The government has spent many millions of dollars in recent decades cleaning up sites contaminated with dioxin and, in extreme cases, relocating residents of entire neighborhoods tainted by the toxin.
But tough new pollution standards proposed by the Obama administration could require additional dioxin cleanups at scores of abandoned factories, military bases, landfills and other locations declared safe years ago, officials say.
If the guidelines receive final approval, federal and state officials will examine sites with known dioxin contamination to identify those needing work and what the work will cost. Among those expected to be reviewed are notorious places such as the former village of Times Beach, Mo., where about 2,000 people were relocated in the 1980s after dioxin-laced waste oil was sprayed on roads to control dust.
The Environmental Protection Agency plan has escalated a decades-long debate over the danger of dioxin, a family of chemical byproducts from industries such as pesticide and herbicide production, waste incineration and smelting. One form of dioxin was in Agent Orange, the defoliant used by U.S. forces during the Vietnam War.
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.
"We're driven by the need to protect against excessive risk of both cancer and non-cancer health concerns," said Mathy Stanislaus, EPA assistant administrator for solid waste and emergency response. "We believe (the current standards) are not sufficiently protective and more stringent numbers are needed."
The Associated Press obtained an EPA list of 92 current and former Superfund locations where records show that dioxin is among the soil contaminants, making them candidates for a review under the new standards.
The sites include Cherry Point Marine Air Station in North Carolina, Camp Pendleton Marine Base in California and the former Rocky Flats nuclear weapons plant in Colorado.
Also listed is an abandoned wood treatment plant in Pensacola, Fla., where more than 350 households had to be relocated in the 1990s.
The notorious Love Canal neighborhood of Niagara Falls, N.Y., where hundreds of families were uprooted, also may rate another look although it was declared clean in 2004 after dioxin and other chemicals were removed or covered, the EPA says.
The EPA also estimates that up to 150 hazardous waste sites not on the Superfund list may have dioxin contamination and will need reviewing.
Since 1998, the agency has regarded dioxin soil concentrations of less than 1,000 parts per trillion (ppt) as safe for residential areas. For commercial and industrial zones, 5,000 ppt to 20,000 ppt has been considered safe.
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.
Cleanups could be ordered anywhere dioxin readings exceed those thresholds, including sites where previous cleanups used less stringent standards.
The 1,000 ppt standard was used for Times Beach's $120 million cleanup completed in 1997 after 265,000 tons of dioxin-contaminated materials were incinerated. Two years later, the abandoned town was converted into a state park.
Marilyn Leistner, the last mayor of Times Beach, said she always suspected the job was inadequate and crews would be back one day.
"My attitude is, 'I told you so. You should have done it in the first place,'" Leistner said.
David Fischer, an attorney with the American Chemistry Council, said: "It could mean a great deal of cost and disruption to communities and municipalities who thought their issues had been resolved. And there will be little if any public health benefit."
EPA officials say the standards are guidelines, not hard-and-fast targets that must be reached at every site. For example, regulators may decide that higher dioxin levels are acceptable when there is little chance of human contact.
The World health Organization says significant exposure to dioxin, which is present to widely varying degrees in soils, river sediments and some foods, can damage human reproductive and immune systems.
The WHO's cancer research arm, part of the National Institutes of Health and the EPA all have designated the most potent form of dioxin as a human carcinogen.
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.
EPA's upcoming health risk assessment is expected to revisit the cancer issue.
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.
The company acknowledges responsibility but has argued with regulators over what cleanup is necesssary.
Spokeswoman Mary Draves declined comment on the proposed standards and said the company wouldn't speculate on how they might affect the cleanup.
But Dow, a leading employer, has many supporters who doubt there is a serious threat.
"The process of cleaning up could cause more trouble than the dioxin itself," said Tittabawassee Township Supervisor Rick Hayes.
Signs along the rivers warn anglers to limit consumption of their catch because of dioxin and other contaminants. Even so, the village of Freeland holds a yearly walleye fishing tournament.
"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."
Associated Press reporter Jim Salter in St. Louis contributed to this story.