"You must take action now. There needs to be a discussion and further investigation," Fields told the board.
The tribe and environmental groups, she said, want the state to intervene.
Mill owner Energy Fuels Resources maintains the mill's radon emissions, which are the subject of constant monitoring, are "de minimus" and within acceptable limits.
"The calculations used by the activist groups and the tribe are wrong. And frankly, their results are preposterous," said Curtis Moore, the company's spokesman. "They are applying an EPA formula incorrectly, with incorrect inputs."
The White Mesa mill's five ponds total 145 acres in area and sit only three miles from the White Mesa tribal community.
A radioactive metal, radium in trace amounts appears in the waste stream from uranium milling and decays into radon gas.
The Clean Air Act has set the emission limit for the radioactive gas at 20 pico Curies per square meter per second. Breaching that limit could threaten people's health within a 50-mile radius.
Environmentalists agree their radon emission numbers seem preposterous, but they are what the federal formula predicts would come off the ponds, given the intense concentration of radium they hold.
"If we are right, this is a public health emergency," said Anne Mariah Tapp, energy program director for Grand Canyon Trust, which has teamed with Uranium Watch on the issue. "At a minimum, they need to be proving us wrong."
At Wednesday's meeting, Utah Division of Air Quality director Bryce Bird noted that the EPA is revising rules for radon emissions.
"Our concern would be: What is the exposure? And what can be done to reduce that?" Bird said.
"There is coordination between two federal agencies and two state agencies," he added. "The only thing we can do is use the existing regulations and wait and see what EPA does."
EPA officials could not comment on the groups' calculations and whether they applied the formulas correctly.