Nevada for the first time has approved regulations for monitoring mercury emissions, and Utah officials and activists on Thursday said they are concerned the rules may not do enough to protect the health of Utah residents who live downwind from Nevada's vast gold mines.

Nevada's State Environmental Commission approved Wednesday a first-of-its-kind regulation in the United States, to require enhanced monitoring, testing, recordkeeping and reporting for precious metals mines in Nevada.

Mercury is a byproduct of the gold-refining process and is commonly released into the air through the mines' smokestacks. Nevada's gold mines were responsible for 86 percent of the nation's total toxic mercury waste in 2003, the most recent year for which figures are available from the U.S. Environmental Protection Agency.

While having any regulation "is a major step forward, we remain concerned that the rule that was passed did not go far enough," said Tim Wagner, who works on energy and mining issues for the Utah chapter of the Sierra Club. He was dismayed that the commission failed to set a target emission rate.

Mercury exposure can cause damage to the central nervous system, particularly for developing fetuses, and has been linked to autism. Mercury can also damage the kidneys and lungs and can impair seeing, hearing, speech and walking. Fetuses are four to five times more sensitive to mercury than adults, according to the Utah Department of Environmental Quality.

Utah recently advised residents not to eat two species of ducks after tests showed they had been contaminated with high levels of mercury in the Great Salt Lake wetlands.

A study undertaken by Kennecott Utah Copper showed the Great Salt Lake has the highest levels of mercury ever found in open water in the United States, according to Bryce Bird, the air-standards branch manager for the Utah Division of Air Quality.

Idaho officials also became concerned after Salmon Falls Creek Reservoir, on the Nevada border, had mercury levels last year that were 10 times higher than had ever been measured in any other body of water in the state.

Utah officials and environmental activists in recent days had written letters urging Nevada to adopt strong rules on mercury emissions.

Bird's division sent a letter to the Nevada commission asking the agency to follow up on the regulation approved Wednesday and to get the mining industry to reduce further toxic emissions.

Salt Lake City Mayor Rocky Anderson also sent a letter to the Nevada commission asking for quick action in regulating mercury emissions and continuous monitoring of them.

The mercury levels in the Great Salt Lake "are tragic," Anderson wrote, given the lake's "unique, thriving ecosystem." The amount of mercury in Utah's air and fresh water is "also likely well above safe limits," his letter said.