Sept, 11, 2001

A woman cries on engravings of names on the 9/11 memorial during ceremonies for the eleventh anniversary of the terrorist attacks on lower Manhattan at the World Trade Center site September 11, 2012 in New York City. (Craig Ruttle, Pool/Getty Images)

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.

(Editing by Sandra Maler and Christopher Wilson)