Typically, asthma comes on quickly after the lungs are exposed to the toxic pollutants or caustic dust that resulted from the blasts, then declines as the lungs heal.
"Ordinarily, when people get exposed to toxic stuff, you get an immediate reaction. We don't expect a delayed effect," Edelman said. "We saw 9/11 responders who got sick pretty quickly. Some got better. Some did not. What we haven't seen are people who present with asthma two or three years later related to 9/11."
Diseases of the spongy part of the lungs, such as mesotheliomia from asbestos exposure, may take years or decades to develop. But asthma is a disease of the airways, which is typically apparent right away.
Researchers said there was no evidence that asthma rates continued to rise after 2005.
"However, the majority of those who developed asthma during or after their World Trade Center exposures have continued to have asthma attacks many years later," Enright said. "Their asthma did not spontaneously resolve within a few months after their WTC exposure ceased."
The American Lung Association has more on asthma.
SOURCES: Norman H. Edelman, M.D., chief medical officer, American Lung Association, and professor, preventive and internal medicine, Stonybrook University Medical Center, Stonybrook, N.Y; Lorna Thorpe, Ph.D., deputy commissioner, epidemiology, Department of Health, New York City; Paul Enright, M.D., professor, medicine and public health, University of Arizona, Tucson; Nov. 3, 2009, presentation, American College of Chest Physicians annual meeting, San Diego