HIBBING - A nearly $5 million, five-year effort to find out why so many Iron Range residents have died from a rare lung disease has traced the deaths to the taconite industry, but stops short of finding a precise source of the problem.

The final report of the University of Minnesota’s Taconite Workers Health Study was made public Monday afternoon at an open house at the Hibbing Memorial Building.

Early results from the study were released in April 2013 and found a definitive correlation between working in taconite mines and processing plants and mesothelioma, an always-fatal lung disease previously associated only with commercial asbestos.

But when researchers tried to mine deeper into the data, and even as they studied hundreds of Iron Range workers’ health histories, they couldn’t answer the question of where the microscopic mineral fibers that caused the mesothelioma are coming from - either commercial asbestos, such as insulation, or from even smaller fibers that occur naturally in the taconite-bearing rock of the eastern Iron Range.

The result leaves unanswered whether or not the microscopic fibers found in taconite rock actually cause lung disease.

“We don’t know for sure,” Jeffery Mandel, University of Minnesota professor and lead researcher on the project, said in a briefing with reporters before Monday’s public meeting.

Because taconite workers were exposed to both long-fiber asbestos and the short fibers in taconite, there is no way to distinguish the causal relationship to disease, Mandel said.

“If you had exposure to the longer fibers (as a taconite worker) you also had exposure to the shorter ones,” he said. “Statistically, we’re not sure what is driving the relationship.”

While it had been widely held that only long mineral fibers caused the disease, Mandel said shorter fibers also might be culprits.

“What we can’t determine is the contribution of each of them” to each victim’s disease, said John Finnegan, dean of the university’s School of Public Health.

Taconite workers have a 3.3 per 1,000 person rate of mesothelioma by the time they reach 80 years old, far higher than the 1.4 per 1,000 person rate for Minnesota as a whole.

Mesothelioma has a long latency period, and often doesn’t show up for 40 years after the initial exposure. Only recently has formal asbestos been removed from the workplace for health reasons. If the unusually high level of lung disease continues, researchers may eventually conclude that the cause comes from the taconite fibers, researchers said.

“So if it keeps showing up, then we know it’s the taconite?’’ asked David Malkar, a member of the Safety Committee at United Steelworkers Local 2660 that represents workers at U.S. Steel’s Keewatin Taconite.

In response, health experts shook their heads yes.

About 100 people attended Monday’s question and answer session with the university experts. But not all were convinced that all has been done to find out why Steelworkers have high rates of lung problems.

“I can’t have a lot of faith in this study,’’ said Robert Bassing, now retired after working for 33 years at U.S. Steel’s Minntac plant in Mountain Iron.

Bassing said researchers didn’t spend enough time talking to workers who had been exposed to commercial asbestos on the job.

Bassing suggested more state money be spent on autopsies of workers who die of lung ailments to see if exposure on the job contributed and to hold taconite companies accountable if the exposure was from the workplace.

Increased heart disease

The study also found an “unexpected” relationship between workers exposed to the small fibers found in the rock and an elevated level of heart disease. Taconite workers appear to have an approximately 30 percent higher rate of heart disease than the general Minnesota population, the researchers said.

Again, however, they were unable to sort out whether the fibers are indeed a risk factor or a coincidence, with the heart disease actually caused by traditional factors such as obesity, smoking or cholesterol levels.

“We weren’t set up to determine that answer,’’ Mandel said.

But researchers strongly urged all industry workers to monitor their heart health and inform their physicians of the possible risk. The study found that nearly two-thirds of all taconite workers surveyed were current or past smokers.

Plants generally safe, but silica levels high

Authors of the study said that taconite plants now appear to be generally safe places to work - noting that extensive air-quality testing found that current fiber levels are below the exposure limits set by the National Institute for Occupational Safety and Health.

Moreover, tests of air quality in Iron Range cities found dust and particulate levels below that of a control monitor at the university in Minneapolis.

But the university experts strongly recommended that any taconite industry workers exposed to dusty conditions should wear respirator masks to filter out small fibers. The study authors said those masks are generally available to workers, but infrequently worn.

Air quality at the facilities wasn’t always as good, however. Study authors said their reviews of past data showed fiber exposure above current levels during past periods.

Taconite iron ore mining is “by its nature a very dusty industry,” said Gurumurthy Ramachandran, professor in the U of M’s Division of Environmental Health Sciences and an author of the final report.

“Given the known hazards in mining, the process of avoiding exposures generated in the mining and processing of taconite ore is critical,” the report concludes. “Exposure avoidance is the most effective way to minimize disease risk.”

The one exception to current acceptable air quality in the plants is silica, for which at least some of the recent air sample testing found levels above federal standards. It’s not clear what role if any silica exposure may play in the elevated level of lung disease.

Silica can be toxic to the lining of the lung.

Silica standards “are still not being met routinely” at the six operating taconite plants, Ramachandran said.

Mandel noted that an unusually high number of Iron Range taconite workers given chest x-rays as part of a health screening for the study - nearly 17 percent - had lung scarring consistent with silica exposure, but that it’s not clear whether the silica caused the scarring.

“It’s definitely higher than it should be,” Mandel said. “We can’t get our hands around it (lung scarring) as being from one type of exposure or another.”

No update on deaths since 2010

Through 2010, about 80 Iron Range mine workers had died from mesothelioma in recent years; 57 of those worked in taconite operations. There’s been no update since then to see if more have died, Mandel said, although one of the study’s recommendations is for the state to pay for ongoing analysis, checking every five years to see how many people have died and whether the problem is getting worse or diminishing.

The $4.9 million Minnesota Taconite Workers Health Study by the university was ordered by the 2008 Legislature in an effort to provide answers after years of questions about why Iron Range mesothelioma rates were so high.

The study made no suggestions on changes in state or federal regulations for mining operations, but the full report has been presented to state lawmakers and state agencies that regulate mining for their consideration.

Highlights of the Taconite Workers Health Study

* The unusually high level of mesothelioma on the Iron Range can be traced directly to workers being exposed over long periods to elongated mineral particles, or fibers at taconite operations, especially in dusty conditions.

* It remains unclear whether the fibers entering taconite workers’ lungs and causing mesothelioma are from commercial asbestos, found in insulation and other industrial uses, or whether the fibers came from the rock where taconite is found, or both.

* The study found an unexpected link between high levels of heart disease and exposure to small fibers found in the dusty environment of mines. Taconite workers screened had a 30 percent or higher rate of heart disease than the general Minnesota population. Researchers say this link needs to be studied further.

* Current air-quality standards tested extensively at all six of Minnesota’s major taconite operations are “below the exposure limit” as set by the National Institute of Occupational Safety and Health for the fibers that might cause lung disease. But data reviewed from past air testing shows there were periods when the fibers were over the current allowable limit.

* About 1,188 taconite workers received health screenings as part of the study, as did 496 spouses of workers. An unusually high percentage of taconite workers given chest X-rays - nearly 17 percent - had lung scarring, although no specific source was identified. The scarring is not necessarily a precursor to lung disease. That compares to 4.5 percent scarring for their spouses, which is within the range for the general population.

Previous conclusions released in 2013:

* Every year a worker spent in the industry increased the risk of mesothelioma by about 3 percent.

* Taconite workers have higher-than-expected risks of all types of cancer and heart disease, the researchers said. That leads them to believe other factors, including lifestyles, may be at work.

* Air quality in communities surrounding the mines is better than in most parts of Minnesota in terms of particulates in the air.

* Exposure to dust from taconite operations is, in general, within safe limits.

* Spouses of taconite industry workers are at no greater risk of contracting dust-related lung diseases than the general public in Minnesota.