The Idaho Department of Environmental Quality Board of Environmental Quality Wednesday pulled back a permit granted last year to Perpetua Resources for its planned gold and antimony mine at Stibnite in Valley County.

The Nez Perce Tribe and multiple conservation groups had appealed the air permit, saying it allowed excessive dust-based toxic air pollution beyond the mine’s boundaries, threatening land users with carcinogens like arsenic. The main focus of the challenge was the permit’s “project-specific adjustment factor for ambient concentration.” State rules do allow an “acceptable risk” level of contaminants released into the air, depending on a variety of factors.

The permit DEQ granted Perpetua allowed greater emissions than generally allowed because of the way expected toxic air emissions (known as Ambient Air Concentration for Carcinogen, or AACC) were calculated over the 16-year predicted life of the mine versus a standard human life span, explained the Idaho Conservation League’s Will Tiedemann.

“It’s kind of like saying, if you were allowed so much arsenic to be exposed to in 70 years, let’s allow Perpetua to emit that much arsenic in just 16 years,” he said.

ICL was among the groups challenging the air permit, and have filed multiple challenges to the proposed mine over other environmental concerns.

DEQ board members ultimately agreed that pro-rating emissions in that way was not a safe calculation.

“DEQ’s project specific adjustment factor creates a new, higher level of cancer risk for 16 years,” said board Vice President J Randy MacMillan during Wednesday’s determination meeting. He said the approved permit did not fit within any measure of ‘acceptable risk’ identified in the state’s air quality rules.

MacMillan spoke at length Wednesday from a prepared statement regarding the board’s reasoning behind remanding that portion of the air permit. The board in March heard oral arguments over the issue, but deliberated in closed executive session before primarily relying on MacMillan to speak publicly.

“By applying the, what I would call short sighted, project-specific adjustment factor to the Stibnite Gold project,” he said. “DEQ created a misleading risk analysis that greatly underestimates the actual cancer risk.”

The board unanimously voted to pull the permit back to DEQ to be reworked based on that finding. It upheld several other aspects of the permit which had also been challenged by the outside groups.

It’s not clear how the decision could affect Perpetua’s planned site operations. The air permit is one of a handful of ‘major’ permits needed to fully operate, in addition to approvals through the federal National Environmental Policy Act (NEPA) review process. A Forest Service spokesman said the final environmental impact statement and draft record of decision for the Stibnite Gold project should be released this summer, with a final record of decision issued by the end of the year. Those dates are only estimates.

Prep work at Stibnite has been underway for several years already, including as of 2022, SuperFund-sponsored cleanup of some previous mining leftovers. Perpetua has long said their plan is to mine the area and fully remediate their impact and the mess previous companies left behind after decades of mining.

Perpetua could appeal the DEQ board’s decision in state court. In a statement, the company characterized the board’s action as remanding only a ‘narrow issue’ for additional review, and said it will work with DEQ to respond. But Perpetua reserved additional reaction until after it receives the full written order from DEQ May 9.

Tiedemann called the board’s decision a big win that validates opposition groups’ concerns about the project.

“It’s maybe not as glamorous or as romanticized as water,” Tiedemann said. “But harmful effects on air quality can be just as significant and harmful to human health and the environment as water pollution. And so I think it’s an important win.”