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October 20, 2003
Volume 81, Number 42
CENEAR 81 42 pp. 26-30
ISSN 0009-2347

Scientists struggle to understand the complex mixture of aerosols released during and after the destruction of the World Trade Center



PLUMES OF SMOKE Fallout dust from the collapse of the WTC towers was composed mainly of construction materials. NYPD PHOTO

With the collapse of New York City's two World Trade Center (WTC) towers on Sept. 11, 2001, more than 1 million tons of dust enveloped lower Manhattan. And fires that lingered at ground zero until December created a plume of smoke initially detectable from space.

Aerosols that could be seen and smelled were inescapable during and long after the attack on the World Trade Center. But aerosols are also transient. Were it not for a handful of scientific groups that quickly mobilized, usually donating time and money to collect ambient air data and dust samples near ground zero, little would now be known about the WTC aerosols.

Many of the scientists who tested and analyzed the WTC air gathered for a symposium at the recent American Chemical Society national meeting in New York City to discuss their results. They described the composition of the fallout dust, estimated where it came from, and proposed how its properties led to the "World Trade Center cough." They described the plume of smoke, what it was made of, and how weather patterns in New York at the time affected its movement. They debated how much of the pollution of lower Manhattan could be ascribed to the plume and how much was native.

The data they accumulated are hard to interpret. "There were a lot of variations in results," said Roger N. Clark of the U.S. Geological Survey (USGS). Different groups were using different instruments. "With different instrumentation, people see different stuff," Clark remarked. "Any one individual data set gives a picture, but it's not the whole picture."

Lack of a whole picture has led to some conflict among scientists and some confusion among the general public about WTC aerosols and how safe they were to breathe. Last month, the Office of Inspector General (OIG) released a report (PDF file) stating that the Environmental Protection Agency didn't have the data to support its pronouncements shortly after Sept. 11 reassuring New Yorkers that the air was safe (C&EN, Sept. 1, page 23). And some scientists argue that there still is not enough known to make that pronouncement. We know that "no one died from inhalation of the dust," said Paul J. Lioy, deputy director of government relations for the Environmental & Occupational Health Sciences Institute (EOHSI), Piscataway, N.J. "But we don't know what all the health effects are."

The WTC disaster was entirely unparalleled, and not all data about the aerosols have been released, even two years later. A full analysis and interpretation will take some time yet. The ACS symposium was just one of the steps that may help scientists come to a consensus on the nature and fate of urban aerosols released on and after Sept. 11.

In the first four hours after two planes hit the WTC towers, a massive cloud of dust and smoke from the collapse and explosions filled the air. "Basically, you had blackout," Lioy said. The dust gradually settled, resuspended, and settled again over the next few days. On the third and fourth days, it rained and the dust in the air diminished. Fires continued to burn and the plume lofted, yet was intermittently pushed down by inversions. On the 13th day, search and rescue was abandoned, diesel engines started up, and the site became a construction zone. A plume of smoke rose from ground zero until the fires were extinguished on Dec. 20.

In a broad sense, the WTC attack generated two different kinds of aerosols: pulverized dust from the collapse of the towers and smoke from the fires in the debris pile. Other pollution sources were affected by WTC activity, notably demolition at the site, which started in mid-October; diesel generator emissions; and traffic pollution.

The dust "was unlike any dust and smoke mixture I had ever seen before," Lioy said. The fluffy, pink and gray powder "was basically a complex mixture of everything that makes up our workplaces and lives." Six million sq ft of masonry, 5 million sq ft of painted surfaces, 7 million sq ft of flooring, 600,000 sq ft of window glass, 200 elevators, and everything inside came down as dust, said Greg Meeker of USGS. The only thing that didn't get pulverized was the WTC towers' 200,000 tons of structural steel. That was just bent, Meeker said.

Lioy and his collaborators examined the dust using microscopic, inorganic, organic, and particle size fractionation analysis. They found plaster, paint, foam, glass fibers and fragments, fiberglass, cement, vermiculite (used as a fire retardant instead of asbestos), chrysotile (asbestos), cotton fibers and lint, tarry and charred wood, and soot.

"Yes, there was some impact of the World Trade Center fires and activity. But it wasn't earth-shattering, and I don't think it poses any serious health risk to the public."

THE ASBESTOS was at first very worrisome. Initial reports claimed that the dust consisted of as much as 20% asbestos--"an enormous amount," according to Meeker. So early on, EPA and the U.S. Public Health Service recruited the help of those at USGS who are expert in testing for asbestos. USGS, in cooperation with the National Aeronautics & Space Administration's Jet Propulsion Laboratory, sent out a spectral imaging instrument that, when flown over the WTC site, determined the chemical composition of ground debris. The AVIRIS (Airborne Visible/Infrared Imaging Spectrometer) essentially does environmental mapping with imaging spectroscopy.

The picture it created wasn't as easy to interpret as USGS had hoped. "We didn't realize the complexity of trying to map a pulverized building in amongst a bunch of other buildings," Clark said. AVIRIS pictures did indicate that localized pockets of exposed asbestos may have existed, he added. But he and his coworkers weren't able to confirm the suspicion with targeted sampling at those sites. And random sampling done around ground zero by a number of groups, including USGS, showed that levels of asbestos generally remained under 1%. Only one of the samples collected by USGS contained roughly 20% asbestos, and that sample came off one of the steel beams that had been coated with asbestos before its use was discontinued.

The USGS team also analyzed WTC dust using scanning electron microscopy (SEM) and X-ray diffraction analysis. Like Lioy's group, USGS scientists discovered a complex mixture of materials: glass fibers (up to 40% in some samples), gypsum (wallboard), concrete, paper, and other construction debris. "I was just amazed at how many glass fibers there were," Meeker said. The high concentration of glass was due partially to windows, but primarily to ceiling tiles. SEM revealed that much of the glass was present as odd-shaped fibers and spheres. "It's not an effect of the collapse," Meeker said. These compositions are compatible with "slag wool," a common component of ceiling tiles and other building materials.

All of the samples were very alkaline. Aqueous suspensions of Lioy and Chen's dust samples ranged from pH 9 to 11.5. Nobody was surprised by the high pH values, which are mainly due to cement dust. CaOH, CaCO3, and CaSO4 from cement, wallboard, and other construction materials permeated the samples.

IN ADDITION, most of the dust particles were relatively large. When environmental health scientists look at aerosols, one of the first things they are concerned about is the size of the particles. "Aerodynamic diameter is what determines where something lodges in the body," said Joseph Pinto, an aerosol scientist with EPA's National Center for Environmental Assessment.


POWDERY CONDITIONS An analysis of WTC dust (top) revealed many odd-shaped glass fibers that come from slag wool, as shown in this scanning electron microscopy image. USGS PHOTOS

If a person breathes in particles having a diameter larger than 10 µm, chances are they won't get past the nose and throat. They are either swallowed or expelled by coughing or sneezing. Particles between 2.5 and 10 µm generally only get as far as the upper airways of the lung. But particles smaller than 2.5 µm can reach the deepest surfaces (the alveoli) of the lung. These are considered to pose the greatest potential harm.

So the designations PM 2.5 (which stands for particulate matter equal to or less than 2.5 µm in diameter) and PM 10 (equal to or less than 10 µm) are not arbitrary. Finding those particular size ranges automatically indicates possible health risks. The mass size distribution of a typical ambient air sample usually shows two peaks: one peak at approximately 7 µm and one at approximately 0.3 µm. Scientists often collect them as the coarse (2.5–10 µm) and the fine (0.1–2.5 µm) fractions.

Most of nature's aerosols (for example, plant fragments or pollen) and mechanically generated aerosols (for example, demolition dust) are in the coarse fraction. Aerosols from other man-made sources--such as diesel emissions or chlorinated aerosols--cluster in the fine fraction. Because the fine fraction can get past the body's defenses into the deep lung, small particles were among the first species that scientists started looking for in the WTC dust.

They didn't find many. According to Lung Chi Chen and George D. Thurston, professors of environmental medicine at New York University School of Medicine, more than 95% of the mass of the dust particles consisted of particles larger than 10 µm and more than 50% consisted of particles larger than 53 µm. Particles that big are often off the radar screen of health scientists, and the government paid little attention to them when studying the WTC dust, they said. The absence of fine particles, and little evidence of widespread asbestos, may have been some of the early evidence that EPA used to justify its statements in early September that the dust was not harmful. However, Chen and Thurston continued, some of the workers at ground zero have complained of what they call the "World Trade Center cough."

The culprit for the cough, Chen and Thurston discovered, was actually in particles larger than 10 µm. Large dust particles, because they were alkaline and caustic, irritated the upper passages of the nose and throat. And though this shouldn't lead to long-term health effects, large, caustic particles and fiberglass caught in the upper airways cause acute short-term effects. Lioy thinks fiberglass was particularly responsible because it was found in the sputum of firefighters 10 months after Sept. 11.

Chen and Thurston were less concerned about caustic particles in the fine fraction because they saw that pH decreased with decreasing particle size. Therefore, the few particles that did get down to the deep lungs were less likely to be alkaline. This jibes with other data presented by Chen that particles between 10 and 53 µm had more construction-related elements (Al, Ca, Mg, Ti, Fe, Zn), while particles smaller than 2.5 µm were more likely to be combustion related (Cl, Pb, and Cu).

Some scientists also studied the dust for evidence of persistent organic pollutants. John H. Offenberg, a chemist who was in the department of environmental science at Rutgers University but is now at EPA, looked for polychlorinated biphenyls (PCBs), organochlorine pesticides, and polycyclic aromatic hydrocarbons (PAHs). Only PAHs showed up in significant amounts. PAHs represented nearly 0.04% of the mass of settled outdoor dust, Offenberg said, and they likely came from the huge amounts of burned and partially burned hydrocarbons present in plastics, woods, and other synthetic products found in offices.

Combustion products continued to be emitted from the debris pile in the ensuing months. Dust was "no longer part of the plume per se after about day three or four because the rains came and washed some of the dust and smoke away," Lioy said. What was left were smoldering fires.

The fires, which began at over 1,000 °C, gradually cooled, at least on the surface, during September and October 2001. USGS's AVIRIS also measured temperatures when it flew over ground zero on Sept. 16 and 23. On Sept. 16, it picked up more than three dozen hot spots of varying size and temperature, roughly between 500 and 700 °C. By Sept. 23, only two or three of the hot spots remained, and those were sharply reduced in intensity, Clark said.

However, Clark doesn't know how deep into the pile AVIRIS could see. The infrared data certainly revealed surface temperatures, yet the smoldering piles below the surface may have remained at much higher temperatures. "In mid-October, in the evening," said Thomas A. Cahill, a retired professor of physics and atmospheric science at the University of California, Davis, "when they would pull out a steel beam, the lower part would be glowing dull red, which indicates a temperature on the order of 500 to 600 °C. And we know that people were turning over pieces of concrete in December that would flash into fire--which requires about 300 °C. So the surface of the pile cooled rather rapidly, but the bulk of the pile stayed hot all the way to December."

Where the smoke went depended on the weather and the wind. "Most of the time, there was very clean air coming into New York throughout September," Thurston said. "I think we all recall the sky on Sept. 11, how clear blue it was." He described how the sun warms Earth during the day, and the warm air rises to create a vertical mixing effect. "The plume would tend to go straight up into the air," he said, and for a large portion of September and October, clean air came in to replace it. This was supported by a detailed reconstructive model of the plume's movement created by Panos G. Georgopoulos at EOHSI.

"But at night, when there was less mixing and the wind speeds dropped, the plume would be lower down," Thurston said. Soot levels at Thurston's sampling sites peaked at night. The plume tended to fall during the night and rise again during the day--that is, except for when a local inversion occurred or a longer term regional inversion settled over the city, notably on Oct. 3, 4, and 5. Many of the groups described the plume "hitting" their sites on one or more of those days because, as Thurston said, the plume was held close to the ground by warm air above it.

"My findings don't show any problem," any worrisome health risk. "But it doesn't mean there wasn't a problem."

BUT EVEN WHEN the plume dropped, not every site registered it. When the plume came down to street and building level, it didn't spread uniformly. It concentrated in narrow streams. Pinto called the roads in between tall New York office buildings "street canyons." The plume could snake through them, concentrating in some areas and dispersing in others, sometimes following wind patterns that were perpendicular to the winds coming into Manhattan. This meant that sometimes the plume would hit sites that weren't downwind at all. "These meandering plumes are very narrow in extent, especially under stable, nighttime conditions," Pinto said.

Whether a sampling site got a hit from the WTC plume depended on weather, location, wind, and time of day. And even when a group detected a spike of sulfur or chlorine, it could be difficult to determine if that came solely from the WTC plume, a local emission, or from down south and west in New Jersey.

Thurston believes that a good portion of the pollution detected at or near the WTC site in fall 2001 was pollution that would have been there in any case. Even without the disaster, air pollution levels like New York City's give a person a lung cancer risk roughly equivalent to that of a nonsmoker living with a smoker, he said.

In one of Thurston's studies, his group collected PM 2.5 samples daily at the downtown NYU hospital (five blocks east of ground zero) and in midtown Manhattan (1st Ave. and 26th St.) from Sept. 14, 2001, through December 2001. They weighed each sample and looked for trace elements using X-ray fluorescence. Then, to pinpoint the interplaying sources of pollution and tease out which source contributed which element on which day, Thurston applied a statistical tool to the data called positive matrix factorization source apportionment.


SMOLDERING Smoke rose from the burning pile from September to December 2001. NYPD PHOTO

The analysis came up with five pollution sources: the WTC plume (which contributed S, Cl, K, Cu, Zn, Pb, and elemental carbon); dust from the WTC collapse (Mn, Cr, and elemental carbon); dust from cleanup demolition at the WTC site (S, Si, Ca, Ti, Fe); residual oil combustion (S, V, and elemental carbon); and traffic dust (Al, Fe, and elemental carbon). The first three, which are all WTC sources, made up 55% of the fine particle pollution in lower Manhattan in September, 33% in October, 21% in November, and about 7% in December, Thurston said. "By October, at sites five blocks away from ground zero, the air was really like other parts of the city. It was, thankfully, abbreviated exposures that people got to this plume--when they did get it."

Other researchers who also took samples at the NYU site and repeated their sampling a year later generally echoed Thurston's conclusions. Maire S. A. Heikkinen and Beverly S. Cohen, both at NYU School of Medicine's department of environmental medicine, tested specifically for fine and ultrafine particles (below about 2.5 µm in diameter). They found that the number and mass concentration of PM 2.5 (in terms of total amount) was similar in 2001 and 2002, with somewhat elevated mass concentrations and fraction of coarse particles in September and October of 2001. Fire and dust-associated elements--such as lead, chlorine, calcium, iron, and elemental carbon levels--went down in 2002 compared with 2001, but concentrations of sulfur and vanadium, which are associated with routine local pollution, stayed about the same.

A few researchers from EPA focused on organic acids and semivolatile organic compounds in ambient air collected near the WTC site between Sept. 26 and Oct. 21. Michael D. Hays at the National Risk Management Research Laboratory, Research Triangle Park, N.C., observed a low ratio of alkanoic acids to alkanes, which is indicative of diesel exhaust. He also saw elevated levels of levoglucosan, from burning cellulose; bromine, from burned fire retardant; and chlorine, which he estimated may come from sea salt.

"We see the WTC emissions," Hays said. "However, there was natural processing [of the acids] and a gradual clearing as time went on." For example, they detected pinonic acid, which originally comes from a wood forest, naturally at some distance from the WTC site. Erick Swartz, who at the time worked for the National Exposure Research Laboratory, also saw a PAH ratio that indicated high diesel emissions and compounds known to be in wood smoke (for example, retene).

He pointed out that he also detected a compound he had never seen before in ambient air samples--1,3-diphenylpropane--which, he said, is associated with plastics. He thinks it is a unique marker for the WTC plume and is concerned there may be other compounds in the plume that people didn't even think to look for.

"Yes, there was some impact of the World Trade Center fires and activity," said Yair Hazi, an associate research scientist in the department of environmental health sciences at Columbia University Mailman School of Public Health, who also took samples at the NYU downtown hospital. "But it wasn't earth-shattering, and I don't think it poses any serious health risk to the general public."

Hazi ran an instrument that collected eight size fractions of particles smaller than 15 µm. He used size-segregated sampling because it can be a reliable method for identifying sources of particles. He wanted to find out if the burning fires and activity at the WTC site impacted the concentration and composition of ambient PM in surrounding neighborhoods. "EPA's 24-hour average limit for PM 2.5 is 65 mg per m3. According to my data, it was not exceeded." Even EPA's lower, more stringent standard of 40 mg per m3, set to protect more sensitive populations, was not exceeded, Hazi said.

HIS ANALYSIS easily picked up the large regional PM 2.5 episode in the New York area between Oct. 2 and 5. He saw a large peak of particles containing sulfur at about 1.0 µm and a smaller sulfur peak at about 0.3 µm. The sulfur in the larger diameter peak had probably traveled from far away to get to New York, Hazi said. Combustion-related particles tend to agglomerate as they move through the atmosphere, so foreign-born aerosols will be slightly larger than local aerosols.

He also saw an unusual peak of sulfur particles in the coarse fraction between Oct. 17 and 21. In mid-October, workers at the WTC site had begun demolition, and Hazi guesses that the peak of large sulfur particles is likely from the resuspension of calcium sulfate particles from gypsum board. Hazi also observed a large increase in the fine fraction for Fe and Mn during December 2001, when WTC workers were using a large number of blow torches on the bent steel.

Hazi looked at Pb, Cu, and Zn as indicators of combustion of WTC materials, such as computer parts, electrical systems, and office furniture. He found more of them in the submicrometer fraction in 2001 compared with 2002.


SPECTRAL MAP Red and yellow indicate where concrete dust fell, and purple shows gypsum wallboard in this USGS map of lower Manhattan made using a visible/infrared imaging spectrometer. USGS PHOTO

Vanadium, which primarily comes from oil burned locally for heating and is found mostly in the fine fraction, changed very little between 2001 and 2002. "I measured and showed it as a validation of the approach."

Overall, "my findings don't show any problem," any worrisome health risk, Hazi said. "But it doesn't mean there wasn't a problem."

Thomas A. Cahill, who leads the DELTA (Detection & Evaluation of Long-Range Transport of Aerosols) group at UC Davis, is more concerned about the possible health risks of the plume from WTC. Cahill first started to wonder about the plume after the rainfall of Sept. 14. "The color of the plume was all wrong," he said. "It was a light blue. My background is atmospheric physics, and the color of the plume tells me a lot. A light blue plume means very fine particles. Clearly, the pile was still hot and was giving off very fine particles." Yet very fine particles, he said, are more characteristic of a very high temperature process, such as a coal-fired power plant, a smelter, or a diesel engine. The pile at ground zero wasn't hot enough to generate such fine particles.

Cahill sent one of his instruments to a colleague in New York who started measuring ambient air on Oct. 2. As Cahill looked at the data, he began to think that perhaps the WTC debris pile was acting like an oxygen-poor municipal waste incinerator, "an enormous ground-level waste incinerator that burned for three months." If there is chlorine in an oxygen-poor municipal waste incinerator, the chlorine can combine with metals in the waste and create volatile compounds (for example, VCl4), which leave with the smoke. "Metals that would normally stay, mobilize and come out of the stack," Cahill said.

The WTC debris is clearly chlorine-rich. Paper, computers, rugs, plastics, and paint all contain chlorine. Cahill's theory is that metals in the pile reacted to form chlorinated species, which rose to the surface, and when these species encountered oxygen, they were transformed to less chlorinated but still very fine particles (smaller than 0.25 µm).

To test his theory, Cahill looked at metals in the pile and compared their volatility temperatures in the presence of chlorine. If chlorine depressed a metal's volatility temperature enough, Cahill predicted that he would detect the element in his ambient air samples. If chlorine didn't affect a metal's volatility, he didn't expect to see it. Metals that don't volatilize include barium and chromium. Though they were present in the pile, Cahill said, he didn't see them. Ni, Pb, V, and Si, however, all form volatile compounds with chlorine, and Cahill observed these metals when the WTC plume hit his sites.

"Every single metal we saw in the fine-particle mode fit our model," he said. For example, vanadium and chromium should be in the pile in equal amounts. But only vanadium showed up in the ambient air samples. Cahill attributes most of the metals that he detected to the WTC plume. "It is a complicated and miserable mix to breathe," he remarked.

NYU's Thurston sharply disagrees with Cahill on his interpretation of the data. The pile could have been acting like a municipal waste incinerator, Thurston said, but he doesn't agree that most of the vanadium and other metals can be attributed solely, or even primarily, to the plume. This sticking point is representative of their larger disagreement about how much of the aerosol pollution in the city in fall 2001 came from the burning WTC heap, as opposed to regional transport. Cahill thinks some of the disagreement can be ascribed to his concentration on very fine particles as opposed to Thurston's concentration on fine particles. "We're like blind men in a room with an elephant," he said.

Jeffrey S. Gaffney, a senior chemist at Argonne National Laboratory, who organized the ACS symposium, would like to put a book together about all the data that were presented at the symposium. He predicted that the research on the WTC dust and smoke will translate to a better understanding of the aerosols we breathe all the time. Yet, given the amazingly rich complexity of the WTC aerosols, it will take a while for scientists to resolve, analyze, and interpret all of the data they collected. Until then, as Columbia's Hazi said, "nothing is carved in stone."


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Copyright © 2003 American Chemical Society

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Related Sites
U.S. Geological Survey (USGS)

Office of Inspector General (OIG)

OIG report (PDF file)

Environmental & Occupational Health Sciences Institute (EOHSI)

National Center for Environmental Assessment

Lung Chi Chen

George D. Thurston

Thomas A. Cahill

National Risk Management Research Laboratory

National Exposure Research Laboratory

DELTA (Detection & Evaluation of Long-Range Transport of Aerosols)

Jeffrey S. Gaffney

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