The CDC sent a large environmental health team to work with FEMA, the EPA and others to help determine what steps need to be taken before the city of New Orleans is safe to reinhabit.

Environmental damage is so severe -- from the stagnant, sewage-stained floodwaters blanketing part of New Orleans to a string of oil spills along the Mississippi River -- that emergency responders and health experts warn it could be weeks before they even know the full extent of the cleanup job that lies ahead.

Jean Kelly, public information officer for the Louisiana Department of Environmental Quality, said the long-term prospect of reoccupying New Orleans is hard to measure right now.

"Until the water is pumped out and until we can test the sludge and everything, we really don't know what the livability aspects are going to be," Kelly said. "And that is probably going to be another two months just to get the water out of New Orleans."

High Levels of Bacteria

Initial tests run on the foul-smelling floodwater taken from Jefferson and Orleans parishes last week turned up high levels of bacteria, such as E. coli and fecal coliform, Kelly said. But readings for heavy metals, pesticides and other toxic materials were low.

Emergency crews who descended on New Orleans and other flooded areas were being warned to wear gloves and boots to avoid contact with contaminated water.

Even Mississippi, which was spared from prolonged flooding after the hurricane, warned people in three coastal counties last week to stay out of water contaminated by animal corpses, fecal matter, garbage and debris.

"There is just so much stuff out there until we can get out and begin doing some assessments and getting water-quality samples, we are encouraging people to stay off the water," said Phil Bass, director of the office of pollution control for the Mississippi Department of Environmental Quality.

Lake Pontchartrain

Meanwhile, Louisiana also is keeping tabs on the condition of water being pumped into Lake Pontchartrain.

Frank Manheim, a former geochemist for the US Geological Survey and now an adjunct professor at George Mason University in Virginia, said pumping the foul water into the brackish estuary could have short-term effects but it should recover provided there isn't any significant toxic contamination.

Because the sewage-laden water will be richer in nutrients, it will create algae blooms, which in high concentrations can create a harmful environment for fish and shrimp, said Manheim, who co-authored a 2002 study on the sediments and contaminants in the Lake Pontchartrain Basin.

The last major hit the lake took was in March 1997, when floodwaters from the swollen Mississippi River were diverted through the lake to prevent flooding in New Orleans. Until all of the water is pumped from New Orleans, it will be hard to say how this will compare.

"The people around Pontchartrain really love that water body and they are going to be very skeptical of people who say, `Don't worry, nothing will happen,'" Manheim said.

Hazardous Waste

One of the more immediate challenges, Kelly said, is disposing of debris and potentially hazardous waste left in the wake of Hurricane Katrina.

"It is impeding a lot of progress to have debris everywhere," she said. "This hurricane made a mess of the Gulf Coast."

Louisiana is temporarily easing restrictions on the kinds of solid waste that can be dumped at landfills and is permitting controlled burning of tree limbs and other debris, Kelly said. Variances also are being issued to wastewater treatment plants to treat the contents of portable toilets.

There are more than 160,000 houses in New Orleans alone that have been underwater, and a number may be uninhabitable. Each of those dwellings has an estimated three to 10 pounds of hazardous household waste. Then there are the tens of thousands of cars that were immersed in the floodwaters.

George Haddow, a former deputy chief of staff for the Federal Emergency Management Agency, said the massive response to the hurricane will likely shift into a recovery mode any day now.

He agreed that the sustained flooding of New Orleans poses a particular challenge.

"The problem is getting the water out and -- of course -- what is in the water is incredible," he said. "It's going to be an incredible cleanup and frankly I am not sure how they are going to do it."

For instance, he asked, do you tear down houses and buildings that were exposed to the floodwater or just remove the sheetrock?

Rebuilding the public infrastructure will be a "multibillion-dollar investment," he said. He questioned whether his former agency, which was widely criticized for its handling of the hurricane response, is the one that should direct the effort.

Oil Spills

Louisiana is using aerial surveillance to monitor oil spills at the Murphy Oil Corp. refinery in Meraux, La., and Bass Enterprises Production Co. near Venice, La., where as much as 78,000 barrels of crude oil flowed from two damaged storage tanks near the Mississippi River, Kelly said.

Murphy Oil spokeswoman Mindy West said the leak from one of its crude oil tanks had been repaired and the spill had been contained.

"We are in the process of cleaning it up, obviously," West said.

Another immediate concern is public health.

Tom Skinner, a spokesman for the Centers for Disease Control and Prevention, said the agency sent a large environmental health team to work with FEMA, the EPA and others to help determine what steps need to be taken before the city of New Orleans is safe to reinhabit.

Last week, they had to get healthcare facilities up and running and began guarding against the spread of infectious diseases. Vaccine was being made available to children arriving at evacuation shelters.

Topping the list of concerns are the types of diseases people contract when living in close quarters -- such as diarrheal diseases -- and vaccine preventable diseases, such as influenza and tetanus, Skinner said.

Healthcare providers also have seen cases of Vibrio vulnificus, a bacterium often found in saltwater that can enter the body through an open wound. A handful have proven fatal.

Spread of Disease

Some health agencies in the Gulf Coast are urging people to use insect repellent to prevent mosquito bites in a region that has seen its share of the West Nile virus.

Eddie Hedrick, emerging infections coordinator at the Missouri Department of Health and Senior Services, said the fear of disease-carrying mosquitoes rising from the stagnant floodwaters may be overblown. Natural disasters -- particularly hurricanes -- tend to "blow things away" for a short period of time, he said.

Another myth, he said, is that dead bodies increase the spread of disease. So far, the death toll from Katrina is guesswork, but officials in Louisiana warned early on that it may climb into the thousands. Preliminary sweeps through New Orleans have shown that initial predictions of 10,000 dead may have been exaggerated.

"Dead bodies just do not pose a serious health threat and are no more likely to carry diseases than a living body," Hedrick said. "The World Health Organization has been trying to debunk that for years."

The most common ailment is likely to be gastrointestinal, he said. People exposed to water containing E. coli, for instance, can become ill and spread ailments easily when living in closed quarters, particularly evacuee centers.

"I know all the shelters are setting up handwashing stations and promoting personal hygiene to prevent transmissions," Hedrick said.