NEW YORK (AP) -- An estimated 500 firefighters might have to retire on disability because of respiratory problems caused by inhaling smoke, dust and other particles after the terrorist attacks last year, department officials reported.

The number of early retirees amounts to about 4 percent of New York's 11,500 firefighters.

Dr. David Prezant, the Fire Department's deputy chief medical officer, spoke in a telephone briefing Monday arranged by the Centers for Disease Control and Prevention, which is publishing a report by him and colleagues in a special Sept. 11 issue of its Morbidity and Mortality Weekly Report.

In the publication, Prezant and colleagues said about 600 firefighters and paramedics remain on leave or limited duty because of respiratory problems or stress.

That includes 358 firefighters and five department paramedics who were on medical leave or light duty as of Aug. 28 because of respiratory illness that appeared after the trade center tragedy. A total of 250 were on leave with stress-related problems. Those numbers include 37 workers with both respiratory and stress problems.

Out of the 300-plus firefighters who developed a severe and persistent condition dubbed "World Trade Center cough," about half are still on medical leave or light duty, or are awaiting evaluations for disability retirement.

The "World Trade Center cough," which includes coughing, wheezing and shortness of breath, was defined for the study as requiring at least four straight weeks of medical leave. Within six months of the attack, it appeared in 332 firefighters and one paramedic; with treatment, about half have recovered and returned to full duty, Prezant said.

He said he expected a higher recovery rate based on previous experience with smoke inhalation, where the level is closer to 90 percent, he said.

Asked why the cough recovery rate is lower, he said it is not known what firefighters were exposed to last Sept. 11, but that tiny particles in the huge dust cloud could themselves be highly dangerous if inhaled. The sheer volume of particles and lengthy exposure over days probably sets the trade center experience apart from ordinary firefighter exposures, he said.

He said symptoms have improved to varying degrees even in the firefighters who have only partially recovered.

"Life is becoming a little more livable," he said. "But they're a long way off from (being) the physically active, athletic firefighter."

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