Twenty-one new cases of mesothelioma have been identified in a group of 69,000 mine workers that the Minnesota Department of Health and the University of Minnesota have been studying since the late 1990s.

Image/National Heart Lung and Blood Institute
Image/National Heart Lung and Blood Institute

Eighty cases of the rare cancer had previously been found in the workers, bringing the total number of cases reported in the workers to 101. The workers were employed in Minnesota’s iron mining industry between the 1930s and 1982. Both the 21 new cases and the previously identified cases occurred in individuals who worked in a number of places across the Iron Range, and were not limited to any one mining company or location.

The primary risk factor for mesothelioma is exposure to asbestos. The illness is generally fatal.

MDH officials stressed that the new cases were not unexpected.

“This form of cancer has an extremely long latency period,” said Dr. Ed Ehlinger, Minnesota Commissioner of Health. “The interval between exposure to the agent that causes the cancer and the time when the cancer appears can be as long as 40 or 50 years, possibly even longer. We have always expected to see additional cases as time went by, in people who were exposed many years ago. We expect to see still more cases going forward.”

New cases also continue to be diagnosed in the general population, Dr. Ehlinger noted, even though commercial use of asbestos began to decline in the late 1970s and early 1980s. Some of the cases may involve people exposed before the general decrease in asbestos use. Others may involve exposure to asbestos that continues to be present in many industries, commercial buildings and residences. Overall, mesothelioma rates in the general population have remained stable.

The first mine worker mesothelioma cases were identified in a 2003 study conducted by MDH, providing the first evidence ever produced that the illness was occurring in mine workers. The study concluded that 14 of the 17 workers diagnosed with mesothelioma in that study had possible or probable exposure to “commercial asbestos.” In two cases, not enough was known about the work history of the individuals involved to determine whether they had been exposed to asbestos.

Commercial asbestos is found in a variety of industrial settings and materials – such as insulation – and is not unique to the mining industry. Removal of commercial asbestos remains an ongoing process in many mining company facilities.

The 2003 study concluded that those first 17 mesothelioma cases were most likely caused by exposure to commercial asbestos, but MDH investigators were not able to rule out a role for the dust generated by taconite processing. The significance of taconite dust and other mineral fibers in causing mesothelioma in the mine workers has been the focus of ongoing concern and investigation, both for MDH and for the University.

The work being done with the mine worker group has proceeded against a backdrop of generally elevated mesothelioma rates in Northeastern Minnesota. Mesothelioma rates in that part of the state have been elevated since at least the late 1980s, when the first data became available from the Minnesota Cancer Surveillance System (MCSS).

MDH has identified a number of factors that may be contributing to the elevated incidence of mesothelioma in that part of the state. In addition to the possible role of mining-related work, they also cite the experience of workers at the former Conwed facility in Cloquet, which made asbestos tile from 1958 to 1974.

To date, 39 cases of mesothelioma have been identified among 5,200 former Conwed workers. New cases continue to be diagnosed in that group.

MDH officials emphasized that, based on the available evidence, the elevation in mesothelioma cases in Northeastern Minnesota is most likely an occupational health concern, and does not reflect any increased risk for the broader community.

They point to the fact that the elevation in mesothelioma rates is only occurring in men, while women in the region have actually been experiencing lower than expected mesothelioma rates. Officials say this pattern suggests that the elevation reflects something people were exposed to in the workplace, rather than community-wide.