NEW ORLEANS — As New Orleans residents begin to re-enter the homes and businesses after Hurricane Katrina, many may face an obstacle more pervasive and possibly more dangerous than mud and rotting wood: mold.

Public-health experts say mold spores could present significant risks and may force thousands of demolitions.

Trillions of spores, exacerbated by the sultry late-summer weather, could sicken the 20 percent of the population that has allergy problems, experts say, and also could be dangerous for older residents, children and people with weakened immune systems.

Furthermore, New Orleans has large populations with high rates of asthma — particularly African Americans and people living below the poverty line — that can be aggravated by mold.

But as the city confronts its enormous rebuilding task, removing mold has been deemed a low priority compared with repairing levees and bridges, clearing streets and restarting the economy.

Officials at the state Department of Health and Hospitals, the agency primarily responsible for mold mitigation, said the department was so overwhelmed with other flood-related work that it could not inspect homes or analyze the potential health risks of mold, beyond disseminating information on its Web site.

After the city's levees were breached, tens of thousands of commercial and residential structures were submerged. Any wall or ceiling above the flood line was at risk of attracting mold spores, which are present in the air and reproduce when there is sufficient moisture and when they have a food source — organic materials such as insulation, mattresses, wallpaper and upholstered furniture.

Dr. Douglas Rice, director of the environmental-quality laboratory at Colorado State University, said each square inch of mold growth could produce 1 million spores. While most people will not feel ill effects after breathing a moderate number of spores, Rice said that even healthy people could come down with sinus and lung infections that could spread to the brain, and that some molds produce toxins that could cause birth defects and cancer.

"You are dealing with literally trillions of spores," he said. "That can overwhelm even a healthy immune system."

But other experts disagree.

Howard Kipen, a professor of environmental and occupational medicine at the Robert Wood Johnson Medical School in New Jersey, said scientific evidence did not bear out links between mold exposure and cancer, birth defects or fatigue.

"It can make a significant percentage of the population sick," Kipen said, referring to those with allergies. "Mold is very nasty. But whether it causes permanent health injury is inconclusive."

Dr. Harold Zeliger, a chemical toxicologist in West Charlton, N.Y., said the mold in New Orleans — after nearly four weeks of uninterrupted growth — could force the widespread demolition of buildings.

"Even if they could be saved structurally," Zeliger said, "it probably doesn't make economic sense to do so."

Despite such predictions, some property owners are trying to clean up. But some advice from state and federal agencies is contradictory, and most of it does not take into account the current circumstances in New Orleans.

The Occupational Safety and Health Administration, for instance, recommends that cleaning be completed within 48 hours but says nothing about what to do if a building was flooded nearly four weeks ago. The agency's Web site states that care should be taken not to spread spores when cleaning, and recommends blocking air vents and not using fans to dry buildings.

However, the state Department of Health and Hospitals advises on its Web site, "where possible, use fans to dry out the building," and does not discuss taking steps to prevent the spread of mold.

Many residents have not followed the recommended precautions. Some said they could not afford special equipment and lacked insurance that would allow them to hire a professional.

"It's a little mold," said Charlie Cooke, 43, who was scrubbing the walls of his home with bleach. "I'd rather get a little sick than wait to have my place cleaned up."

About 8 inches of water had flooded the first floor of Cooke's house near downtown; his sofa was spotted with fungus, and gray and black spots of mold covered the walls, which had turned from white to yellow. He wore gloves but not a mask, and was barefoot.

"You pick your poison, I suppose," he said.