The researchers, whose findings were reported in the online edition of the http://www.pnas.org/cgi/doi/10.1073/pnas.0404499101Proceedings of the National Academy of Sciences estimated that exposure to certain carcinogens released at Ground Zero only added a one person per 100 million increase in lifetime risk for individuals who lived or worked in the area. The carcinogens studied were polycyclic aromatic hydrocarbons (PAHs), compounds formed by the incomplete combustion of organic materials such as fuel, wood, and even cooked meat. They often attach to particulate matter such as soot and circulate in the atmosphere, where they can be inhaled.
Stephen M. Rappaport, PhD, co-author of the study and professor of environmental health at North Carolina, said the team focused on PAHs because these compounds are always associated with burning organic materials and cancer risk. That connection was first shown in 1775 when exposure to PAH-containing soot among British chimney sweeps was linked with the appearance of squamous cell carcinomas. Since then, PAHs have also been associated with human cancers of the skin, lungs and bladder.
"Whenever you burn hydrocarbon fuels you produce PAHs," Rappaport said. "But there was no research on risk of exposure to PAHs following the collapse of the World Trade Center."
Following the collapse of the World Trade Center's twin towers, fires burned through Dec. 20, 2001, fueld by approximately 91,000 liters of jet fuel from the airliners that crashed into the buildings. The fires spread to an estimated 100,000 tons of organic debris, 490,000 liters of transformer oil, 380,000 liters of heating and diesel fuel, and gasoline from several thousand automobiles stored beneath the towers, the researchers said. The fires, plus emissions from construction machinery operated at the site after the collapse, created between 100 and 1,000 tons of PAHs that circulated and settled primarily in lower Manhattan.
The researchers analyzed 243 air samples that had been collected from four areas surrounding the collapse site for 200 days following Sept. 11. Three of the sites were at the fence line of Ground Zero; the fourth was on the 16th floor of an office building about 0.5 km from Ground Zero. The investigators found air concentrations of PAHs on Sept. 14 ranged from 1.3 to 15 ng/m3. By day 200, PAH levels had returned to background levels, comparable with the range of 0.03 to 0.50 ng/m3 typically seen in Los Angeles.
The researchers estimated that over a 70-year lifetime, 15.6 people per 100 million will develop cancer due to baseline PAH levels. The new data suggest 16.7 people per 100 million will develop cancer due to elevated PAH exposure related to the World Trade Center disaster, the researchers said.
While the findings offer some reassurance, Rappaport cautioned that they pertain only to individuals with jobs or residences in lower Manhattan, not to emergency personnel and construction and demolition crews working at Ground Zero, who could have had more intense interaction with PAHs.
Rappaport also noted that the study focused only on outdoor air, which is transient in nature. Sizable amounts of PAH-containing soot remains trapped by the buildings in the area, he said.
"People living in lower Manhattan had a great deal of contamination in their apartments; the dirt and soot blew in and can still be there," Rappaport noted. "Also, these materials could be recycled throughout the buildings by the heating and air-conditioning systems."