*Image courtesy: © Thinkstock photos/ Getty Images

*Text Courtesy Reuters

(Reuters) - The 70,000 surviving firefighters, police officers and other first responders who raced to the World Trade Center after the attacks of September 11, 2001 will be entitled to free monitoring and treatment for some 50 forms of cancer.

The National Institute for Occupational Safety and Health announced on Monday that responders as well as survivors exposed to toxic compounds from the wreckage, which smoldered for three months, will be covered for cancer under the Zadroga 9/11 Health and Compensation Act.

The act, which also covers responders and survivors of the 9/11 attacks on the Pentagon outside Washington, was signed into law by President Barack Obama on January 2, 2011.

The decision addresses concerns over the rising health toll for emergency workers in the wake of the attacks, when aircraft slammed into the World Trade Center's Twin Towers in New York and the U.S. military command center in northern Virginia.

It "marks an important step in the effort to provide needed treatment and care to 9/11 responders and survivors," said Dr. John Howard, administrator of the World Trade Center Health Program established by the Zadroga law.

"We have urged from the very beginning that the decision whether or not to include cancer be based on science," New York City Mayor Michael Bloomberg said in a statement.

The decision "will continue to ensure that those who have become ill due to the heinous attacks on 9/11 get the medical care they need and deserve."

Illnesses related to the September 11 attacks have caused an estimated 1,000 deaths. Last week, the New York City Fire Department etched nine more names into a memorial wall honoring firefighters who died from illnesses after their work at Ground Zero, bringing the total to 64.

Cancers to be covered include lung and colorectal, breast and bladder, leukemias, melanoma and all childhood cancers.

The program had already covered respiratory diseases such as asthma and pulmonary fibrosis, mental disorders including depression and post-traumatic stress disorder as well as musculoskeletal conditions.

But researchers have known that responders and survivors, including local business owners and residents, were exposed to a complex mixture of chemical agents, including human carcinogens.

That mix included combustion products from 20,000 gallons of jet fuel, 100,000 tons of organic debris, and 100,000 gallons of heating and diesel oil.

Pulverized building materials created a toxic pall of cement dust, glass fibers, asbestos, crystalline silica, metals, polycyclic aromatic hydrocarbons, polychlorinated biphenyls, pesticides and dioxins - "a total of 287 chemicals or chemical groups," the WTC health program reported in 2011.

"They did a magnificent thing, showing not only scientific acumen but also a generosity of spirit," said Dr. Michael Crane, director of the WTC health program at Mount Sinai School of Medicine in New York City.

QUESTIONS ABOUT A CANCER LINK

While scientists knew from the start that responders were exposed to toxic chemicals, it was not obvious they had caused cancer.

For many cancers, the time between exposure to a carcinogen and the appearance of a malignancy can be 20 years or more. That has cast doubt on whether cancers detected in the years after the attacks were caused by exposure to the toxic chemicals.

The health of first responders has also been intensely monitored, raising questions about whether an elevated rate of cancers reflected closer scrutiny, not a true increase. Also, data on potentially cancer-causing agents in the air around the WTC wasn't collected until four days after the attacks.

"Nobody knows to this day what was in that cloud," said Mount Sinai's Crane. "Trying to assess the risk from an unknown exposure is incredibly difficult: we don't know what people actually breathed."

To confuse matters further, neither of two major reviews, in 2006 and 2007, found "any epidemiologic evidence for a causal association between September 11, 2001, exposures and cancer," the WTC health program reported last year.

As a result, the program concluded that there wasn't enough evidence to add cancer to the list of covered conditions.

What changed?

On March 31, the program's science advisory committee wrote to Howard noting that 15 compounds in the smoke, dust and gas at the WTC site are classified by the International Agency for Research on Cancer (IARC) as known to cause cancer in people.

Thirty-seven are classified by the U.S. National Toxicology Program as "reasonably anticipated to cause cancer in humans."

In addition, the science advisors noted that many responders and survivors had high levels of inflammation, which recent research has linked to an elevated risk of cancer.

They therefore recommended that the program cover cancers that met any of three criteria: cancers caused by any 9/11 compound which the IARC classifies as a human carcinogen, cancers where high levels of inflammation have been documented, and cancers that epidemiology studies suggest that responders are at higher risk for than the general population.

The last category includes multiple myeloma and non-Hodgkins lymphoma, which have been reported in unusually high levels for New York City firefighters who worked at the WTC site.

Malignancies caused by compounds in the debris include respiratory system cancers, from the nose to the lungs. They have been linked to arsenic, asbestos, beryllium, cadmium, chromium, nickel and silica dust, all of which were in WTC air.

Cancers of the esophagus, stomach, colon, rectum and liver have been linked to tetrachloroethylene, asbestos, lead or polychlorinated biphenyls, also in the toxic cloud and dust.

A 9/11 responder or survivor who seeks treatment for any of the covered conditions must be "certified" by a physician at one of the WTC health program centers, such as Mount Sinai's.