In addition to the 2,749 people killed at the World Trade Center (WTC) on September 11, how many more deaths and disabilities will result from the toxic pollution released in the city? Books have been written to add up all the sources and amounts of pollutants, including asbestos, silica, lead, volatile organic compounds, polychlorinated biphenyls, dioxins, and mercury.

Scientists from Carolina and the U.S. Environmental Protection Agency (EPA) have recently added a piece to the puzzle by finding a way to measure the levels of chemical compounds — polycyclic aromatic hydrocarbons (PAHs) — generated by the combustion of fuels and other organic matter. PAHs were created by the WTC explosions involving about 91,000 liters of jet fuel; 490,000 liters of transformer oil; 380,000 liters of heating and diesel oil; and 100,000 tons of organic debris. The resulting fires that lasted well into December emitted PAHs, as did the diesel engines used to remove an estimated 1.5 billion kilograms of rubble through May 2002.

PAHs can cause cancers of the lung, bladder, and skin. They also affect the development of babies — making them smaller and more likely to be born premature and with birth defects. Although the EPA had collected air-quality samples around Ground Zero in the weeks after the 9/11 disaster, virtually no scientists were analyzing PAH levels. Why? Because comprehensive measurements of PAHs require a bulky instrument (the size of a dorm refrigerator) and a connection to an electric power source. In the chaotic aftermath of 9/11, the EPA was not able to install these specific machines on site. In January 2002, Joachim Pleil, an EPA scientist working on the Carolina campus, was talking with EPA colleague Alan Vette, who had collected many general air-quality samples at Ground Zero. When the conversation turned to the absence of PAH data, Pleil recalls, “I said, ‘What about all those archived samples? Maybe we can do something with those.’” Working with environmental-sciences and engineering professor Stephen Rappaport in UNC-Chapel Hill’s School of Public Health, Pleil developed a methodology to analyze the archived WTC samples — taken for other purposes — for particle-bound PAH levels.

Working with other Carolina and EPA colleagues, Pleil and Rappaport described their conclusions from the WTC samples in the August 2004 Proceedings of the National Academy of Sciences. The Carolina-EPA study documented PAH levels at four sites close to Ground Zero from September 23, 2001, to March 27, 2002. The researchers used these data to attribute sources (first fires and then diesel engines) of the PAHs. This is rare in environmental studies, Rappaport says.

The Carolina-EPA project is likely to inspire more studies because this type of sample is readily available. The EPA routinely uses filters that capture particles smaller than 2.5 micrometers to monitor air quality in more than a thousand locations across the country. “The methodology was not particularly novel. What was novel was just the idea to do it,” Rappaport says. “If you were talking to chemists who measure these PAHs, they would say ‘Oh, whatever.’ But if you were talking to environmental scientists who never measure PAHs because the traditional method is way too cumbersome, then they might say, ‘Wow, you mean I could use these Particle Matter 2.5 filters that I’ve been collecting for years?’ And we would say, ‘Yeah, you could do it.’”

Because pre-9/11 New York City PAH measurements were not available, the report compares its findings from 243 Ground Zero air samples to PAH levels from downtown Los Angeles and the coal-burning region of Teplice in the Czech Republic. The Carolina-EPA study predicts that on the third day after September 11, PAH concentrations in New York City were similar to those observed in Teplice in winter, “where some of the highest outdoor PAH concentrations in the world have been reported,” they write. These levels were as much as 214 times those found in New York City on the two hundredth day after the disaster. In their measurements of benzo(a)pyrene — which Pleil calls the “poster child of PAH carcinogens” — the researchers predicted 2.2 nanograms of benzo(a)pyrene per cubic meter of air for day three. Smoggy Los Angeles air has just 0.11 nanograms of the same chemical per cubic meter of air.

The researchers’ report expresses concern about children who were exposed to 9/11 PAHs in the womb, particularly in the vulnerable time during the first month of pregnancy. So far, at least one adverse effect, smaller birth weights compared to other New York City babies, has been documented in WTC pregnancy studies.

The scientists did not comment on the increased health risks of first responders, who are assumed to have higher exposures than what was captured on the air-pollution filters. Instead, this report calculates the increased PAH-initiated cancer risks for Manhattan residents and workers not involved in cleanup work. Following the standard risk assessment developed by the EPA and the California Air Resources Board, the Carolina-EPA team compared the additional risks to the risks of ordinary New York City exposures, with both exposures averaged over a seventy-year lifetime.

“We saw that the risk from September eleventh PAHs was only elevated very slightly: one in one hundred-million,” Rappaport says. “So it really wasn’t very much. That’s because even though the levels were quite high just after September eleven, they were only high for about thirty days. Over [an average life span of] seventy years, that thirty days is not very significant.”

A Sierra Club spokesperson, Suzanne Mattei, takes issue with this risk calculation but expresses appreciation for the overall contribution of the Carolina-EPA study. “This is not necessarily an accurate risk comparison for a short-term intensive exposure incident,” she wrote in a Sierra Club report. In an interview she says, “A high-intensive, short-term exposure could have a different impact than a low-level exposure spread out over time. Nobody really knows for sure the answer to that question.” She also cautions that these hazards cannot be estimated in isolation from other toxic exposures since metals and other toxins maximize PAH damage. The Carolina-EPA report itself states that the team could not factor the possible effects of asbestos, silica, and additional carcinogens into its cancer calculation.

Now Pleil and Rappaport are turning their attention to indoor air quality in lower Manhattan, where office workers and residents still fear that 9/11 pollutants circulate in the buildings.

“Grey dust filled all the spaces that were anywhere near [Ground Zero] and apparently you can still go [inside buildings] and look on top of shelves and in duct work leading to heating and air conditioning systems and you can still find this stuff,” Rappaport says. “So people are justifiably concerned that they’re still being exposed to this mixture of chemicals.” Because the Carolina-EPA team found signature PAH patterns, they can probably determine if 9/11-related PAHs are present. They are currently seeking funding for such investigations.