There is new proof toxic mercury travels downwind from Nevada gold mines, says an environmental group that conducted its own tests.

The Idaho Conservation League said one day's testing with a portable mercury monitor showed air coming from the mines' direction has more mercury in it - sometimes at exceptionally high levels. A few samples detected about 700 nanograms of mercury per cubic meter of air. Five is a natural background reading.

For the league's Justin Hayes, the numbers themselves are not the issue. The trends they suggest are.

"Our data pretty conclusively proves there is mercury in the air," he said.

So far, regulators and the mining companies do not agree. Since the Boise-based environmental group has not released the data publicly, they have not had a chance to review its methods and results.

Mercury is a well-studied neurotoxin that builds up in the food chain. High exposures can harm the human brain and nervous system, especially in the unborn.

Concern about mercury swelled in Utah this spring when scientists reported they had found some of the highest mercury ever measured in the Great Salt Lake. In Idaho, high concentrations prompted a statewide mercury advisory for fish.

Now the question is: How is the mercury contaminating the streams and lakes of the two states?

The U.S. Environmental Protection Agency's San Francisco office points to the global atmosphere. Mercury can be carried around the world on high air currents and drop to the ground by way of rain or on the air, said David Jones, associate director for waste management in that office.

He said his agency is performing its own ground-level tests to better understand the Nevada mine's role. Of Hayes' data, he said: "Any data that's accurate is obviously useful to the discussion so that we can have an informed discussion."

The Idaho Department of Environmental Quality has another explanation for the high mercury levels. It points to studies by a Nevada researcher whose work shows pockets of natural mercury in the ground easily becomes airborne in the region's hot, dry climate.

But forest fires and ground-level ozone pollution can make tracing the source of mercury difficult, said Cheryl Heying of the Utah Division of Air Quality. State regulators from Utah, Idaho and Nevada are working with the EPA and the mining industry to conduct tests that might answer the questions. And, in that spirit, they all welcomed any new information to help solve the puzzle.

"We'd really like to see that data," said Heying.

"I would like to see that data to assess it," agreed John K. Mudge, director of environmental affairs in North America for Mining Corp., which owns one of the mining sites the Idaho Conservation League monitored.

Hayes' group took its measurements July 25, stopping upwind and downwind of mines outside of Carlin, just south of the Idaho border. They used a specialized machine between the size of a bread box and a microwave oven. The machine measured wind speed, wind direction, temperature and mercury levels at various sites. They found similar natural background readings to those measured in Michigan, where environmental regulators have used the same device since 2001 to track mercury sources from industry and even broken thermometers.

Joy Taylor Morgan of the Michigan air quality agency said readings 10 and under were considered "normal," while a mercury source was suspected when the monitors detected 20 nanograms per cubic meter. She called the Idaho group's 713 reading pretty significant.

Glenn Miller, a University of Nevada mining expert, praised the environmental group's work, despite its limitations. He was briefed on it and called the data the first of its kind.

"It was important data," he said, "and the question ought to be asked why hasn't anybody else been doing that."

Hayes said he will be briefing regulatory agencies on the data. He brushed off the suggestion that it would be fodder for his group's threatened lawsuit against the EPA for failing to better regulate the mine's mercury emissions.

Rich Haddock of Toronto-based Barrick Gold Corp. downplayed the results. He, like the Newmont Mining executive, noted that gold mines have been reporting mercury emissions for years and have been working successfully since 2001 to curb them through a voluntary program.

"We realize there is man-made mercury added to the global pool," he said. "And, to the extent we can, we are working to mitigate our effect, reduce our impact."

fahys@sltrib.com