Dust in the Wind. (Focus)
Taylor, David A., Environmental Health Perspectives
On a blistering June day in West Africa, standing on the bank of the Senegal River, one might look east and see what appears to be a blood-red curtain billowing the length of the horizon. When the curtain arrives, daylight disappears, and the land is covered in a dark red, gritty-tasting night. Going inside offers little protection--wind-blown dust soon penetrates shutters and plaster cracks, leaving a thin red layer everywhere; rooms feel like mine shafts. This is a dust storm in Africa's arid Sahelian band, the southern fringe of the Sahara Desert from Mauritania to Chad.
Every year, Sahelian dust storms, which can rise up to 4 kilometers into the sky, launch masses of dust--estimated from 500 million to over 1 billion tons--into the atmosphere. By comparison, Mount Pinatubo's massive 1991 volcanic eruption sprayed just 29 million tons of comparable ash.
Eight weeks before a Sahelian storm reached Florida last June, a dust cloud from another direction made headlines. In April 2001, two dust storms from Mongolia's Gobi Desert passed over the Aleutian Islands to the Pacific Northwest, penetrating North America. Between April 14 and 18, the first, large storm--at one point roughly the size of Japan--blanketed Colorado with a fine, whitish coating. A second, smaller storm dumped on the Pacific Northwest region the following week. Storms from both Africa and Asia appear to be growing more frequent.
Although dust itself is small--most particles range in diameter from 0.01 to several dozen micrometers--the subject is huge. Hannah Holmes, author of The Secret Life of Dust, states it boldly: "It regulates the weather. It influences the climate. Its powers seem unlimited.... This stuff makes the world go around." Dust can also sway the fates of nations and wars: recently a broadcast series of daily satellite images of dust plumes in Asia was stopped because some of them originated over Afghanistan, and knowledge of their whereabouts had military value, according to an unnamed official at the National Oceanic and Atmospheric Administration (NOAA).
Increasingly, the global dust "budget," as scientists refer to the total amount of matter in global circulation, is attracting scrutiny for its changing patterns, its possibly far-reaching effects on climate and ecosystems, and its potential impacts on public health. Dust and other particulate matter that occurs naturally (from volcanic eruptions, forest fires, live vegetation, and sea spray) makes up 90% of airborne aerosols; about 10% of aerosols in the atmosphere are caused by humans, mainly as residues from automobile and industrial exhausts.
That 10% used to be the main concern for health, but researchers see growing importance in naturally occurring dust storms. Dust motes averaging less than 2.5 [micro]m in diameter are in the range of particles that research is showing may have serious health consequences. In addition to the mineral composition of the particles themselves, dust can carry with it a variety of hitchhikers including bacteria, fungi, and chemical contaminants, all of which may adversely affect health and the environment.
Students of Storms
The naturalist Charles Darwin was on the trail of dust ill the early 1830s. Aboard the HMS Beagle when it docked in the Canary Islands, Darwin noted the common sight of dust accumulation on ships during the Atlantic passage, where it could foul the instruments and cover the decks in a fine, reddish film. Darwin speculated on where the dust came from and where it ultimately settled. Since then, others have followed his lead.
One fellow observer is Joseph M. Prospero, director of the University of Miami's Cooperative Institute for Marine and Atmospheric Studies. Like his Shakespearean namesake, Prospero knows about storms. He has studied dust storms from the Sahel and their trail across the Atlantic for nearly 40 years.
Prospero first became interested in dust while studying the mineralogy of deep-sea sediments. …