The EPA is expected to make a final decision this fall on the new standards. But congressional critics and chemical companies say the agency is acting hastily and should wait until it completes a reassessment of dioxin's health effects in the coming months.
"They're proposing these sweeping changes to regulations without giving us an idea of how many sites will be affected, how many homes will be affected, what the economic impact would be," said Rep. Dave Camp, a Republican whose Michigan district includes a 50-mile-long watershed polluted with dioxin from a Dow Chemical Co. plant.
EPA officials say they want to move ahead because they are convinced dioxin is hazardous at lower concentrations than previously thought. If necessary, they say, the standards can be adjusted later.
The proposed revisions would drop the safe levels to a fraction as much _ 72 ppt for residential areas and 950 ppt for commercial and industrial sites.
"The dioxin is real, it's pervasive, it's toxic," said Michelle Hurd Riddick, a member of a local environmental group called the Lone Tree Council. "But some people just want to look the other way."
Cleanups could be ordered anywhere dioxin readings exceed those thresholds, including sites where previous cleanups used less stringent standards.
But the National Research Council recommended in 2006 that EPA conduct more research, noting that the agency's conclusion had relied on occupational and animal studies where doses are higher than those to which people typically would be exposed.
The debate is playing out along the Tittabawassee and Saginaw rivers in Michigan, where sediments and floodplains were polluted last century with dioxin from a Dow plant in Midland.