Researchers will also never know how many people were exposed. "We're never going to know how many people were exposed and that makes it difficult to quantify the extent of the potential health problems," Geyh said.
Nevertheless, the people who definitely were exposed need to be followed for the next decade or even two decades, Geyh said. "We need to follow them into the future to document what happens to their health," she said. "We're not saying that there is a problem or isn't a problem. We just need to make sure we know what is happening with these people and if we understand what is happening with this well-defined group, it will make it easier for other people who feel like they're having problems related to exposure."
Visit the World Trade Center Health Registry for more on the health consequences of 9/11.
SOURCES: Alison Geyh, Ph.D., assistant professor, environmental health sciences, Johns Hopkins Bloomberg School of Public Health, Baltimore; Len Horovitz, M.D., pulmonary specialist, Lenox Hill Hospital, New York City; May 31, 2007, New England Journal of Medicine