Fifteen years after the September 11 attacks researchers are still trying to understand the full impact of the toxins that were created when the towers fell. Our Health Reporter Erin Billups looks at the growing number of cancer diagnosis among responders.

In recent months, the World Trade Center Health Clinic at Mount Sinai Hospital has seen 30 to 40 responders walking in for treatment each week — for the first time.

"They had a very, very difficult mixed exposure of really awful stuff and I think now we're beginning to see the consequences of that," said Director of The WTC Health Program Clinics at Mount Sinai, Dr. Michael Crane.

More than 5,400 responders and survivors have been diagnosed with a 9-11-related cancer.

A new study from the World Trade Center Health Registry found that melanomas, non-Hodgkin's Lymphoma, prostate, thyroid and breast cancers are being found among responders in significantly higher numbers than the general population.

And in an increasing number of cases like Kathleen Kelly's- a trauma nurse diagnosed in March with breast cancer — it's more aggressive.

"It's very odd looking," said Kathleen Kelly, a 9/11 volunteer responder. "Not something that you normally see and that's really what began to make my husband Mike and I think, I wonder if this is a sequel from the World Trade Center."

Hours after the attack Kelly joined her husband, a police sergeant, at the site to care for responders.

"I could treat them there and allow them to go back to the pile which is what they wanted to do," she said.

Dr. Crane says exposure to the cocktail of toxins has made some of these cancers more difficult to treat.

"It wasn't just one substance, it wasn't just asbestos fibers, it wasn't just aromatic hydrocarbons, it was a whole gimmish of everything going right into people's lungs," Dr. Crane said.

Crane says he's also seen a steady number of rare cancers, some growing faster than normal, raising questions for researchers.

"One of the theories is that something about the exposures accelerated the process, or made the process more aggressive so that the tumor spreads more quickly," Dr. Crane said.

Kelly's tumor and part of her breasts were removed. She has had reconstructive surgery and chemotherapy, and she is now receiving radiation therapy — and still she doesn't regret volunteering.

"This was an unprecedented event," she said. "The days after were just people doing what they needed to do to try and save people."