June 6, 2001 Most residents of the parched southwestern United States accept dust as an unavoidable fact of desert life. The silty powder that settles from the air on to desktops, beneath beds, and into noses is viewed by many as nothing worse than a common annoyance. But, in recent years, dust has gone from being a benign nuisance to major health hazard, as scientists have discovered harmful chemicals and microorganisms hitching a ride on the airborne particles.
Storms in places as distant as China and Africa have generated public attention with dust clouds that travel across oceans to North America, bringing with them living bacteria, fungi, heavy metals, and other pollutants.
Researchers at Arizona State University now say that dust generated in our own back yards could be equally dangerous. Dust blowing from local industries and agricultural fields has the potential to carry cancer-causing pesticides and toxic heavy metals, says ASU geologist William Stefanov. Though the fine dirt that settles into homes across the Phoenix area may look harmless, chronic inhalation of contaminated dust could lead to increased risk for cancer or heavy metal poisoning, leaving Arizona residents to wonder: Do you know where your dust has been?
In collaboration with ASU geology Professor Philip Christensen and University of Pittsburgh geologist Mike Ramsey, Stefanov is using images taken from space to map the movement of dust in Arizona. The maps can then be used to determine areas where health risks are most likely, and where scientists should do additional monitoring. He will present the first results of this project at the spring meeting of the American Geophysical Union in Boston on May 30, 2001.
Stefanov uses images taken by Landsat 7, a satellite operated jointly by NASA and the United States Geological Survey, to identify the areas where winds are most likely to pick up dust, carry it through the air, and eventually drop it off. From the Landsat 7 pictures, Stefanov can differentiate native soil from urban areas, concrete, agriculture, and grass. After using remote sensing to categorize the land use types, Stefanov double-checks them from the ground or by using aerial photographs.
Areas with dry, exposed soil, such as industrial and agricultural areas, are the land use types most likely to produce dust. Cities, with their smooth, paved surfaces, are areas where dust gets blown through without settling. Vegetated, grassy areas, such as golf courses, cause the wind to slow down and deposit its dusty load.
"The biggest problem comes when there's a large-scale disruption of the surface, like a construction project," says Stefanov. "When the soil has been broken up, the fine material, where a lot of these pesticides and heavy metals might be, are free to be picked up by the wind." Frequent watering of the ground surface during construction helps to minimize transport of this dust. Agriculture and industry may contribute to the health dangers of dust by introducing toxic substances into the soil, Stefanov explains. "Pesticides and herbicides can be applied for years in some areas, and that material doesn't just disappear in the soil. It has a very long residence time. Pesticides that were applied 20 years ago can still be there." The toxins adhere to the soil particles and are carried on the wind along with the dust. When the dust is inhaled, the pesticides and heavy metals are taken in, too.
Microorganisms such as fungi and bacteria also often take up residence in the soil and, when the earth is disturbed, get carried on the wind. Valley fever, an infection caused by a soil-borne fungus, is particularly common in the southwestern United States, especially during late summer, when soils are driest.
In his presentation at the American Geophysical Union conference, Stefanov will present data on the sources and sinks for dust in the Nogales, Arizona area, where he conducted the first tests of the use of remote sensing to map dust transport. Stefanov plans to apply the same techniques to mapping dust sources in the Phoenix area. Stefanov started mapping in Nogales because the region is growing quickly and, because construction generates lots of dust, he anticipated that dust transport could be a problem there. The Landsat 7 maps showed that construction sites are actually fairly minor contributors to dust in the region's air, whereas industrial sites are the most productive dust sources. Thickets of mesquite trees are settling-out areas for the dust.
In the rapidly expanding Phoenix area, growth is most rapid at the urban fringe, where agricultural fields are being replaced with residential and industrial developments. These are likely to be prime areas for the release of pesticides on airborne dust. Most of the dust traveling through the city of Phoenix comes from the surrounding mountain ranges. During summer, air currents coming from the Gulf of Mexico blow in from the south, passing Tucson en route. During winter, winds originate over the Pacific Ocean, traveling over the Sierra and Cascade Mountain ranges before reaching Phoenix. The particles carried on these winds are believed to be the source of most of the region's soil.
"Dust has been known as a health hazard for at least 20 years," says Stefanov. "In the Valley, there's certainly a potential health risk." Because the damaging health effects of breathing dust depend on the amount of exposure, those most at risk are people living or working near a major dust source. "If you drive through a dust cloud and breathe some dust, it's probably not going to hurt you," Stefanov says. "But if you live down-range of an industrial site or some area where dust is always being generated and you're always breathing this stuff, then you might have something to worry about."
Recently, public concern about dust-borne pathogens grew when marine biologists blamed the rapid demise of Caribbean corals on attacks by a fungus carried in Saharan dust. Droughts in Africa are causing larger, more frequent dust storms, some of which produce clouds of fine dust covering thousands of square miles. Trans-Atlantic winds can carry the dust to North America and the Caribbean. And, according to another recent study, the sandy particles kicked up in the dust storms may actually further reduce rainfall, exacerbating the drought conditions that caused the storms and fueling a vicious cycle.
Corals may not be the only victims of exposure to toxic dust. Last fall, samples of dust carried from the Sahara to the U.S. Virgin Islands were shown to harbor heavy metals, bacteria, fungi, and what appeared to be viruses. Some researchers suspect that pathogens in the dust clouds are responsible for the high rates of asthma in the United States and the Caribbean.
Major storms in the Gobi desert of Mongolia and China have also thrown dust and other pollutants, including arsenic and toxins from burning fossil fuel, into air currents headed across the Pacific Ocean. This year, one massive cloud reached the western United States before dispersing.
The Landsat 7 satellite was launched by NASA in April 1999. Images are archived, processed, and distributed by the U.S. Geological Survey (USGS), which is also responsible for day-to-day operations of the satellite.
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