changing trends in US mesothelioma incidence

Aims: To report the temporal pattern and change in trend of mesothelioma incidence in the United States since 1973.

Methods: The Surveillance, Epidemiology, and End Results (SEER) programme of the National Cancer Institute has since 1973 provided annual age adjusted incidence for mesothelioma in representative cancer registries dispersed throughout the USA. SEER data are analysed to describe the trend of male mesothelioma incidence in the USA.

Results: The US male mesothelioma incidence data indicate that after two decades of increasing incidence, a likely decline has been observed since the early 1990s, when a highly significant change in the upward course occurred.

Conclusions: Increasing male mesothelioma incidence for many years was undoubtedly the result of exposure to asbestos. The high mesothelioma risk was prominently influenced by exposure to amphibole asbestos (crocidolite and amosite), which reached its peak usage in the 1960s and thereafter declined. A differing pattern in some other countries (continuing rise in incidence) may be related to their greater and later amphibole use, particularly crocidolite. The known latency period for the development of this tumour provides biological plausibility for the recent decline in mesothelioma incidence in the USA. This favourable finding is contrary to a widespread fear that asbestos related health effects will show an inevitable increase in coming years, or even decades.

     

Mesothelioma in an Individual Following Exposure to Crocidolite-containing Gaskets as a Teenager

Abstract

Mesothelioma is considered a signal tumor for asbestos exposure and typically occurs decades after first exposure to asbestos. Tissue analysis often indicates past exposure to mixed types of asbestos. This report describes the case of a 58-year-old man who developed mesothelioma after reported exposure to crocidolite from asbestos-containing gaskets beginning at age 16 during three summers during high school and for approximately four hours per day during the last semester of his senior year. He had no further known exposure to asbestos. Analytical transmission electron microscopy analysis of digested tissue samples revealed elevated levels of crocidolite asbestos fibers and the presence of crocidolite cored ferruginous bodies. This case is unique in that it establishes that relatively short and/or intense exposures to crocidolite asbestos traumatically released from a previously classified Category 1 nonfriable asbestos-containing material (NESHAP) was confirmed via tissue burden analysis years following the historically defined exposures.     

Role of occupational asbestos exposure in Hungarian lung cancer patients

Objective: What is the frequency of occupational asbestos exposure among patients suffering from malignant respiratory tumours and how many of these tumours are associated with asbestos in Hungary? Methods: An internationally established questionnaire with 29 questions, covering the most characteristic activities of asbestos exposure at the workplace was completed for 300 patients with respiratory malignancies, i.e. 297 patients with lung cancer and three with mesothelioma of the pleura. From the questionnaire, the smoking habits were estimated and cumulative asbestos exposure was assessed in fibre-years. Additionally, lung X-rays were classified and the national data on the incidence of malignant pleura mesothelioma were analysed. Results: A cumulative asbestos exposure of 25?fibre-years or more was detected in 11 patients with lung cancer (4%) and in each of the three patients with pleural mesothelioma (100%). In a further 72 patients (24%), cumulative occupational asbestos exposure was assessed as below 25?fibre-years (between 0.01 and 23.9?fibre-years). In this group, car and truck mechanics, and installation and construction workers using asbestos-cement were registered. Among patients with an asbestos exposure of 25?fibre-years or more, six asbestos-cement production workers were observed, among them the three mesothelioma cases. A weak but significant association between positive X-ray findings and exposure estimates could be demonstrated. Additionally, results of the lung tissue fibre counts by scanning transmission electron microscopy were available for 25 of the lung cancer patients. A good correlation was observed between the asbestos fibre counts and the assessment of cumulative asbestos exposure. In Hungary, 84 cases of pleural mesothelioma were registered in 1997 and 73 in 1998. These numbers correspond to an annual incidence of about one new case per 100,000 inhabitants older than 15?years. Conclusions: The annual incidence of lung cancer in Hungary is about 6,000. Since in our series of lung cancer patients about 4% were observed, which could be accepted as representing occupational disease because of a cumulative exposure to 25?fibre-years or more, the annual asbestos related lung tumour incidences may be estimated to be approximately 150 or more. The proportion of nearly two estimated cases of lung cancer per case of pleural mesothelioma corresponds to international experience. Up to now, lung cancer cases only exceptionally have been registered as occupational diseases, i.e. they were seriously under-diagnosed in Hungary. For improving this situation, diagnostic assistance by a self-interview with a questionnaire covering the working history for all newly diagnosed lung cancer patients would be helpful.     

Asbestos fiber analysis in 27 malignant mesothelioma cases.

The asbestos body counts per 5 gm wet lung tissue in 27 (23 pleural and 4 peritoneal) malignant mesothelioma cases derived from 19 autopsy and 8 surgical cases were, according to our own criteria, low level exposure in 13 cases (48.2%), moderate level exposure in 2 cases (7.4%), and high level exposure in 12 cases (44.4%). In our previous study on 235 consecutive autopsy cases, the low level exposure was considered to be environmental, the moderate level was secondary or blue collar, and the high level was occupational. In the present study, about half of the cases examined (44.4%, high level exposure) are closely related to some occupational asbestos exposure and the other half (48.2%) to environmental exposure. The type and size of asbestos fibers from the 12 cases of high level exposure were analyzed and the characteristics were compared with those of cases of low level exposure without lung cancer or mesothelioma. Most fibers analyzed (98%) were longer than 5 microns and thicker than 0.10 micron by our counting rules. In the control group, predominant fibers were tremolite or actinolite. In all the 11 pleural mesothelioma cases, the content of amosite fibers was significantly higher than in the controls. In one case of peritoneal mesothelioma, incipient asbestosis was found and the predominant fibers were crocidolite. It is suggested that the presence of amosite and crocidolite is linked to mesothelioma. The mean lengths of amosite and crocidolite, as detected by our resolution capabilities, were 36.0 and 20.9 microns, and the mean diameters were 0.51 and 0.27 micron, respectively. Both amosite and crocidolite fibers had high aspect ratios (94.2 and 115.4).     

Mortality of workers manufacturing friction materials using asbestos.

A mortality (1942-80) study was carried out on 13460 workers of a factory producing friction materials. The only type of asbestos used was chrysotile, except during two well-defined periods before 1945 when crocidolite was used, and over 99% of the population was traced. Compared with national death rates there were no detectable excesses of deaths due to lung cancer, gastrointestinal cancer, or other cancers; 11 deaths were due to pleural mesothelioma. A case-control study was carried out on deaths due to mesothelioma; this showed that eight workers had been exposed to crocidolite and another was possibly exposed intermittently to crocidolite. The other two had been employed for most of their working lives outside the factory, and their mesotheliomas could not be definitely attributed to exposure to chrysotile. Limiting the study to cases and controls who had exposure to 5 fibres/ml of chrysotile asbestos it was found that five of the six cases compared with two of the 10 controls had also been exposed to crocidolite. The probability (1:36) of this occurring were there no association with crocidolite is most unlikely. A case-control study was also carried out on deaths due to lung cancer and gastrointestinal cancer to investigate the dose-response relationships between these tumours and exposure to chrysotile. Measured and estimated fibre concentrations were available for the different jobs over the period of the study. No dose-response relationships were observed, but the exposures were low with only 5% of men accumulating 100 fibre-years/ml. The experience at this factory over a 40-year period showed that chrysotile asbestos was processed with no detectable excess mortality.     

Asbestos-related mesothelioma: factors discriminating between pleural and peritoneal sites.

Up to the end of 1980, 144 confirmed cases of mesothelioma were identified among employees of an organisation using asbestos in manufacturing and insulation. The primary site was peritoneal in 74 cases, pleural in 66, and undetermined in four. All employees had been exposed to amphibole asbestos, and evidence from different factories confirmed the predominant role of crocidolite in the production of mesothelioma. The ratio of pleural to peritoneal sites showed a continuous change when related to the year of first exposure, varying from 5:1 pleural to peritoneal before 1921 to 1:3 after 1950. The strong temporal relationship appeared to reflect progressive dust suppression, including the non-fibrous dusts present in insulation materials and perhaps also the degree to which the fibres had been opened. Other predisposing factors were related to the degree of individual exposure, the peritoneal site being associated preferentially with longer and heavier exposures.     

Cancer in ex‐asbestos cement workers in Israel, 1953–1992

A cohort of 3,057 male workers employed in an asbestos‐cement plant using 90% chrysotile‐10% crocidolite, located in Northern Israel, was followed from 1953–1992 for incidence and mortality from cancer. In the years 1978–1992, the cohort had an elevated risk for all malignant neoplasms combined (n > 153, SIR > 117, ns), lung cancer (n > 28, SIR > 135, ns), mesothelioma (n > 21; SIR >5000, p > .0001), unspecified pleural cancer (n > 5; SIR > 278 , P > .0001), and liver cancer (n > 7, SIR 290, ns). Risks for colo‐rectal (n > 19; SIR > 79, ns), bladder (n = 12, SIR 69) and renal cancers (n > 5, SIR 104) were less than expected. Risk for mesothelioma showed a sharp risk gradient with duration of exposure, increasing from 1 per 625 for those employed less than 2 years to 1 per 4.5 workers employed over 30 years. The ratio of mesothelioma to excess lung cancer cases was 2.9 to 1, or 3.6 to 1, if pleural cases of unspecified origin were included; the pleura to peritoneum ratio of verified mesothelioma cases was 20 to 1. This atypically high ratio of mesothelioma to excess lung cancer cases is suggested to be the combined result of high past asbestos exposures in the workers and their low prior risk for lung cancer, and possibly, relatively early smoking cessation in relation to asbestos exposure. Am. J. Ind. Med. 35:1–8, 1999. © 1999 Wiley‐Liss, Inc.     

The quantitative risks of mesothelioma and lung cancer in relation to asbestos exposure

Mortality reports on asbestos exposed cohorts which gave information on exposure levels from which (as a minimum) a cohort average cumulative exposure could be estimated were reviewed. At exposure levels seen in occupational cohorts it is concluded that the exposure specific risk of mesothelioma from the three principal commercial asbestos types is broadly in the ratio 1:100:500 for chrysotile, amosite and crocidolite respectively. For lung cancer the conclusions are less clear cut. Cohorts exposed only to crocidolite or amosite record similar exposure specific risk levels (around 5% excess lung cancer per f/ml.yr); but chrysotile exposed cohorts show a less consistent picture, with a clear discrepancy between the mortality experience of a cohort of xhrysotile textile workers in Carolina and the Quebec miners cohort. Taking account of the excess risk recorded by cohorts with mixed fibre exposures (generally<1%), the Carolina experience looks uptypically high. It is suggested that a best estimate lung cancer risk for chrysotile alone would be 0.1%, with a highest reasonable estimate of 0.5%. The risk differential between chrysotile and the two amphibole fibres for lunc cancer is thus between 1:10 and 1:50.Examination of the inter-study dose response relationship for the amphibole fibres suggests a non-linear relationship for all three cancer endpoints (pleural and peritoneal mesotheliomas, and lung cancer). The peritoneal mesothelioma risk is proportional to the square of cumulative exposure, lung cancer risk lies between a linear and square relationship and pleural mesothelioma seems to rise less than linearly with cumulative dose. Although these non-linear relationships provide a best fit ot the data, statistical and other uncertainties mean that a linear relationship remains arguable for pleural and lung tumours (but not or peritoneal tumours).Based on these considerations, and a discussion fo the associated uncertainties, a series of quantified risk summary statements for different elvels of cumulative exposure are presented.     

Mortality of workers manufacturing friction materials using asbestos

A mortality (1942-80) study was carried out on 13460 workers of a factory producing friction materials. The only type of asbestos used was chrysotile, except during two well-defined periods before 1945 when crocidolite was used, and over 99% of the population was traced. Compared with national death rates there were no detectable excesses of deaths due to lung cancer, gastrointestinal cancer, or other cancers; 11 deaths were due to pleural mesothelioma. A case-control study was carried out on deaths due to mesothelioma; this showed that eight workers had been exposed to crocidolite and another was possibly exposed intermittently to crocidolite. The other two had been employed for most of their working lives outside the factory, and their mesotheliomas could not be definitely attributed to exposure to chrysotile. Limiting the study to cases and controls who had exposure to 5 fibres/ml of chrysotile asbestos it was found that five of the six cases compared with two of the 10 controls had also been exposed to crocidolite. The probability (1:36) of this occurring were there no association with crocidolite is most unlikely. A case-control study was also carried out on deaths due to lung cancer and gastrointestinal cancer to investigate the dose-response relationships between these tumours and exposure to chrysotile. Measured and estimated fibre concentrations were available for the different jobs over the period of the study. No dose-response relationships were observed, but the exposures were low with only 5% of men accumulating 100 fibre-years/ml. The experience at this factory over a 40-year period showed that chrysotile asbestos was processed with no detectable excess mortality.     

Mesothelioma in an individual following exposure to crocidolite-containing gaskets as a teenager

Mesothelioma is considered a signal tumor for asbestos exposure and typically occurs decades after first exposure to asbestos. Tissue analysis often indicates past exposure to mixed types of asbestos. This report describes the case of a 58-year-old man who developed mesothelioma after reported exposure to crocidolite from asbestos-containing gaskets beginning at age 16 during three summers during high school and for approximately four hours per day during the last semester of his senior year. He had no further known exposure to asbestos. Analytical transmission electron microscopy analysis of digested tissue samples revealed elevated levels of crocidolite asbestos fibers and the presence of crocidolite cored ferruginous bodies. This case is unique in that it establishes that relatively short and/or intense exposures to crocidolite asbestos traumatically released from a previously classified Category 1 nonfriable asbestos-containing material (NESHAP) was confirmed via tissue burden analysis years following the historically defined exposures.     

Inhalation carcinogenesis from various forms of asbestos

Rats, rabbits, guinea pigs, gerbils, and mice were exposed to the inhalation of chrysotile, crocidolite, or amosite for 2 years. Mean atmospheric concentrations were 47.9–50.2 mg/m³, but only 0.08–1.82% of the dusts retained fibrous morphology during the dissemination procedure which involved hammer milling. Trace contamination especially by chromium and nickel was also increased. Light microscopic fiber counts per ml chamber air were 54 (chrysotile), 1105 (crocidolite), and 864 (amosite). A fibrogenic response to these dusts was observed in all five animal species, the severity corresponding to the extent of exposure (with reaction to chrysotile frequently very slight). Gerbils developed frequent alveolar proteinosis. Mice developed spontaneous papillary carcinomas in the lungs. Disregarding the latter species, carcinogenic response to asbestos inhalation was restricted to rats and occurred in all three exposure groups. There were 2 lung cancers and 1 pleural mesothelioma after chrysotile inhalation; 4 lung cancers after crocidolite inhalation; and 1 lung cancer and 2 pleural mesotheliomas after amosite inhalation. These cases constituted 7–9% incidence of malignancy among rats with adequate survival record. Hypotheses of asbestos carcinogenesis are reviewed and it is suggested that different etiologic principles may be involved in the causation of lung cancer and of pleural mesothelioma.     

The mortality of amphibole miners in South Africa, 1946-80.

A cohort was established in 1981 of all 7317 white male employees in the amosite and crocidolite mines in South Africa whose names had appeared in the personnel records (initiated between 1945 and 1955) of the major companies. Some of the men had been employed as early as 1925, but only 8% had had more than 10 years of service. Three subcohorts were defined: 3212 men whose only exposure to asbestos was to amosite; 3430 exposed to crocidolite; and 675 to both amphiboles. No deaths or losses to view occurred before 1946, and 5925 men (81%) were known to be alive at the end of 1980. Losses to view numbered 167 (2%), and there had been 1225 deaths (17%), an excess of 331 over the number of deaths expected on the basis of the mortality of all white South African males. The fibre related excesses were of mesothelioma, lung cancer, and other respiratory diseases, but there were other excesses perhaps mainly related to socioeconomic factors including lifestyle. When cause of death was determined according to "best evidence" (after study of clinical, radiological, biopsy, and necropsy reports in conjunction with the death certificate), there were 30 deaths due to mesothelioma (22 pleural, six peritoneal, two other) and 65 due to cancer of trachea, bronchus, and lung. Various analyses of these deaths showed that crocidolite had higher toxicity than amosite for lung cancer and this was most pronounced for mesothelioma; there can now be no question that crocidolite is far more dangerous than amosite at least in so far as mesothelioma is concerned. Nevertheless, crocidolite induced mesothelioma appeared only in men who had been exposed for long periods, at least 12 months, but on average about 15 years.     

The additional risk of malignant mesothelioma in former workers and residents of Wittenoom with benign pleural disease or asbestosis

Aims: To examine the hypothesis that people with benign pleural disease or asbestosis have an increased risk of malignant mesothelioma beyond that attributable to their degree of asbestos exposure. Methods: Former workers and residents of the crocidolite mining and milling town of Wittenoom are participating in a cancer prevention programme (n = 1988). The first plain chest radiograph taken at the time of recruitment into the cancer prevention programme was read for evidence of benign pleural disease and asbestosis, using the UICC classification. Crocidolite exposure of former workers was derived from employment records and records of dust measurements performed during the operation of the asbestos mine and mill between 1943 and 1966. Based on fibre counts, exposure for former residents was determined using duration of residence and period of residence (before and after a new mill was commissioned in 1957) and interpolation from periodic hygienic measures undertaken from personal monitors between 1966 and 1992. Cox proportional hazards modelling was used to relate benign pleural disease, asbestosis, asbestos exposure, and mesothelioma. Results: Between 1990 and 2002, there were 76 cases of mesothelioma (56 of the pleura and 20 of the peritoneum). Cases had more radiographic evidence of (all) benign pleural disease, pleural thickening, blunt/obliterated costophrenic angle, and asbestosis than non-cases. Adjusting for time since first exposure (log years), cumulative exposure (log f/ml-years), and age at the start of the programme, pleural thickening (OR = 3.1, 95% CI 1.2 to 7.6) and asbestosis (OR = 3.3, 95% CI 1.3 to 8.6) were associated with an increased risk of peritoneal mesothelioma. There was no increased risk for pleural mesothelioma. Conclusion: The presence of benign pleural disease, in particular pleural thickening, and asbestosis appears to increase the risk of mesothelioma of the peritoneum, but not of the pleura beyond that attributable to indices of asbestos exposure in this cohort of subjects exposed to crocidolite.     

Evaluation of the pleural malignant mesothelioma patients with the relation of asbestos exposure

There has been a recent increase in the number of patients with malignant mesothelioma in parts of Japan where Naval shipyards were located during World War II. This can be attributed to asbestos exposure during work in the shipyard. We have studied eight patients who were seen between 1984 and 1986 in Kure Kyosai Hospital. All had a history of asbestos exposure, seven in a Naval shipyard. In these seven cases, the latent period between exposure and the appearance of mesothelioma was 43-49 years. Quantification of asbestos bodies in the lung indicated a high concentration in all patients, giving further weight to the concept that asbestos is an etiologic factor in the genesis of mesothelioma. The major types of asbestos fibers were crocidolite and amosite. The study of structural features of these two types may provide a clue to the pathogenesis of the disease.     

The mortality of amphibole miners in South Africa, 1946-80.

A cohort was established in 1981 of all 7317 white male employees in the amosite and crocidolite mines in South Africa whose names had appeared in the personnel records (initiated between 1945 and 1955) of the major companies. Some of the men had been employed as early as 1925, but only 8% had had more than 10 years of service. Three subcohorts were defined: 3212 men whose only exposure to asbestos was to amosite; 3430 exposed to crocidolite; and 675 to both amphiboles. No deaths or losses to view occurred before 1946, and 5925 men (81%) were known to be alive at the end of 1980. Losses to view numbered 167 (2%), and there had been 1225 deaths (17%), an excess of 331 over the number of deaths expected on the basis of the mortality of all white South African males. The fibre related excesses were of mesothelioma, lung cancer, and other respiratory diseases, but there were other excesses perhaps mainly related to socioeconomic factors including lifestyle. When cause of death was determined according to "best evidence" (after study of clinical, radiological, biopsy, and necropsy reports in conjunction with the death certificate), there were 30 deaths due to mesothelioma (22 pleural, six peritoneal, two other) and 65 due to cancer of trachea, bronchus, and lung. Various analyses of these deaths showed that crocidolite had higher toxicity than amosite for lung cancer and this was most pronounced for mesothelioma; there can now be no question that crocidolite is far more dangerous than amosite at least in so far as mesothelioma is concerned. Nevertheless, crocidolite induced mesothelioma appeared only in men who had been exposed for long periods, at least 12 months, but on average about 15 years.     

Malignant pleural and peritoneal mesotheliomas in former miners and millers of crocidolite at Wittenoom, Western Australia

Methods: A group of 6493 men and 415 women who had worked at the former Wittenoom crocidolite mine and mill at some time between 1943 and 1966 have been followed up throughout Australia and Italy to the end of 2000.

Results: The cumulative number of mesotheliomas up to 2000 was 235 in men (202 pleural, 33 peritoneal) and seven (all pleural) in women. There had been 231 deaths with mesothelioma (9% of known deaths).

Conclusions: The number of deaths in men with mesothelioma between 1987 and 2000 was at the low end of the predictions made earlier based on the number of cases to 1986. If this trend continues, it is predicted that about another 110 deaths with mesothelioma will occur in men by 2020.

     

The epidermiologic relationship between pleural mesothelioma and asbestos exposure.

This paper describes an investigation of 85 notifications of either probable or possible mesothelioma to the Finnish Cancer Registry from 1953 through 1969. The investigation covers characteristic epidemiologic features and the possibility of the mesothelioma being connected with occupational or other exposure to asbestos. The incidence rate of pleural mesothelioma was estimated at 1.1 per million per year. The male/female ratio was 1.3:1, which greatly differed fsrom that for malignant neoplasms of the bronchus or lung in Finland in 1960. The mean age at dealth from mesothelioma was over 7 years lower than that for bronchial carcinoma. The ratio of the crude incidence rates for the urban and rural populations was 4.2; the corresponding ratio for cases of carcinoma oopulations was 4.2; the corresponding ratio for cases of carcinoma of the bronchus or lung was 1.2 in Finland in 1960. THE LAST OCCUPATIONS OF THE 82 DECEASED PERSONS, OBTAINED FROM THE DEATH CERTIFICATES, WERE DIVIDIED INTO THREE CATEGORIES ACCORDING TO POSSIBLE ASBESTOS EXPOSURE. Exposure was present or probable in 9 (11.0) cases, 28 (34.2 %) had a possible exposure, and in 33 (40.2 %) cases exposure was absent or unlikely. The occupation of 12 (14.6) persons was unknown. Additional information of possible exposure history was obtained by interviewing the relatives of 10 mesothelioma patients. For half of the persons a definite, although in some cases trivial, exposure to asbestos could be ascertained. For the other five persons no exposure, either occupational, neighborhood or domestic, to asbestos could be traced. The residental distribution of the 85 persons with mesothelioma revealed no clustering of cases.