The Natural Resources Defense Council recently conducted tests on samples of sediment around the once-submerged city.

Wilma Subra, an environmental chemist contracted by the council, said arsenic, chromium, lead, barium, cadmium, mercury and hydrocarbons all were found in now-powdery sediment left behind by Katrina.

"The most widespread chemical in excess of the (US) Environmental Protection Agency's residential standard was arsenic," Subra said. "It was exceeded in every single (tested location) in Louisiana."

The average contamination in New Orleans was 30 times the EPA residential standard, she said.

Subra and council officials called for a massive cleanup of all sediment, claiming residents are at risk of maladies ranging from skin and respiratory irritation to cancer.

State and federal officials continue to perform testing to determine when to give an "all clear" to residents regarding sediment contamination.

The EPA has identified 14 sites around New Orleans where toxin levels warranted further review. Several of those sites were also pinpointed by the Natural Resources Defense Council, including the heavily damaged Lower Ninth Ward on the city's east side. But the EPA says it sees no cause for alarm. The agency specifically tested sites it predicted could have the worst contamination and so far results have been in the "acceptable" range, spokeswoman Phillippa Cannon said.

The final decision for an "all clear" designation would come from the state, she said.

Louisiana Department of Environmental Quality toxicologist Tom Harris said the group's findings were fundamentally flawed because arsenic levels were naturally above the EPA's residential standard in Louisiana and elsewhere.

"I have never personally seen soil sample levels come back below the residential screening level for arsenic," Harris said. "I know that's an extreme statement, but it's absolutely true. It's a naturally occurring (element) you can find everywhere.

"People are getting the impression we can't move back to a certain area because of arsenic, without understanding that they're going to have to deal with it if they moved to Shreveport, Louisiana; Jackson, Mississippi; or Dallas, Texas."