A federal health official’s ruling has cleared the way for 50 different types of cancer to be added to the list of sicknesses covered by a $4.3 billion fund set up to compensate and treat people exposed to the toxic smoke, dust and fumes in the months after the Sept. 11, 2001, terrorist attacks.

The decision, released on Friday, came as a vindication for hundreds and perhaps thousands of people who have claimed — often in the face of resistance from public health officials — that their cancers were caused by their exposure to the dust cloud and debris thrown up by the attacks.

It will allow not only rescue workers but also volunteers, residents, schoolchildren and passers-by to apply for compensation and treatment for cancers developed in the aftermath of the attacks. The cancers will not be officially added to the list of covered illnesses until after a period of public comment and review that could last several months.

The decision, by Dr. John Howard, director of the National Institute for Occupational Safety and Health, comes despite a current absence of evidence linking the attack to cancer, causing some skepticism among epidemiologists. It also reduces the amount of money for people suffering from ailments more conclusively linked to the Sept. 11 attacks, namely lung and other respiratory sicknesses.

And it poses a number of logistical challenges, since it will be difficult if not impossible to separate people who developed cancer as a result of ground zero from those who would have gotten the disease anyway, and because many cancer diagnoses are likely to be made years after the deadline for applying for compensation passes in 2016.

Representative Carolyn B. Maloney, Democrat of New York and a primary sponsor of the 2010 law that set up the fund, said that she recognized those concerns but that the decision was the correct one. “I think it’s an important statement that the country’s going to take care of the workers and people who are there to save the lives of the people of the city,” she said.

One cancer patient who typifies the fraught nature of the decision, Ernest K. Matthews, 62, said he developed lung cancer in 2008 and had part of his right lung cut out. He was part of a crew that cleaned elevators for the Merrill Lynch building next to ground zero after the terrorist attack. He was also a smoker.

But he said he had been able to walk up six flights of stairs carrying a heavy tool bag without catching his breath before Sept. 11. He developed breathing problems soon after, he said.

“It’s a good day,” Mr. Matthews said Friday. “Look at all the people that suffered and lost their lives, sacrificing for the cleanup. It took so long for them to decide to help the people that were suffering.”

Dr. Howard’s decision represented an about-face from assurances by the federal government immediately after the terrorist attacks that there was nothing in the air to be worried about. In July 2011, Dr. Howard himself said there was not enough scientific or medical evidence to link cancer to Sept. 11.

But in a lengthy report explaining his decision, Dr. Howard said that a New York Fire Department study published last fall in the British medical journal The Lancet, which showed that firefighters exposed to ground zero toxic substances had about 20 percent higher rate of cancer than firefighters who were not exposed, had provided a strong foundation for a conclusion that some cancers had been caused by exposure to the World Trade Center debris.

Beyond the Lancet study, he said, he had relied on recommendations made in late March by a scientific and technical advisory committee consisting of experts from the fields of cancer, environmental medicine, toxicology and epidemiology as well as neighborhood activists and union officials. He fully adopted the committee’s recommendation that 14 broad categories of cancer, encompassing 50 specific types, should be deemed as related to the attacks.

Among the cancers Dr. Howard approved are some of the most common, including lung, breast, colon, trachea, esophageal, kidney, bladder, skin, thyroid, blood and ovarian cancers. Dr. Howard also approved childhood cancers, which are relatively rare, because children are more susceptible to toxic substances.

People with covered cancers who lived, worked or attended school in Lower Manhattan — generally the area below Canal Street — between Sept. 11, 2001, and May 30, 2002, would be able to apply for compensation for their economic losses, pain and suffering. Until the decision on Friday, the only ailments approved for compensation were mainly respiratory and digestive ones. Survivors of patients who have died, as well as people caught in the dust cloud downtown on the day of the attack, may also apply.

The amount of compensation will depend on the severity of the illness and duration of exposure, as proven by records like employment or housing documents, or city personnel records for fire, police and other public workers.

The new rules would apply to Pentagon and Shanksville, Pa., responders as well, and it allows those cancer patients to tap into a treatment fund to pay for medical costs not covered by insurance.

Mayor Michael R. Bloomberg, who has consistently deferred to scientific rulings on health hazards at ground zero, said that his administration had called for periodic reviews of the medical evidence on cancer, and that the decision was “an important step.”

The advisory committee had said that 70 known or potential carcinogens, including asbestos, arsenic and formaldehyde, had been found in the smoke, dust and fumes from the disaster, that 15 of those were known to cause cancer in humans, and that 37 were “reasonably anticipated” to cause cancer.

The advisory committee considered but rejected, by a 14-to-3 vote, the notion of adding all cancers to the list. It explicitly rejected pancreas, brain and prostate cancers, for various reasons. The committee could meet again to discuss other potential additions to the list.

The broad sweep of the committee’s recommendation raised some eyebrows among epidemiologists, several of whom have said that it appeared the committee was appealing to societal concerns that the cancer patients not be left out of the fund.

“Clearly this was a difficult decision, and primarily motivated by concern for a sympathetic population,” said Dr. Alfred I. Neugut, an oncologist and professor of epidemiology at the Mailman School of Public Health at Columbia. “The scientific evidence currently is certainly weak; whether future evidence bears out the wisdom of this decision will have to be seen.”

Dr. Howard made a nod toward a concern of some epidemiologists that because cancer was a common disease, it would be hard to distinguish who got cancer because of Sept. 11 from those who did not. He also said that hard scientific data conclusively linking Sept. 11 to cancer might take years to obtain.

“Requiring evidence of positive associations from studies of 9/11-exposed populations exclusively does not serve the best interests” of the patients, he wrote.