6 Years Later, 9/11 Health Questi...(gh the study found. Just under 63 perce...),Health

And at least two people have died from illnesses experts have linked to 9/11 exposures. Felicia Dunn-Jones, a 42-year-old lawyer, succumbed to a disease resembling sarcoidosis five months after the attacks, and James Zadroga, a 34-year-old New York City police detective, died of pulmonary disease early in 2006.

Other studies have also suggested at least short-term respiratory effects, including a New York City Department of Health study released last month that found first responders to the attack now have a risk for asthma that is 12 times that of the general population.

But the available data may never be adequate to reveal the whole picture, experts said.

"First of all, we know nothing about the types of contaminants that were present in the days following the event, because there was no monitoring in place," Geyh said.

Her team's study, published in May in the New England Journal of Medicine, found relatively high levels of fine particles under 2.5 micrometers in diameter in air samples taken at Ground Zero in late September and October, 2001. These tiny particles can lodge deep in the lungs, potentially causing health problems for years to come.

Geyh said the U.S. Environmental Protection Agency wasn't able to install air monitoring equipment at the Trade Center site until near the end of September 2001. And she stressed that no one sample can ever give an adequate picture of the overall air quality. "It varied day to day," she aid. "We hypothesize that that had to do with how aggressive the fires were burning, or how aggressive the debris-removal activity was occurring."

Stellman, who has testified before the U.S. Senate on Ground Zero air quality and the clean-up effort, said getting a fix on the exact level of contami
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