At a mayoral debate in January, candidates vied to outdo one another in their dire warnings of a growing threat to public health that they said was hurting families all over New York City. Soon afterward, public and private groups pledged that millions of dollars would be spent to fight the scourge. And last month, Congressional representatives from New York called upon the federal government to help resolve what they called “this emerging crisis.”

The unifying villain of the moment: mold. Erupting in fuzzy streaks of brown and black in houses flooded by Hurricane Sandy, mold has become a constant topic of conversation among elected officials, health professionals and residents of waterlogged communities in the months since the storm.

The threat of widespread infestation has provoked a mix of specific health concerns and more abstract feelings that the forward momentum of recovery is being undermined, one patch of fungus at a time.

The debate about how best to attack the problem has been hampered by the somewhat ambiguous science on its harmfulness. Many studies link mold to the aggravation of respiratory illnesses, but there are no conclusive findings that mold is toxic.

Despite anecdotal reports of mold sickening homeowners in flooded communities like the Rockaways, there have been no unusual upticks of the respiratory illnesses that it is said to worsen, according to the city health department. That finding was echoed in New Jersey, where the number of illnesses in areas devastated by the storm is in line with parts of the state unaffected by the storm, according to state health officials.

That uncertainty has elicited increasingly pitched comparisons to the lack of clarity about air-quality concerns around ground zero in the aftermath of the 2001 terrorist attacks — which were subsequently shown to be substantial.

“We did not adequately react in the aftermath of 9/11 to the potential health hazards to the toxins in the air,” said Representative Jerrold L. Nadler, a Democrat whose district covers parts of Manhattan and Brooklyn, who is pushing for federal help with mold removal.

“It was initially denied and ignored, and thousands were sickened,” he said of the ground zero air-quality problems. “We should learn from that and not repeat the experience.”

Even to skeptical officials, whether or not a public health crisis is emerging matters less than disaster victims’ belief that they have suffered more trauma.

“The fact that it’s not a major health threat, frankly, from my perspective, is beside the point,” said Caswell F. Holloway, the city’s deputy mayor for operations. “It doesn’t mean that mold is a good thing or that there’s no reason to address it.”

There is evidence that breathing in mold and its tiny spores can worsen existing respiratory problems like asthma, exacerbate allergies and sicken those with weakened immune systems, according to the World Health Organization. Moldy conditions may also cause symptoms like wheezing and coughing in healthy people, according to the Institute of Medicine. But the language in these and other studies is guarded and stops short of saying that mold sickens the healthy.

In coastal communities, where the scent of saltwater in the air is a central attraction, mildew and other types of mold have long been facts of life, an annoyance battled daily with mundane weapons like a spray bottle of bleach. Mold spores, which are constantly wafting in the atmosphere, grab a toehold on moist surfaces.

Hundreds of thousands of houses in New York City, coastal New Jersey and on Long Island were damaged or destroyed by the storm, a vast majority of them by floodwater. Each wet wall and damp beam is a potential petri dish for mold — wherever moisture lingered, the spores blossomed.

Dr. Hylton Lightman, a pediatric asthma and allergy specialist who sees about 100 patients a day at his Total Family Care practice in Far Rockaway, Queens, said that since the storm, cases of hacking coughs and wheezing have increased so much that he began conducting a survey to see if those patients had flooded homes. The correlation, he said, was nearly exact.

In Sue Nurnberger’s flooded home in Amityville, on Long Island, black mold marbled the exposed studs of her destroyed first floor. Her husband, Ray, is frail, suffering from terminal liver cancer, so she spent $3,500 for a team of cleaners who removed the mold with toothbrush-sized instruments, in addition to thousands of dollars she spent on repairs she said her insurance did not cover. Construction work has halted at her house because of a dispute with her insurer and the contractor, and she has enlisted the New York Legal Assistance Group, which is helping her get the repairs finished. There is limited assistance available to people facing moldy homes. The Federal Emergency Management Agency provides money for mold treatment if mold is visible when an agency inspector looks at the house for damage. But because mold blossoms over time, sometimes it only becomes apparent long after inspectors have left.

Plans were recently announced in New York to use private donations and some of the $1.8 billion dollars of initial federal relief money to help clean up mold. A plan to treat 2,000 severely moldy homes as part of a $15 million dollar public-private partnership between the Mayor’s Fund to Advance New York City, the American Red Cross and the Robin Hood Foundation is under way. Groups like Queens Congregations United for Action, a consortium of 40 faith-based organizations that pushed for aid have estimated the number of potentially moldy homes in the city at 30,000.

Free classes on dealing with mold began to be held around the city last month as part of the public-private partnership. Participants were soothed when experts told them about simple methods to tackle mold, and how to evaluate companies hired to test for and remove it.

The subject of a company’s qualifications can be confusing, as shown by the recent call by Bill de Blasio, the city’s public advocate and a Democratic candidate for mayor, for the city to dispatch licensed mold specialists to treat houses. Only a handful of states issue such licenses, and New York is not among them.

The concern about mold is a replay of the aftermath of Hurricane Katrina, which hit New Orleans and other parts of the Gulf Coast in 2005. Some studies found as many as half the flooded homes contained mold, and fear rose that relief workers, as well as residents, were not adequately protecting themselves against breathing in its particles.

At a recent class held at a synagogue in Far Rockaway, Yashika Bratton, a schoolteacher, was surprised to learn that scrubbing mold with dish soap was one recommended remedy. Ms. Bratton said she was skeptical about whether mold was a health risk, but expected to spend several thousand dollars hiring a contractor to clean it up because she had a new baby, Daniel, who was born the day before the storm.

“That’s not a chance I’m willing to take,” she said.