Objectives An excess of mesothelioma has been found in iron ore miners in northeastern Minnesota. Miners of taconite, the current form of ore mined, face a number of potentially hazardous exposures. The intensive processing of taconite ore combined with the unique and variable geology of the Mesabi Iron Range create the potential for exposure to natural amphibole fibres, cleavage fragments, and respirable dust. Exposure differed in natural ore (hematite) mining where minimal processing was required. We performed a case-control study to evaluate the risk of mesothelioma associated with taconite mining employment.
Methods Mesothelioma cases were identified through the Minnesota Cancer Surveillance System (MCSS) and death certificates from a cohort of 68,737 miners employed between the 1930s and 1982. Four controls of similar age were selected from the mining cohort for each case using an incidence density sampling protocol. Odds ratios and 95% confidence intervals were estimated with conditional logistic regression for years employed in all iron ore mining, taconite mining, and the earlier natural ore mining.
Results Eighty cases of mesothelioma were identified in this cohort. There is a slight elevation of mesothelioma risk for each additional year of work in iron ore mining (OR = 1.02, 95%CI 1.00–1.04) and taconite mining (OR = 1.03, 95%CI 1.00–1.07). No association was observed for years of employment in natural ore mining.
Conclusion These initial findings indicate a potential relationship between mesothelioma and employment in taconite mining and processing. The extent to which these cases are caused by exposure to taconite dust components, including cleavage fragments and non-fibrous amphiboles found in some zones of the Mesabi Range, or commercial asbestos will be explored in future analyses.