The floodwaters in New Orleans still pose a health risk because of dangerous levels of sewage-related bacteria and toxic chemicals, according to government test results released Wednesday.

A minimal number of air pollutants, such as methanol (search), isobutylene (search) and freon (search) have been found, but at levels Environmental Protection Agency officials say do not worry them. Federal agencies aren't predicting when the city will be habitable again.

"This is one of the biggest environmental challenges in our agency's history. Since we haven't seen anything of this scale before, it's hard to make specific predictions," said Eryn Witcher, an EPA spokeswoman. The agency's top priority is a quick cleanup that is done well and protective of people's health, she said.

The chemical samples were drawn from more than 100 pollutants on Sept. 4 and Sept. 6 by EPA and the Louisiana Department of Environmental Quality (search). Like the previous tests, they turned up high levels of chemicals such as hexavalent chromium, arsenic and lead.

Thallium (search) also was detected at one sampling location at levels slightly elevated, but not enough to harm the public.

Young children are most susceptible to illness because their immune systems still are developing. However, the EPA said the amount of chemicals found in the water would pose a risk to children only if a child were to drink a liter of floodwater a day.

Still, officials from the EPA and the Centers for Disease Control and Prevention strongly urge people not to wade in or drink the standing water. If contact can't be avoided, soap and water to clean exposed areas should be used.

The second tests of chemicals and other pollutants also turned up elevated levels of E. coli (search) and other coliform bacteria (search) that can cause vomiting, diarrhea, stomach pain and fever.

It was not immediately clear how the test results might impact plans to begin moving people back into the city. New Orleans Mayor Ray Nagin has said the test results will be a factor in determining how quickly residents are allowed to return.

The agency planned to discuss the test results Wednesday afternoon.

EPA cautioned, however, that people trying to return to homes and businesses after Hurricane Katrina may be exposed to potentially life-threatening hazards such as leaking natural gas lines and carbon monoxide poisoning from using fuel-burning equipment indoors.

"During a flood cleanup, failure to remove contaminated materials and to reduce moisture and humidity may present serious long-term health risks from micro-organisms, such as bacteria and mold," the agency advised.

Among the precautions EPA urges, once local authorities authorize a return to homes and businesses, are to:

—Be aware of possible combustible or explosive gases from broken fuel lines or decaying materials.

—Open all windows when entering a building and leave immediately if you smell gas or hear it escaping. Avoid creating any source of ignition.

—Dry out the building and avoid standing water that is a breeding ground for microorganisms and insects such as mosquitoes that can spread West Nile Virus. Contamination also can result from breathing water vapors or mists from contaminated water.