Asbestos
The adverse effects of asbestos were first observed in the early 1900s and the relationship to mesothelioma was suggested in the 1940s. One of the earliest reports linking mesothelioma to occupational asbestos exposure came out of the medical clinic at anasbestos mine in Canada. At a scientific meeting in 1952 Cartier, then in charge of the industrial medical clinic at Thetford Mines, Quebec, reported eight cases of respiratory cancer, two of which he described as pleural tumors. He declared that two such rare cancers in a small series of only eight cases suggested an occupational origin.
By 1960 the scientific community generally recognised asbestos as a cause of mesothelioma. However, the issue was far from settled. In a national survey of mesothelioma in Canada from 1960 to 1968, McDonald and colleagues found a history of asbestos contact in a relatively small proportion of cases – mostly in insulation and allied trades rather than in the asbestos-producing industry. They surveyed pathologists across the country to find all cases of mesothelioma after 1959. They found 165 cases (111 pleural,47 peritoneal, 3 both and 4 pericardial). Updates in subsequent years showed aslightly increased association with asbestos exposure, but still lower than expected. When pathologists reviewed the cases the diagnosis was confirmed in only about 50 per cent of the cases, among whom the incidence of asbestos exposure was also higher.
A follow-up analysis added a chrysotile mining industry cohort and two small groups of employees in gas mask factories to the survey data. They found 254 fatal cases ofmesothelioma (181 men, 73 women) in Quebec from 1960 to 1978. They were able to obtain occupational and residential histories for the majority, and found that only about 40 per cent of the male cases and 5.4 per cent of the female cases were attributable to occupational asbestos exposure (asbestos manufacture, production, insulation, heating trades, shipyards, and construction). Six people probably had household exposure. The intervals between first employment and death from mesothelioma were longer for miners and millers than for manufacturing workers. All the miners and millers had pleural mesothelioma, while the factory workers included 8 with peritoneal mesothelioma. The incidence did not clearly increase over this time period. Subsequent studies showed an increased percentage of cases attributable to asbestos exposure. Ruffie and colleagues conducted a retrospective study of 332 patients diagnosed with pleural mesothelioma at several teaching hospitals in Ontario andQuebec between 1965 and 1984. They found 396 patients with mesothelioma, including 332 of the pleura. Of the patients with pleural mesothelioma, 262 were menand 70 were women. Ages ranged from 22 to 85 years. About 60 per cent of those with history available had identifiable asbestos exposure. Only 3 were known to have had household exposure and 68 per cent of men and 17 per cent of women had identifiable occupational exposure.
With more detailed occupational histories, more exposures may have been found. In Sherbrooke and Quebec City, almost all of the exposed patients came from the asbestos mining areas of Asbestos and Thetford Mines – mostworking in asbestos processing, as opposed to mining. In Ontario, an important sourceof exposure was from asbestos-cement factories, where 21 cases were observed in acohort of 535. Twenty-five per cent of patients with mesothelioma in Ontario wereimmigrants who had asbestos exposure in their country of origin. In 1994 Spirtas and colleagues published a study aimed at defining the attributable risk of asbestos exposure in the United States. They used the Los Angeles County Cancer Surveillance Program, the New York State Cancer Registry (excluding New YorkCity), and 39 large Veterans Administration hospitals to identify people diagnosed with mesothelioma from 1975 to 1980. They identified 208 cases of pathologically confirmed mesothelioma. Controls were 533 people who had died of other causes. They interviewed immediate family members to obtain asbestos exposure history. Among men the attributable risk for asbestos exposure was 88 per cent, among women it was 23 per cent(although the confidence interval was very wide at 3–72%).
The increasing incidence of mesothelioma in the United States is primarily due to the increased incidence amongmen, thus probably reflecting more occupational exposure. Most patients with mesothelioma do have a history of exposure to asbestos, although it may have been brief and remote in time. The variability in percentages ofcases with identifiable asbestos exposure may be related to several factors: incompletehistory taking, unknown or hidden occupational exposures, or environmental exposure.