In the most thorough analysis yet of the dust and smoke blown through lower Manhattan after the collapse of the World Trade Center, researchers at UC Davis on Monday described unprecedented clouds of very fine particles that should be considered in evaluating rescue workers' and residents' health problems.
"No one has ever reported a situation like the one we see in the World Trade Center samples," said UC Davis researcher Thomas Cahill, an international authority on the constituents and transport of airborne particles. "The air from Ground Zero was laden with extremely high amounts of very small particles, probably associated with high temperatures in the underground debris pile. Normally, in New York City and in most of the world, situations like this just don't exist."
Cahill, a UC Davis professor emeritus of physics and atmospheric sciences and a research professor in applied science, heads the UC Davis DELTA Group (for Detection and Evaluation of Long-range Transport of Aerosols), a collaborative association of aerosol scientists at several universities and national laboratories. The DELTA Group has made detailed studies of aerosols from the 1991 Gulf War oil fires, volcanic eruptions, global dust storms, and most recently Asia.
The Manhattan air samples were collected at the request of a U.S. Department of Energy scientist from Oct. 2 through mid-December, by a DELTA Group air monitor placed on a rooftop at 201 Varick St., one mile north-northeast of the trade center complex.
The samples were collected continuously in eight separate size modes from coarse (12 micrometers diameter) to ultra-fine (0.09 micrometers diameter). Regional meteorology showed that the site at Varick Street could have received material from the World Trade Center site about half the time.
DELTA Group analysts used a suite of seven techniques including synchrotron-induced X-ray fluorescence, scanning transmission ion microscopy and proton elastic scattering analysis, and soft beta mass measurements and scanning electron microscopy.
Airborne particles and health
The DELTA team analyzed the samples for the unique chemical and physical signatures of dozens of substances. They also looked for organic, or carbon-based, compounds from burning wood, plastic and carpets. And they tested for glass shards and asbestos.
Some of these materials are known to cause health problems in some people when they are inhaled in sufficient amounts. Sulfur, for example, can irritate lung tissues. Lead can damage the central nervous system. Some carbon-based compounds can cause cancer. So can the mineral asbestos.
Coarse particles are typically filtered by the nose or coughed out of the throat and upper lungs. In some individuals, they can cause allergic reactions such as dry eyes; nose, throat and skin irritation; coughing, sneezing and respiratory distress. They can also cause or aggravate breathing problems, such as asthma.
But very fine particles can travel deep into human lungs. Such small particles may have no immediate apparent health effects in moderate concentrations, but they typically are removed from the lungs through the bloodstream and heart, increasing the possibility of health impacts.
Very fine particles
There were numerous events when winds lasting six to eight hours carried unprecedented amounts of very fine particles to the sampling site. In the largest spike, the DELTA Group analysis found 58 micrograms per cubic meter of very fine particles in one 45-minute period - "an extremely high peak," Cahill said.
Cahill said the very fine particles contained high levels of sulfur and sulfur-based compounds. The very fine particles also contain high levels of very fine silicon, potentially from the thousands of tons of glass in the debris.
Cahill noted that even those large amounts were likely to be smaller than those present at Ground Zero and some other parts of Lower Manhattan, since weather data show that typically only part of the dust clouds traveled directly over the sampling site.
"Even on the worst air days in Beijing, downwind from coal-fired power plants, or in the Kuwaiti oil fires, we did not see these levels of very fine particulates," Cahill said. The amounts of very fine particles, particularly very fine silicon, decreased sharply during the month of October.
Similar to the high concentrations of very fine particles, virtually all the air samples from the trade center site carried high concentrations of coarse particles - those about 12 micrometers to 5 micrometers in diameter.
"These particles simply should not be there," Cahill said. "It had rained, sometimes heavily, on six days in the prior three weeks. That rain should have settled these coarse particles." The finding suggests that coarse particles were being continually generated from the hot debris pile.
Metals, asbestos and glass
Many different metals were found in the samples of very fine particles, and some were found at the highest levels ever recorded in air in the United States. Lead was present at low levels in fine and very fine particles.
Metals present in very fine particles in relatively high concentrations were iron, titanium (some associated with powdered concrete), vanadium and nickel (often associated with fuel-oil combustion), copper and zinc. Mercury was seen occasionally in fine particles but at low concentrations.
Although some asbestos was used in the buildings for fireproofing and in floor tiles, and the DELTA group checked for it carefully, they found very few asbestos fibers, even in the very fine particles.
All evidence indicates that ambient air in New York City now is little influenced by the World Trade Center collapse, especially since the fires are out and the debris pile has cooled.
UC Davis DELTA Group
DELTA Group scientists analyzing the trade center samples included Cahill; engineering professor James Shackelford, chemistry professor Peter Kelly, assistant research engineer Steve Cliff, and laboratory and field manager Michael Jimenez-Cruz, toxicologist.
Key collaborators from other institutions were Kevin Perry, an assistant professor of meteorology at the University of Utah, Salt Lake City; Graham Bench, a project investigator at Lawrence Livermore National Laboratory; and Jodye Selco, a visiting professor at UC Davis and a professor of chemistry at the University of Redlands.
The DELTA Group's previous projects include studies of the smoke from the Kuwaiti oil fires and two international ACE studies (for Aerosol Characterization Experiment), which are designed to shed light on the roles of aerosols in global weather, water pollution and human disease.
DELTA Group currently is continuing analyses of 2001 ACE samples - taken from sites from Asia to western North America - as well as studies of Alaskan air quality, the nature of diesel-vehicle exhaust, the impacts of air pollution on human health in the Central Valley, and the environmental health of the Lake Tahoe Basin.