Dust blows in from the ends of the Earth, even beyond

Tuesday, September 14, 2010; HE03

Dust. It's not just the bane of anyone trying to keep a tidy home. It can support life, or smother it. It can change continents.

Dust's composition varies a lot from one dusty place to another. The only common theme is that each speck is minute, no larger than about half a millimeter across. Thanks to that small scale, dust particles are not completely under the sway of gravity. Other physical forces -- static electricity, magnetism and wind currents, for example -- can help grains cling to objects or keep them aloft for days.

In your home, dust is mainly organic material, including flakes of dead skin, bits of fabric and the excrement of dust mites, which are ticklike creatures that literally eat dust and are tiny enough to be dust particles themselves. (Makes you want to grab a cleaning rag, doesn't it?)

By contrast, the dust on the hood of your car typically comes loaded with pollen grains, toxic particles of soot that exhaust pipes have spewed out, and bits of pavement that have been worn down and kicked up into the air. Metro commuters encounter other kinds of dust: Subway stations around the world have been found to contain high levels of metallic particles that have been ground off the rails or off trains' wheels and brakes.

There's dust in space, too -- so much of it, in fact, that a fine mist of extraterrestrial dust falls steadily to Earth. A couple of years ago, researchers traced much of the incoming material to iron-rich asteroids orbiting between Mars and Jupiter. Budding scientists can Google "collecting micrometerorites" to learn how to capture a few grains using little more than a magnet and a glass of water or a sheet of paper.

In agricultural regions, the main ingredient of dust is soil, often mixed with soil bacteria, fungal spores, globs of pesticides and the like. This earthy stuff has global significance because there's an enormous amount of it out there and it tends not to stay put.

It blew its way into America's history books with the disastrous Dust Bowl of the 1930s, when massive dust storms raged across the Great Plains. Hundreds of thousands of people fled the plains, many died of dust-related pneumonia, and great black clouds of dust traveled as far as Boston and Washington.

Dust Bowl-like phenomena are more common worldwide than many people realize. Last September, a massive dust storm blew out of central Australia and briefly enveloped most of that country's eastern coast. In Sydney and other cities, the sky turned orange, flights and ferries were canceled, and emergency-room visits spiked, according to news reports.

The Australian government estimates that the storm involved more than 16 million tons of airborne material, some of which ended up in the ocean.

Elsewhere, major dust storms are an annual occurrence. By one estimate, nearly 1 million tons of wind-blown dust from the Gobi Desert descends on Beijing every year, adding to the air pollution that the city itself generates. Even more blows out to sea: Studies in places as far away as Hawaii have found that dust from Asia can be a major constituent of the soil on Pacific islands.

Health experts worry about the effects of inhaling dust of all sorts.

The fine-grained particles get sucked deep into the lungs, where they can cause mischief. People with respiratory disorders such as asthma are at particular risk. Surprisingly, so are people with cardiovascular problems. Tiny particles in auto exhaust have been found to pass through the lungs and enter the bloodstream. The particles' effects in arteries may be one reason people who have recently been in traffic face an elevated risk of heart attack.

Environmental scientists, meanwhile, have linked dust to both positive and negative ecological effects. Wind-borne dust often injects valuable nutrients, including iron, into far-flung ecosystems. But dust has been also associated with toxic algal blooms and mass fish deaths as well as the decline in some coral reefs.

Northwestern Africa is a cauldron of dust production. Every year, vast plumes of reddish dust rise up from the arid Sahel region, which fringes the southern edge of the Sahara, and blow out over the Atlantic toward the Americas on prevailing winds.

This flow of dust has increased dramatically over time. According to a study published in July, one jump occurred in the mid-1800s, when commercial agriculture took off in the region. The dust flux has hit even higher levels since the 1970s, when the Sahel entered a prolonged period of drought that has not entirely abated today.

These days, by one estimate, Africa's dust export to the Atlantic is about 240 million tons a year. More than half of it falls into the ocean. But massive amounts waft westward for a week or so and eventually make landfall in the Amazon, Caribbean and southeastern United States.

Consequently, much of the soil in the tree canopy of the Amazon rain forest is of African, not South American, origin. This imported raw material lands on the crooks of trees, for example, and supports entire ecosystems of aerial plants that take root up there and live high above the forest floor. Africa's red dust coats the Caribbean's islands, too.

And at times, African dust hangs so thick in the air over Florida that the state has been in violation of federal air-quality standards for particulate matter. According to one scientific analysis, moreover, about half the mass of African dust that's airborne in southern Florida during the summer months is in particles so small -- 2.5 micrometers or less across -- that they can be inhaled deep into the lungs.

But as you tidy up for the cooler days ahead, it's worth noting that the dust you sweep off your porch may have traveled farther this summer than you did.

Harder is general manager of health and science at U.S. News & World Report.

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