Scientists say some of that air pollution can drift back to this country and contaminate lakes and rivers, undercutting aggressive efforts to keep mercury out of the environment.

The federal government estimates that U.S. firms exported at least 276 tons of mercury last year. It moves overseas through a little-known network of purifiers and brokers that operates without government oversight and faces few questions about what happens to the silvery metal once it is sold.

Environmental regulators acknowledge they know more about used motor oil and scrap tires than they do about the mercury trade, in part because it is considered a commodity and isn't subject to the same handling and tracking laws as hazardous waste.

But as policymakers become more aware of the dangers of mercury exposure, particularly for young children and women of childbearing age, they are focusing more attention on curbing sources of mercury pollution. Last month, U.S. Sen. Barack Obama (D-Ill.) introduced legislation that would bar American mercury exports.

"This is a problem that is impacted by things happening all over the world," Obama said in a recent interview. "But we can make an enormous difference."

Combined with a European Union proposal to block mercury exports, the U.S. effort could shrink global supplies of the metal and drive up the cost enough to encourage alternatives, Obama said.

The number of U.S. companies that use mercury in industrial processes or products is declining, a trend driven by concerns that once the metal gets into the atmosphere it can pose serious threats to public health.

Mercury pollution that falls into oceans, lakes and rivers is converted into a neurotoxin that becomes more dangerous as it moves up the food chain from fish to people. The federal government estimated last year that 410,000 babies are born each year at risk for mercury poisoning because of high levels in their mothers' bodies.

Illinois and other states have encouraged the move away from mercury with laws phasing it out in thermometers, school laboratories and industrial scrap. States also are taking steps to discourage mercury-laden garbage from being disposed of in landfills.

But there still is robust demand for mercury in developing countries, where it is used by small-scale gold-mining operations, thermometer manufacturers and chemical plants.

The demand is great enough that most mercury collected through U.S. recycling programs is sold instead of stored away. The U.S. Geological Survey estimates that most of the 276 tons exported by American companies went to Brazil, Mexico, Peru and Vietnam.

Total exports likely are much higher because U.S. metal mines aren't required to report what happens to mercury dug out of the ground in the same places as gold, silver, copper and lead, said William Brooks, who follows the mercury trade for the Geological Survey.

The world market soon will be flooded with even more mercury. Two American chemical plants that use large amounts of mercury to create chlorine are shutting down. Obama is pushing a second bill that would require six other chlorine plants to close or switch to mercury-free technology by 2012.

Those plants turn salt, or sodium chloride, into chlorine gas and caustic soda by pumping a briny solution through electrified vats of mercury. The industry had more than 2,600 tons of mercury on hand at the end of 2005, according to the Chlorine Institute, a trade group.

Environmental groups and a coalition of state officials are urging the federal government to store the metal safely instead of allowing it to be sold to other countries. The government already stores about 4,400 tons that had been stockpiled by the Defense Department.

The U.S. Environmental Protection Agency has agreed only to study whether the nation's commodity-grade mercury should be taken off the market.

"Outside of a handful of people, our role in this international trade has been completely off the radar screen for people in Washington," said Linda Greer, director of the environment and health program for the Natural Resources Defense Council. "We should be part of the solution, not part of the problem."

In recent years the federal government's policies have focused largely on a Bush administration proposal to limit mercury emissions from coal-fired power plants, the world's largest manmade source of the metal. Coal plants are responsible for about half of the 3,000 tons of mercury churned into the atmosphere each year, according to the United Nations Environment Program.

The next-largest source is gold mining in developing countries, which releases about 1,000 tons. UN officials say much of the mercury used to extract gold from ore comes from the U.S. and Europe.

U.S. mines long ago switched to mercury alternatives. But it still is used in other countries as an inexpensive method to remove gold from ore.

Lured by rising gold prices, some 15 million people across the world are engaged in small-scale mining, the UN says. The most common extraction process is to mix an amalgam of mercury and gold-laden ore. Mercury is released into the atmosphere when the mixture is heated to separate the gold.

Some of the toxic pollution lands nearby. But scientists studying weather patterns and the movement of mercury through the atmosphere have found the metal also can travel to places far from the source.

"People are being poisoned," said Marcello Veiga, chief adviser to the UN's Global Mercury Project. "Right now there is no reason to care about this mercury getting into the air because it is so easily accessible."

Another major source of mercury emissions is the chlorine industry. Although most U.S. chlorine plants now use technology that does not require mercury, the others on average release five times as much mercury into the atmosphere as the average coal-fired power plant.

Obama said he introduced the bills that would ban exports and force the remaining chlorine plants to close or switch methods in response to a Tribune series last year that documented the health risks posed by mercury-contaminated fish.

"We know there is a cost-effective alternative," he said. "It sets the stage for us to tackle the more important issue of attacking emissions from coal-fired power plants."

Neither bill is likely to pass this year. Obama said he introduced them now to draw more attention to mercury-related problems and to pressure the EPA and other agencies to act.

He acknowledged that questions remain about what to do with the excess mercury. Chlorine plants that already have closed or switched methods have sold their supplies to similar operations in other countries, some of which have few if any environmental regulations.

"Some of that mercury is polluting the same parts of the world where we get most of our seafood," said Jackie Savitz, campaign director for Oceana, an environmental group that has been pressuring U.S. firms to use mercury-free technology.

One of the top suppliers in the recycled mercury market is D.F. Goldsmith Chemical and Metal Co., an Evanston company that occupies a low, red-brick building in a quiet neighborhood. The firm at 909 Pitner Ave. obtains the mercury from scrap brokers, purifies it to meet industry standards and ships it around the globe.

Bob Goldsmith, the company's owner, said in an interview that he doesn't always know what happens to the mercury he sells. But he assumes most of it ends up overseas.

Five years ago, the company tried to ship 20 tons of mercury from a shuttered chlorine plant in Maine to a similar plant in India. Dock workers refused to unload it, though, and the shipment was returned to the U.S.

Goldsmith said the chlorine company took back the mercury and eventually sold it on the international market through different brokers.

The incident highlighted how difficult it can be to keep the toxic metal off the market, Goldsmith said. He argued that stockpiling mercury will only encourage more mining for it in other parts of the world.

"If you're trying to solve the mercury issue, that isn't going to do it," Goldsmith said. "We're providing a service by giving people an outlet for their mercury. Obviously we're in business to make a profit, but I like to think we're providing a service to the community."

Read the Tribune series: "The Mercury Menace" at http://www.chicagotribune.com/mercury

Michael Hawthorne: mhawthorne@tribune.com