A new Nature Sustainability study solves a long-running ice-core puzzle and shows how a profitability rule kept cleaner technology off a giant smelter for about a decade

A single copper operation in southwestern Montana sent enough arsenic into the air a century ago to expose roughly 50.5 million Americans, about 55 percent of the U.S. population at the time, to levels associated with higher lung cancer risk, according to a study published Nov. 10 in Nature Sustainability. Another 600,000 Canadians, about 11 percent of that country's population, were also downwind.

The source was the Anaconda Copper Mining Company's operations near Butte and its massive smelter in Anaconda. Researchers led by the Desert Research Institute (DRI) found that the company waited about a decade to install electrostatic precipitators, a pollution-control device patented in 1908 and adopted quickly by many other Western smelters.

The finding matters beyond history. The smelter's footprint is still being cleaned up today, and the study offers a measured, data-backed example of how regulatory loopholes can shift the cost of pollution onto the public for generations.

Ten Ice Cores and a Century-Old Puzzle

For years, ice-core scientists had noticed something odd. Arsenic, bismuth and antimony in Arctic ice rose sharply into the 1910s and then fell, even as overall industrial pollution kept climbing through the 20th century.

DRI ice-core scientist Joe McConnell told Eos that the early peak simply did not fit the expected pattern, which is what sent his team looking for a single dominant source.

The team analyzed 10 ice cores from Greenland, the Russian Arctic and northern Canada. Arsenic was measured directly in four cores, and bismuth, which tracked arsenic closely during that era, served as a stand-in for the other six. Arsenic in central Greenland peaked around 1914 at about 15 times preindustrial levels, and heavy-metal pollution rose roughly 15-fold between about 1886 and 1918.

When the researchers lined up Anaconda's historical production records against the ice, the curves matched almost perfectly, and lead isotopes in the ice pointed the same way, according to DRI's announcement. The levels dropped sharply after 1919, which the team attributes to precipitators that cut heavy-metal emissions by about 65 percent.

Ruling Out Tacoma, Sudbury and the Keweenaw

Because arsenic and lead stay airborne for only days, comparing cores across the Arctic helped the team narrow down possible sources. Several other North American operations were considered and set aside.

The Tacoma smelter in Washington state was too small to produce the levels seen in the ice. Lead isotope ratios from mining around Sudbury, Ontario, did not match. Michigan's Keweenaw Peninsula mines produced mostly native copper, so their emissions of these particular metals were relatively low.

Combining ice data, atmospheric modeling and mine records, the team estimates Anaconda released about 11,000 tons of arsenic and 4,400 tons of lead per year during two to three decades of peak production. "Everything points to Anaconda," McConnell said in DRI's release.

Karl Kreutz, a University of Maine Earth scientist not involved in the study, told Eos the work was a careful piece of detective work and the first attempt he had seen to trace arsenic this way.

A Profitability Rule That Delayed Cleaner Air

The policy lesson is the most striking part of the paper. Under Montana policy at the time, McConnell told Eos, polluters "only had to install pollution controls if it was profitable to the polluter." That standard let Anaconda postpone precipitators for roughly 10 years — even as other Western smelters adopted them.

The researchers estimate that installing controls a decade earlier would have prevented an additional 60,000 tons of arsenic and 24,000 tons of lead from spreading across much of North America. About 1.4 million Americans and Canadians, including all residents of Montana and Wyoming, were exposed to lead above 1 nanogram per cubic meter, a level the team links to an estimated loss of 2.6 IQ points.

For comparison, recent U.S. and European arsenic levels run 0.1 to 0.5 nanograms per cubic meter, 12 to 60 times lower than the historical exposures. The World Health Organization's maximum for chronic exposure is 6 nanograms.

The evidence has clear limits — and the authors state them. This is a reconstruction based on ice cores, atmospheric modeling, and historical production data, not a measurement of anyone's health. The researchers are not epidemiologists and describe health effects as potential rather than proven. The study also did not fully assess arsenic that settled onto crops, soils, and water. What it does establish, with multiple independent lines of evidence, is the timing, source, and approximate scale of the pollution.

Butte and Anaconda Still Live With the Legacy

The contamination is not only in Arctic ice. In 1983, the EPA placed the Anaconda Co. Smelter site on the Superfund National Priorities List. The site covers more than 200 square miles in Anaconda-Deer Lodge County, and cleanup of arsenic and lead in residential and commercial soils has continued for decades under EPA oversight, with Atlantic Richfield Company conducting the work. Farmers, ranchers, and the U.S. Forest Service sued the company long ago over crop damage, livestock poisoning, and forest loss.

For residents of the Anaconda and Butte areas today, the practical steps are local. Households with questions about yard soil can contact the EPA's Montana office or the county's community programs, and parents of young children can ask a pediatrician about blood lead testing. Families elsewhere in the country face no new risk from this study, since the emissions it describes ended generations ago.

The exposure is not ancient history for many families. The researchers note that the parents and grandparents of many Americans alive today breathed this pollution, and Science News reported the team's estimate that the company emitted roughly 30 tons of arsenic per day at its peak.

The team hopes epidemiologists will use the new records to study long-term health effects. For policymakers, the takeaway is concrete: pollution controls tied to a company's profits left a whole continent breathing the difference, and that history is now preserved in the ice.

What Readers Want to Know

What did the study find? Arctic ice cores show an arsenic spike from about 1886 to 1918 that researchers traced to the Anaconda Copper Mining Company's operations in Montana.

How many people were exposed? About 50.5 million Americans and 600,000 Canadians lived where modeled arsenic exceeded 1 nanogram per cubic meter, roughly 55 percent of the U.S. population at the time.

Why did pollution drop after 1919? The smelter installed electrostatic precipitators, which the ice record suggests cut heavy-metal emissions by about 65 percent.

Does the study prove people got sick? No. It estimates exposure. The researchers say health effects were potential and call for epidemiologists to study them.

Is the area still contaminated? Yes. The Anaconda Co. Smelter is an EPA Superfund site where soil cleanup has continued for decades.

Should people outside Montana worry today? No new action is needed. The emissions ended long ago, and modern arsenic levels are 12 to 60 times lower.