Business owners and residents returning to New Orleans this weekend face an array of potentially dangerous conditions, including polluted drinking water, broken traffic signals, a shortage of hospital beds, an antiquated 911 emergency call system, contaminated soil and virtually no food, according to health, public safety and environmental officials.

Officials acknowledged the deteriorated state of their city in a briefing Friday, and warned that people entering in the next few days would come at their own risk. Nevertheless, they maintained it was important to demonstrate that New Orleans intends to return to its previous vibrant condition.

"Beginning Saturday, we will be on the path of bringing New Orleans back," said Col. Terry Ebbert, director of homeland security for the city. "We feel it's vitally important for this city to stay alive, for commerce to start moving in the right direction."

But state and federal officials, as well as other disaster recovery specialists, voiced concerns that the return of several thousand people immediately, and many more within a week, raised the specter of a second crisis. Public services are scant, and officials will begin to open areas on Monday that were once home to about 180,000 people.

"We're all very nervous about an overwhelming influx of people coming in and the potential health threats that exist," Mike McDaniel, secretary of the Louisiana Department of Environmental Quality, said in a briefing Friday. Others detailed concerns about unsafe levels of E. coli bacteria, oil and gas in the sediment left behind as the floodwaters of Hurricane Katrina are pumped from the city.

Just three hospitals in the region are operating, and the disaster wiped out Charity Hospital, the only one in the area licensed to handle the most severe cases, known as "Level 1" traumas.

"When you place all those people back in the city without a health care infrastructure, it is a risky proposition," said Peter Deblieux, an emergency physician and the last person to leave Charity after a four-day siege. "We're going to have a second disaster in this city."

As soon as people return to a city with no traffic lights and attempt to make repairs to property, there will be a wave of car accidents, broken bones, severed limbs, heart attacks and dehydration, he warned.

For now, telephone calls to the city's emergency 911 system are routed manually by workers with radios. The city's automated system, which handled more than 2,000 calls a day, was destroyed by the storm.

And if just three inches of rain falls on the Crescent City, flooding will again occur in "so many of the areas we've now emptied and are dry," Ebbert said. Despite Mayor C. Ray Nagin's announcement Thursday, Ebbert would not commit to opening the historic French Quarter by Sept. 26.

Although lights are flickering in parts of downtown and crews are removing downed trees, abandoned cars and rooftops tossed aside by Katrina, most of New Orleans still resembles a city that was abandoned in a hurry and inherited by heavily armed troops in camouflage.

Around the convention center, where more than 20,000 people lived in squalor for several days, traffic lights shone red and green simultaneously Friday. A building nearby was spray-painted: "Raw Sewage Danger."

Nagin, who aides said was unavailable Friday, has said he hopes to convert the center into a temporary retail building, but for now it stands empty.

After finding elevated levels of harmful bacteria, gas and oil in 18 samples of the dirt that now coats large swaths of the city, officials at the Environmental Protection Agency Friday recommended "avoiding all contact with sediment." Touching or inhaling the dust and dirt "may cause adverse health effects," said Bill Farland, acting deputy assistant administrator for science.

Cleanup crews must contend with a massive spill in St. Bernard Parish from Murphy Oil, along with gasoline that has leaked from 350,000 flooded vehicles, 50,000 stranded boats and numerous storage tanks, environmental secretary McDaniel said.

About 160,000 homes probably will be demolished, raising fears about asbestos and lead paint in the older buildings; the standing water has heightened concerns about illnesses such as West Nile virus that are transmitted by mosquitoes, he added.

The few people in the city -- cleanup crews, journalists and some hotel staff -- are totally self-sufficient. They carry in food, water, portable generators, tents and barbecue grills. The Sheraton Hotel and two W hotels, all owned by Starwood Inc., are reopening only because the company brought in its own water, food, security and waste disposal tanks, said regional vice president Kevin Regan. Eight trucks deliver 250,000 gallons of water every day to keep air conditioners cooling, toilets flushing and dishwashers running, he said.

"We still don't have a definitive time frame for when the city water will be usable," he said.

Providing housing for construction crews and hotel employees has been manageable, Regan said. But having experienced firsthand the bedlam and violence that followed in Katrina's wake, he is worried that New Orleans is not ready for average residents to descend en masse.

"There are no grocery stores, not many hospitals, gas lines broken. It could create major chaos," he said. "It's great to say we're getting New Orleans back to work, but let's talk about the water, the sewer, medical, food. They don't know what they're coming back to."

At a late afternoon briefing from a new command center in the Hilton Hotel, Ebbert displayed a chart rating the condition of services such as water and power in the three neighborhoods scheduled to reopen by Monday. Categories were marked green for good, amber for improved or red for not functioning. Only one of the dozen categories -- security -- was shaded green, largely because 2,000 military personnel are supplementing the depleted police department. In the central business district and Uptown area, city leaders rated the sewer, water, building inspection and housing as red.

As people enter through two checkpoints, they will be handed a two-page warning that begins: "You are entering at your own risk." The document recommends getting a tetanus vaccine, wearing steel-toed boots, and not inhaling mist from pumps or dehumidifiers. In stark black-and-white type, the paper outlines the harsh realities of life in the city once known as the Big Easy.

"The sewage system has been compromised," it reads. "Standing water and soil may be seriously contaminated. Avoid carbon monoxide poisoning. Proceed with extreme caution, especially around downed power lines."

By issuing extensive warnings, "the city is not going to have any liability" if someone is injured as a result of faulty traffic signals or inadequate emergency rescue services, said City Attorney Sherry Landry.

"We do want to offer them the opportunity" to return, she said. "But we want to make sure it's an informed decision."

Dwanda Tyler pauses while cleaning in the French Quarter in preparation for the return of residents and business owners to New Orleans.Wayne Shannon, left, Lionel Green and Brian Massey, right, all of New Orleans, clean Bienville Street in the French Quarter, slated to reopen Sept. 26. Dwanda Tyler, who works for Omni Pinnacle Emergency Response Specialist, sweeps up debris in the Quarter.