One of the largest asbestos plants in the world between 1912 and 1980 was located in Manville, in Somerset County, New Jersey. The plant employed up to 3500 people at one time, and manufactured asbestos products for more than 70 years. The plant primarily used chrysotile asbestos to produce multiple products. Beginning in the 1970s, the factory moved away fromusing asbestos products. Enterline and Henderson in 197256, studied a cohort of 1348 men who retired from this company between 1941 and 1967. In the whole group they found onlyone case of mesothelioma. They published a follow-up report in 1979, which included all deaths up to 1973. For the entire period the cohort had an overall mortality rate 20.4 per centhigher than that of all United States white males. The excess was due almost entirely to cancer and diseases of the respiratory system. They found 5 cases of mesothelioma in the cohort, including one that had been missed previously.In contrast to Enterline’s finding of only a mildly increased incidence of mesothelioma, two reports indicating a marked increase in cases of mesothelioma surfaced from a hospital in the vicinity of the Manville plant.
In 1967 a surgeon in Somerset hospital reported 17 cases of mesothelioma (9 pleural and 8 peritoneal) over a three year period. He explained the high concentration by the close proximity of the asbestosmill, 3 km from the hospital. There was also a large asbestos dump adjacent to a residential area. Radiologists from the same hospital also reported 10 cases, all with some occupational asbestos exposure. Several years later, the surgeons had treated another 36 cases, and searching through the medical records of neighboring hospitals they found an additional 19 cases, making the total 72. All of the cases where a history was available had an identifiable asbestos exposure in the nearby asbestos mill, except for 2 women who had environmental exposure. Over the years from 1951 to 1972 the incidence increased yearly from 1 per year to 8–9 per year.The discrepancy between these two studies is easy to explain, and highlights one of the difficulties with epidemiological studies of rare diseases with long latency periods. Enterline’s inclusion criteria required that members of the cohort had retired at a certain time, and had lived at least until age 65. They missed all the cases of mesothelioma in people who had already died, or who had not retired.This plant was also known to be extremely dusty, supporting the theory that the degree of exposure is a determining factor in the incidence of mesothelioma. Employees were quoted in the hospital charts as complaining ‘It was like working in a snowstorm’, and ‘I worked in an asbestos fog 20 minutes daily for 22 years’. In 1984 McDonald and colleagues conducted a study of 3641 workers employed for one month or more in a friction products plant in Connecticut from 1938 to 1958.
Like the Manville plant, this factory also used almost exclusively chrysotile asbestos. By 1983, of 3513 employees traced, 1267 had died; death certificates were obtained for 1228. No cases of mesothelioma were identified. These results support the amphibole hypothesis. However, exposures to asbestos in this plantwere probably very low since there were no deaths attributed to asbestosis, which is a disease marker for high exposures.