WASHINGTON, D.C. -- Large quantities of dust, originating in Africa, are blown across the Atlantic Ocean each summer and constitute up to one half of breathable particles in the air over Miami, Florida, according to a new study. African dust can on certain days push the total number of airborne particles above the limit set by the U.S. Environmental Protection Agency under the Clean Air Act.
The study, by Joseph M. Prospero of the University of Miami'sRosenstiel School of Marine and Atmospheric Sciences, will appear in the July 20 issue of Journal of Geophysical Research, published by the American Geophysical Union. Describing himself as a "champion of dust," Prospero reviewed 23 years of measurements of airborne particles, or aerosols, at a coastal site in Miami. He believes the impact of African dust is comparable throughout the southeastern United States. His study was funded in part by the National Science Foundation.
Prospero emphasizes that there is nothing new about the transoceanic transport of African dust. He says it has been occurring on a geologic time scale, giving the example that soil composition in Bermuda is consistent with African dust sources and shows little evidence of North American inputs.
The EPA's PM (for particulate matter) standard is based on the total mass of particles measuring 2.5 microns or less that may be observed in a 24 hour period. Particles of this size can penetrate deep into the lung, where they are deposited and react with lung tissue. The standard does not distinguish among various types of particles, which include pollution and local dust along with African dust. Prospero says that manmade photochemical pollution in the eastern and southeastern United States is at its highest in summer, the same time that most of the African dust arrives. The dust causes a dense haze, but people who are unaware of the dust might mistake it for local pollution haze, he says.
In assessing an area's compliance with its PM 2.5 standard, EPA may exclude exceptional natural events, such as volcanic eruptions, wild-land fires, and high-wind events. This last category would cover dust, but EPA is referring only to local dust whipped up by unusually high winds, as in a storm. During African dust events, wind speeds are close to average and provide no clue as to the cause of the high PM 2.5 concentrations. Other indicators must be used to identify the presence of African dust.
EPA's standards are established on the basis of health issues. Prospero notes that very little research has been done into the health effects of ambient, or "normal," dust in the air, as contrasted with dust in coal mines and other contained industrial environments. Further, it has not been shown which, if any, specific components of dust may affect human health. Various mechanisms have been proposed by which airborne particles might damage lung tissue, one of which involves iron-rich coatings on particles. Prospero notes that African dust particles contain large amounts of iron as a coating, providing its characteristic red-brown color. Once dust is deposited in the lung, the iron coating would likely be released to the lung tissue.
Historically, most studies of dust have focussed on its role in forming deep sea sediments. Prospero had argued for years, he says, that dust must also play a role in climate forcing, observing that it was the most prominently visible particle constituent in satellite images. He says that in the last few years, a number of researchers have been investigating dust as an element in climate change. Although dust is "natural," Prospero says, much of it is liberated into the air the result of poor land use practices and could therefore be classified a pollutant. He expresses satisfaction that after all these years, dust--or at least the study of it--is finally gaining some respect.
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