The Minnesota Department of Health and the University of Minnesota’s joint study has identified a total of 101 cases of deadly mesothelioma in iron ore workers.

Asbestos is a naturally-occurring mineral. It is comprised of millions of microscopic, glass-like fibers. These fibers make exceptional thermal insulators. Since asbestos transfers heat very poorly, it is a nearly-ideal insulation material. Additionally, asbestos is functionally fireproof, even at industrial temperatures. During the Industrial Revolution, the useful properties of asbestos led to the material’s widespread adoption. However, it rapidly became apparent that working with asbestos could present serious health risks, including mesothelioma.

Mesothelioma is a particularly deadly form of cancer. Often mistaken for a lung cancer, mesothelioma actually starts in a membrane called the mesothelium, a sort of protective sac that encloses the chest cavity. Roughly 90 percent of mesothelioma patients die within two years of their diagnosis, but new treatments involving heated chemotherapy drugs may help extend the life expectancy of many patients suffering from mesothelioma.

One unsettling feature of mesothelioma is that it has a long latency period. It can take years for mesothelioma to appear after an exposure to asbestos; in fact, mesothelioma lawsuits are still being filed by plaintiffs that were exposed to asbestos while building warships for World War II. So even though asbestos was phased out in the United States in the 1970s and 1980s, patients are still developing mesothelioma.

In this findings of this six-year study, researchers found that iron ore miners in Minnesota have developed mesothelioma. Northern Minnesota in general has a higher rate of mesothelioma, possibly due to industrial work in that area, but cases were found throughout the state. It is possible that iron miners in Minnesota mined iron in asbestos-rich mines, and developed mesothelioma from these exposures. The study also found dozens of cases among workers in plants that produced asbestos-bearing ceiling tiles. At one time, these asbestos-bearing ceiling tiles were widely used because their insulating properties meant lower heating and cooling bills. On top of this, they were fireproof. But thousands of employees in Minnesota may have been exposed to carcinogenic asbestos fibers in the process of making these tiles.

Researchers found that iron miners had a higher rate of several other heart and lung diseases, in addition to mesothelioma. Former iron miners were 30 percent more likely to die from heart disease than their peers. Researchers recommended that miners digging for taconite ore—a type of iron ore that may appear alongside asbestos—should take extra care and wear protective equipment. It is not clear if taconite itself may cause mesothelioma, or if the material co-occurs with asbestos. In general, nearly all cases of mesothelioma can be traced to asbestos exposures.