Asian pollution drifts over North America.

Faster than mail traveling from Beijing to Seattle, air pollution and dust from China can speed across the Pacific Ocean and blanket broad swaths of North America, according to measurements made during the past 2 years.

"[This] is the first time that anyone has ever documented that pollution from one continent can make it all the way to a downstream continent," says Dan Jaffe of the University of Washington-Bothell. Jaffe and members of other research teams presented the new data this week at a meeting of the American Geophysical Union in San Francisco.

By the time Asian pollution crosses the Pacific--which takes from 4 to 10 days--it typically does not rival the strength of home-grown grime spewing out of tailpipes and chimneys in North America. Nonetheless, these results provide a vivid demonstration that environmental problems in one country can reach nations on the other side of the globe. "We have to recognize that there is no `away.' Everybody's garbage goes somewhere," says Jaffe.

The Asian pollution rides over the ocean principally during springtime, when strong winds cut a path to North America. Jaffe's group first detected a clear burst of pollution on March 29, 1997, at a research site located on Cheeka Peak in Washington, near the westernmost tip of the contiguous United States. Measurements of air coming from the Pacific showed a jump in the concentrations of carbon monoxide, hydrocarbons, and other pollutants from fossil-fuel combustion. A meteorological computer model that tracks winds indicated that the polluted air had started in Asia 6 days earlier.

An even larger shipment of Asian pollution arrived in North America late last April. A series of strong dust storms in China lifted 140 million tons of fine soil particles into the atmosphere, where they were swept up by winds moving east, says Douglas L. Westphal of the Naval Research Laboratory in Monterey, Calif. The dust cloud appeared on satellite images, which showed the plume crossing the Pacific toward North America, he says.

It took a week for the dust to reach western North America, where it turned the sky milky white, says Thomas A. Cahill of the University of California, Davis. In late April, the Asian dust was so thick that the concentrations of fine particles in the air at the usually pristine site of Crater Lake, Ore., equaled 40 percent of the EPA daily allowable limit for the United States.

Along with the dust came measurable quantities of arsenic, copper, lead, and zinc. Air concentrations of these metals rose across the western United States on April 29. At Crater Lake, they reached more than 10 times their typical values, says Cahill. The heavy metals came from smelters in Manchuria, he concludes, because the Asian dust passed over that region before heading toward North America. There are no sources of such pollutants near Crater Lake, Cahill says.

Atmospheric scientists have previously recognized that dust from Asia or Africa can reach North America, but the recent data provide the first firm evidence that pollution travels that far. There are also hints that American pollution sails across the Atlantic Ocean and lands in Europe, but clear-cut proof of that connection has yet to emerge, says Cahill.

The pollutants crossing an ocean typically do not present a threat because their concentrations are small in most cases, say the researchers. "We would expect that there would be low health impacts, generally," says Jaffe. Yet in certain instances, such as the April case, winds can carry substantial quantities of unwanted foreign material across the seas, he says.