Great Lakes Article:

As the Bush administration presses ahead with a market-based plan to let power companies swap their rights to emit mercury, scientific and economic uncertainties leave an important question unresolved: whether the plan would leave "hot spots" with extremely high levels of mercury around the country.

That question will be among those explored on Wednesday at hearings in Chicago, Philadelphia and Research Triangle Park in North Carolina, as the Environmental Protection Agency moves to draw up mercury regulations for coal-burning power plants by the end of the year.

Mercury emissions from coal-burning plants are not regulated under federal law, though the Clinton administration had moved toward strict regulation by classifying mercury as a hazardous pollutant under the Clean Air Act in 2000. Hazardous air pollutants, which include asbestos and lead, are generally subject to strict controls at each source, a requirement intended to bring down the level of pollutants everywhere.

The Bush administration wants mercury to fall under a less stringent section of the Clear Air Act that governs pollutants like those that cause smog and acid rain, which are not as toxic to humans.

Under the current proposal, power plants would buy and sell the rights to emit mercury into the air; the administration says this trading system is intended to cut mercury emissions by 70 percent by 2018. Critics say that developing technology will make it feasible to reduce mercury by 90 percent on a faster timeline.

Environmental groups argue that mercury, a neurotoxin that can harm fetuses and young children, should be subject to plant-by-plant controls because it is acutely toxic even at low levels and because it tends to concentrate nearer its source than other air pollutants do.

According to the agency's own models, in-state sources produce more than 50 percent of the mercury pollution in 8 of the 10 states with the highest concentrations of mercury.

"There are criteria for deciding which pollutants should be traded, and should not be traded and mercury fails most of these criteria," said Michael Shore, who works with Environmental Defense, a group that has previously backed pollutant trading.

Industry groups have sought a trading system, arguing it is more efficient than uniform mandatory controls. Industry and E.P.A. economists estimate that it would cost $40,000 to $70,000 to remove each pound of mercury, with the technology now available. Power plants emit about 48 tons of mercury annually.

Administration officials say that concerns about hot spots are unfounded. "Based on the information we now have, we don't think there is a concern on hot spots," said Jeffrey Holmstead, the E.P.A. assistant administrator for air.

He noted that despite similar concerns, problems did not arise after a sulfur dioxide trading program was created by the 1990 Clean Air Act amendments to combat acid rain. In fact, that system is widely considered a success by both industry and environmental advocacy groups, reducing the overall level of emissions ahead of schedule at a fraction of the costs estimated when the proposal was passed. Hot spots did not occur in large part because the dirtiest power plants were the ones that had the greatest incentive to clean up.

Environmental groups, though, say the most dangerous areas for mercury are not necessarily the ones with the dirtiest power plants since mercury enters the food chain by converting into an organic form, called methylmercury, as it falls into lakes and rivers.

Power plants produce mercury in two forms: a stable form of elemental mercury that travels long distances and a reactive gaseous form, which falls to earth faster.

"Hot spots come from high proportion of reactive gaseous mercury," said David Krabbenhoft, a mercury researcher at the United States Geological Survey. "You wouldn't get hot spots with releases of elemental mercury."

Scientists and engineers say the reactive form of mercury is easier to remove from power plants, which may address some local concerns about hot spots. Another safeguard is that states can impose their own controls.

"If states felt like they had vulnerable ecosystems, they could opt out of a national trading program," said Robert Stavins, a professor of environmental economics at Harvard's John F. Kennedy School of Government. Indeed, while 43 states have advisories for mercury levels in fish, only 14 of them, concentrated in the Great Lakes region and the Northeast, have advisories covering all state lakes and rivers.

Some believe the debate should be less on the appropriateness of trading than on the overall limit and timeline. E.P.A. staff and Democrats have endorsed hybrid controls that combine trading with plant-by-plant controls.