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NEW YORK (CNN) -- The city allowed people to return to Manhattan after the collapse of the World Trade Center towers even though officials were told the air was not yet safe, according to an internal memo from a New York City Health Department official.

The October 6, 2001, memo states that the city Office of Emergency Management -- called OEM -- and the Department of Environmental Protection -- referred to as DEP -- disagreed over the air quality following the September 11, 2001 terror attacks. But it suggests commercial interests trumped safety concerns.

Kelly McKinney, associate commissioner of the health department, wrote that the mayor's office was under pressure from building owners and business owners to open more of the "red zone."

"According to OEM, some city blocks north and south of Ground Zero are suitable for re-occupancy. DEP believes the air quality is not yet suitable for re-occupancy. I was told the mayor's office was directing OEM to open the target areas next week," McKinney wrote.

In a letter dated the day before the memo was written, the U.S. Environmental Protection Agency told the city's health department that there were concerns about worker safety at the World Trade Center site.

"In addition to standard construction/demolition site safety concerns, this site also poses threats to workers related to potential exposure to hazardous substances," including building materials, hazardous materials stored in the buildings and combustion products emitted from the smoldering rubble, the letter states.

The two documents are proof that "EPA officials were aware of the severity of the danger at Ground Zero in contrast to their public statements at the time," said Joel Kupferman, an environmental lawyer with the Environmental Justice Project who obtained the documents through Freedom of Information Act requests.

Calls to the New York City Health Department were referred to the New York City Law Department.

Gary Shaffer, deputy chief of the department's World Trade Center unit, said in a statement that New York "stuck firm to requiring that before a site could be opened, there had to be two consecutive days of asbestos fiber counts below the DEP level of concern."

"All Mr. McKinney's memo shows is that even when these stringent standards were met, reopenings were still discussed before being finalized," the statement says.

The documents were made public two days after Mount Sinai Medical Center released a study showing that 70 percent of World Trade Center rescue workers have developed serious and persistent respiratory illnesses from exposure to toxic dust. (Full story)

Also, Rep. Jerrold Nadler, D-New York, introduced legislation Thursday that would give new federal health benefits to first responders, recovery workers and residents of lower Manhattan who have developed 9/11-related illnesses.

The 9/11 Comprehensive Health Benefit Act would extend Medicare benefits to people suffering from medical or mental health problems associated with exposure to toxic chemicals at Ground Zero.

A companion bill will be introduced in the Senate by Hillary Rodham Clinton, D-New York.