To breath the cleanest air on Earth you have to hover the world's largest body of water.

Mid-Pacific Ocean air is probably the cleanest still found anywhere because it has not been over a pollution-producing continent for perhaps two or three weeks, says Brian Ridley, an atmospheric chemist at the National Center for Atmospheric Research in Boulder, Colo.

During that time, he explains, it has been cleansed by rain and other climatic processes.

By contrast, "there's no clean air left over the North Atlantic," Daniel Albritton, director of the National Oceanic and Atmospheric Administration's aeronomy laboratory. "The Atlantic is a small lake compared with the Pacific and the distance form pollution sources on land is two to three times shorter."

Winds in the upper Northern Hemisphere tend to blow from west to east, putting the Atlantic in the path of one of the world's worst polluters, the industrialized United States, Albritton says.