In issuing the Clean Air Mercury Rule this past March, the Bush administration hoped to ease health concerns about mercury from coal-fired power plants. The White House enacted a "cap and trade" approach to reduce emissions of the element nationwide by about 20 percent in five years and 70 percent by 2018. In formulating its rule, the administration noted that power plants emit only 48 tons of the metal every year--just a small fraction of the total amount of mercury in the atmosphere. Mandating further emission cuts, it argued, would not solve the problem of human exposure to the neurotoxin.

Eleven states and four public health groups are challenging this approach, arguing that cap-and-trade does not address areas particularly vulnerable to mercury pollution. Not so, says the Environmental Protection Agency. When the cap-and-trade proposals were announced, the EPA's head of air regulations, Jeffrey Holmstead, said, "We don't think there will be any hot spots." The hot-spot standoff arises from big gaps in mercury science, according to environmental researchers, and the lack of comprehensive data on mercury deposition means that a consensus about emissions control will not likely emerge soon.

Theoretically, mercury should pref-erentially rain down in areas near to power plants. But attempts to determine the fallout have proved incomplete. For instance, the Mercury Deposition Network, which measures the metal in rainwater in many parts of the country, does not account for mercury particulates that settle dry onto vegetation, a form of deposition that could be equal to the wet variety, according to Oak Ridge National Laboratory scientist Steve Lindberg.

And just because a region receives above-average deposition doesn't mean that it will have high levels of methyl mercury, the form that builds up in long-lived predatory fish such as trout, pike, tuna and swordfish. "The areas with the most problems may not have the highest levels of deposition," explains mercury expert David Krabbenhoft of the U.S. Geological Survey branch office in Middleton, Wis. Indeed, the Southeast has greater measured deposition than the Northeast, but both regions have serious methyl mercury problems.

A partial explanation for this dichotomy is the process by which elemental mercury becomes methyl mercury. For mercury to get methylated and enter the food web, it must be processed by bacteria that thrive on sulfate, a sulfur compound. This means that dissolved organic matter and sulfur enhance methylation, as do acidic waters such as those in the Northeast. The methylation process changes the conclusions drawn simply from examining mercury fallout. For instance, Krabbenhoft recently completed a study of New England lakes from locations near Boston and up to Maine. Mercury emissions and deposition are highest in the urban area, but methyl mercury in fish is low. The fish problems occur in Vermont and New Hampshire, where the conditions are right for methylation. "What do we want to protect?" he asks. "If it's beautiful fishing spots like these lakes, then we need to look at more than deposition."

Scientists finally understand methylation well enough to map out vulnerable areas at a national level, according to Krabbenhoft, who is currently working on such a map for the EPA. The work should, for the first time, combine deposition and vulnerability to identify unambiguous hot spots--regions where it makes the most sense to take action to limit the effects of mercury. Scientists hope that this information and other advances may resolve the hot-spot debate and demonstrate the wisdom or folly of the administration's approach to mercury regulation.