But many workers say they couldn't get respirators at first. And when they did, they were cumbersome and often clogged, making them useless. Within weeks, most of them were feeling minor effects of inhaling the dust. But as the months wore on, some - like one group of first responders 60 Minutes interviewed - began to come down with more severe and unusual symptoms.
"I have chronic lung disease. I have the nodules in my right lung. The pleural lining, which is thickening, the pleurisy, which is a pain in my back that's constant, 24 hours a day, seven days a week," one said.
"I ended up getting a lung infection, nose infection," another said.
And a third says, "My throat still burns every now and then. Basically right now, the biggest part of my problem is breathing."
Initially, many doctors were skeptical that a link could be made to the dust at ground zero. But not Dr. Prezant. He started to investigate. Based on routine breathing tests, he knew the lung capacity of every firefighter in the department before 9/11.
He found that firefighters who worked at ground zero lost a significant amount of lung function - an average of 12 years. While 60 Minutes was visiting, firefighter Garrett Barbosa, who spent two months on the pile, was being tested.
"How has this affected your life?" Couric asks.
"From Superman to Jimmy Olsen. It's a big difference," he replies.
Dr. Prezant's findings were taken seriously, published in the country's top medical journals. "We're never going to be able to get this stuff out of people's body in terms of the dust and the concrete that is down there," Dr. Prezant says.
No one knows that better than firefighter Robert Ryan, a longtime tri-athlete who spent four months on the pile. Two years later, he got the call for a routine fire. Strapping on 100 pounds of equipment, he ran up five flights of stairs.
"We opened the door and we were going in to do a search and collapsed on the floor," he remembers. "At first, I thought maybe my mask had malfunctioned but that's not what happened. I was just laying there, gasping, just gasping for air and it just wouldn't go into my body."
Doctors told him he had lost about 40 percent of his lung function since Sept. 11 - so much that he was forced to give up his lifelong dream of being a fireman.
Ryan says the hardest thing for him was telling his son he was leaving his job. "He just looked at me and said, 'Daddy you're not gonna be a fireman any more?' And I said, 'Daddy will always be a fireman. I just can't - I'm, just can't be in the fire department any more.' So, yeah, that was … somebody stabbed me through the heart."
Ryan is only one of the 750 firefighters who are so ill they can no longer work. And according to department records, five years after Sept. 11, at least 3,000 firefighters - that's 25 percent of the department - still have respiratory problems.
"As a physician, had you ever seen anything like this?" Couric asks Dr. Prezant.
"What you've never seen as a physician is such a large percentage of people all have the same problem, all right? And all have affected their lifestyle so dramatically," he says.
As more and more people have become sick, many workers are questioning if more could have been done to protect them on the pile. At the time, many of them remember being told the air was okay. And they remember being told that by the head of the EPA at the time, Christine Todd Whitman.
On Sept. 18, Whitman said, "I'm glad to reassure the people of New York that their air is safe to breathe."
What led her to make that determination?
"Everything that the scientists were telling us. That the air - ambient air quality in lower Manhattan, this was not about the pile, this was about lower Manhattan - the readings were showing us that there was nothing that gave us any concern about long-term health implications," she says. "That was different from on the pile itself, at ground zero. There, we always said consistently, 'You got to wear protective gear.' "
"Many of the first responders told us all they heard was the air is safe and it gave them a false sense of security. Do you think you emphasized it enough in hindsight?" Couric asks Whitman.
"You know, it's hard to know when people, people hear what they wanna hear. And there's so much going on that maybe they didn't make the distinction," she replies.