Mercurial airs: tallying who's to blame.

Each year, human activities throughout the United States release an estimated 341 tons of gaseous mercury into the air, according to an Environmental Protection Agency draft report that inventories the toxic pollutant. This report constitutes the first comprehensive attempt to gauge airborne mercury releases nationwide in almost 20 years - a time during which industries that use and products that contain the metal have changed dramatically.

Fossil-fueled power plants have long been recognized as the single largest industrial source of airborne mercury. The pollutant has rendered fish throughout large portions of North America a serious threat to human health (SN: 3/9/91, p.152). The inventory links 36 percent of the anthropogenic releases -- emissions from human activities -- to naturally occurring mercury contaminants in the fuel used at such power plants: about 117 tons per year to trace levels in coal and 4.4 tons per year to the mercury released from burning oil.

Three other types of combustion facilities play a major role, the new EPA report notes. Every year, incinerators burning mercury-laced municipal trash and medical wastes contribute 64 tons of mercury each, it says, while commercial and industrial boilers together spew out another 30 tons of the highly toxic metal.

Though the U.S. government controls mercury releases from a select group of industries, no federal limits exist for what the inventory points to as the biggest contributors - power plants, incinerators, and other combustion sources. Together, these account for an estimated 83.6 percent of all industrial and residential mercury releases to the air. Moreover, because incinerators emit an extremely soluble form of the metal, most of the mercury they discharge can wash out of the air - and into the food chain -- more effectively than can the .mercury released by power plants.

But controls on some relatively big mercury polluters could emerge soon. Over the past 2 years, the Natural Resources Defense Council and the Sierra Club Legal Defense Fund (SCLDF), both in Washington, D.C., have sued EPA to force the agency to develop mercury controls for combustion facilities. Last December, EPA entered into a tentative settlement with the groups, notes Howard Fox of SCLDE

Perhaps as early as September, EPA intends to issue revised emission standards for municipal incinerators; these will explicitly target mercury emissions. Somewhat later, the agency will propose controls for medical-waste incinerators, notes EPA's Robert Martineau. "I don't have any idea what the requirements will be," he says, "but mercury emissions will be considered."

Here the new inventory may prove helpful, argues Marjorie J. Clarke, a New York City-based consultant active in drafting state incinerator limits on mercury A lack of data on the role of medical-waste incinerators as a source of mercury has hampered efforts to impose controls on these facilities, Clarke notes. But if, as the new inventory suggests, garbage and medical waste pose the same threat, she says, "then you would want to attack them with the same vigor."

What about power plants? Congress has charged EPA with studying mercury emissions from them. Pending results, these facilities are exempt from federal mercury controls. Under the 1990 revisions to the Clean Air Act.

Overall, the new inventory says, 40 percent of the mercury in air may trace to natural sources -volcanic eruptions and vapors from mercury-laced rocks, soil, and water. However, EPA also concedes that "natural sources" is a bit of a misnomer, since these releases include some revolatilization of "yesterday's anthropogenic emissions."

Indeed, the inventory's global figure for natural mercury emissions seems greatly exaggerated and its numbers for many industrial contributions low or absent, according to Eva Voldner, who analyzes such data for Environment Canada's Atmospheric Environment Service in Downsview, Ontario.