The problem, says Thomas Cahill, an air pollution expert and U.C Davis Professor, wasn't so much that the immediate dust and debris from the towers,

CAHILL: "The stuff that happened next was very bad. All the fluorescent light, all the computers, were cooking off and making a fine mist. So the thing is these people were trying to take the building apart, standing on top of what is fuming chemical factory,  that was still burning beneath their feet."

Cahill says the judgement of federal officials was clouded in their rush to react and recover bodies. He believes they should have focused on putting the fire out before exposing thousands of workers to deadly toxins.

CAHILL: "I'm furious, and in fact I joined as a plaintiffs expert with the New York fire department and people. And if the case had gone to trial last year, and if the case had gone to trial last year I would have happily testified. Fortunately a settlement was reached in January to pay the health damage of these people indefinitely."

The recent settlement, Cahill says, is result of ten years of hard work by scientists and advocates for ground zero workers.