Like almost all lawsuits that end in a settlement, the one that covers thousands of Ground Zero workers completely satisfies no one in its attempt to be impossible for sickened workers to resist.

In the settlement with the city and more than 140 contractors and subcontractors, Ground Zero workers will receive up to $657 million.

But other than some attorneys, no one would get a windfall for injuries that may have been caused by working in the choking dust of the fallen World Trade Center towers after the terror attack of Sept. 11, 2001.

And in the absence of certainty about whether anyone - the City of New York or any of its many subcontractors - was negligent in exposing workers to harmful conditions, it's unclear if anyone is escaping liability for those illnesses, legal experts said.

Indeed, the near impossibility of figuring out who - if anyone - could be held liable for the illnesses is why Congress established the World Trade Center Captive Insurance Co. and funded it with $1 billion to pay claims against the city and its subcontractors.

"It is often very hard to know if an occupational illness is caused by the workplace," said Anthony Sebok, a professor at Yeshiva University's Benjamin Cardozo School of Law in Manhattan. "There can be many reasons for pulmonary disease."

Similarly, after a cleanup as massive as that of Ground Zero, in which dozens of companies took part, it's hard to know if the city or any of those companies should be held liable for specific illnesses.

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The case of police Det. James Zadroga, who died in 2006 after working at Ground Zero, shows that. The results of post-mortem exams differed sharply on the source of the particles that scarred his lungs.

Such cases ring especially true for some who believe the litigation shouldn't have been allowed by U.S. District Judge Alvin Hellerstein. The state Defense Emergency Act should have shielded the city from liability, said James Copland, director of the Manhattan Institute's Center for Legal Policy.

But once the case was allowed, all the variables steered the case in the only direction it could go, he and others said.

"When you throw all these unknowns in, you have the makings of a settlement," Sebok said.

It's much the same circumstances that dictated a $4.85-billion settlement in 2007 in the lawsuits against Merck over its painkiller Vioxx, which was believed to cause strokes and heart attacks, Sebok said.

In that case, as in this one, Sebok said the link between specific illness and a cause was difficult to prove, yet the sheer number of illnesses was compelling evidence of a problem.

Copland agreed it was inevitable the Ground Zero case would settle, simply because litigation would just add to the already massive legal fees. Lawyers on both sides stand to make a total of $400 million.

The only other option at this point is protracted litigation, said Kenneth Feinberg, the special master of the fund for Sept. 11 victims. This settlement is the best possible deal for Ground Zero workers, he said.

"The only reason these victims were not covered by the [victims'] fund is because their injuries came after the 9/11 fund expired - months and years later," he said. "That injustice is now being rectified by this settlement."

Determining how much a plaintiff gets as a result of the Ground Zero litigation requires wading through dozens of pages of guidelines and the plaintiff’s medical records. The factors include:

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-In which of four “tiers” a plaintiff’s illness or injury is classified. Tier 1 is for people who either have no medical records or are not yet ill but fear they will be in the future. Tier 4 is for those who died from exposure to Ground Zero contaminants or suffer severe diseases affecting breathing or blood cancer. Tiers 2 and 3 cover less severe cases.

-A system that assigns points for various illnesses, which varies depending on the severity of the illness or, for deaths, on the likelihood that it was caused by Ground Zero exposure.

-Multipliers, which can increase or decrease a plaintiff’s point total. Older plaintiffs or those who smoke heavily would see their point totals cut significantly. Younger plaintiffs’ points would increase.