By Joseph Ax

NEW YORK (Reuters) - A decade after the September 11 attacks a growing number of rescue workers and first responders are suffering from various illnesses that they ascribe to their exposure to toxins near the disaster sites.

The volume of litigation generated by the attacks has been extraordinary, due to the international impact, the number of victims and defendants, and the complexities in assessing damages and assigning liability.

The numbers are striking: September 11-related lawsuits include more than 12,000 plaintiffs, 1,000 defendants, 600 lawyers and millions of documents.

Because of a federal law that took effect shortly after the attacks, all litigation related to September 11 damages must be filed in federal court in Manhattan. The judge who was randomly selected to preside over the first filed case, District Judge Alvin K. Hellerstein, has since absorbed virtually all September 11 litigation. He has consolidated the cases into four "master cases."

The following is a summary of each master case's history and its current status.

CASE NUMBER: 21-MC-97

PLAINTIFFS: Survivors of those killed in attacks; people injured in attacks

DEFENDANTS: Airline companies

HISTORY: The case consolidated 95 wrongful-death and personal-injury claims against American and United Airlines, which operated the hijacked airplanes. Those suits represented 96 victims, 85 of whom died in the attacks.

The vast majority of victims' families were barred from suing the airlines after opting to receive payment from the September 11 Victim Compensation Fund, a federally financed fund that was offered as an alternative to litigation and doled out more than $7 billion.

Most of the cases in 21-MC-97 were settled under the guidance of Sheila Birnbaum, the court-appointed mediator. Birnbaum is now the special master of the second 9/11 Victim Compensation Fund.

While the precise aggregate settlement amount is sealed, Birnbaum has put the total at around $500 million.

WHAT REMAINS: Mary Bavis, the mother of Mark Bavis, who died when United Airlines 175 struck the World Trade Center's south tower, has sued United and its security contractor, Huntleigh USA Corp. The trial -- which will be the first in all September 11-related litigation -- is scheduled to begin on November 7.

CASE NUMBER: 21-MC-101

PLAINTIFFS: Property & business owners and their insurers

DEFENDANTS: Airline companies

HISTORY: Approximately 80 plaintiffs -- loosely categorized into 21 plaintiff groups, such as developer Larry Silverstein's World Trade Center Properties -- filed suit against the airline companies.

By 2010, eighteen of the groups had settled their cases with the help of John Martin, a retired federal judge who served as mediator. The total settlement amount -- $1.2 billion -- represented 28 percent of the groups' claim of $4.4 billion in damages.

WHAT REMAINS: Silverstein claimed damages in excess of $16 billion, and Cantor Fitzgerald, the financial-services firm that lost more than 600 employees in the attacks, sought close to $1 billion.

In 2008, Hellerstein ruled that the World Trade Center Properties losses are limited to the fair market value of its buildings, which he said was $2.8 billion as of April 2001, when Silverstein leased four skyscrapers at the site for that amount.

He also ruled that Cantor's claim impermissibly included lost profits resulting from its employees' deaths, which limits the firm's potential damages.

Both cases are pending.

CASE NUMBER: 21-MC-100

PLAINTIFFS: First responders and clean-up workers at Ground Zero

DEFENDANTS: City of New York, Port Authority of New York and New Jersey, various contractors

HISTORY: In the months after the attacks, many of the thousands of rescue and emergency personnel who worked in the World Trade Center wreckage developed respiratory and other illnesses they ascribed to exposure to toxins in that environment.

21-MC-100 consolidated the claims of 10,563 plaintiffs, represented by more than 300 lawyers, against approximately 170 defendants. In 2010, 10,116 of the plaintiffs agreed to settle their claims with the city, which will pay at least $637 million. Hellerstein rejected an earlier deal, in part because he felt the lawyers' fee were too high.

The settlement divides the plaintiffs into four tiers based on the severity of their illness. Those in Tiers 1 to 3 will receive fixed payments ranging from $3,250 to $11,000. Tier 4 members, the most serious cases, will receive five- to seven-figure amounts.

Other defendants have also agreed separately to settle: the Port Authority is paying $47.5 million, for instance, while three contractors whose employees worked at the Fresh Kills landfill in Staten Island, where much of the World Trade Center debris was taken, have collectively agreed to pay $24.3 million to those workers.

The individuals who opted into the settlement are also permitted to apply to the new 9/11 Victim Compensation Fund, but any claims will be lessened by the amount of settlement funds they receive.

WHAT REMAINS: Approximately 260 cases are pending.

CASE NUMBER: 21-MC-102

PARTIES: First responders and clean-up workers at buildings near Ground Zero

DEFENDANTS: City of New York, building owners and managers, contractors

HISTORY: Under New York state labor law, workers who suffer injury while working inside buildings without proper safety equipment are allowed to sue the building's owner and manager, as well as any contractor that may have hired their company. But workers' compensation laws prohibit them from suing their own employer.

Approximately 1,500 workers have sued more than 350 defendants associated with 176 buildings in a multiple-block radius around ground zero.

WHAT REMAINS: Settlement talks were put on hold after the new 9/11 Victim Compensation Fund earmarked $2.8 billion for workers and first responders. The plaintiffs in 21-MC-102 will have 90 days from the beginning of October 2011 to decide whether to apply to the fund or continue litigating. Unlike the plaintiffs in 21-MC-100, who settled prior to the fund's creation, the plaintiffs here must choose one or the other.

(Reporting by Joseph Ax; Editing by Jesse Wegman and David Storey)