US scientists say satellite data show nearly half the dust suspended high in the atmosphere over North America comes from other continents, mostly from Asia. The researchers said 70.5 million tons of foreign aerosols, consisting of naturally occurring dust as well as man-made pollution, arrive over North America every year. Human and natural processes in North America produce 76.1 million tons of aerosols on their own, or 52 percent of the total. \"[70.5 million tons] is a big number: half. I wasn\'t expecting anything like that,\" researcher Lorraine Remer of the University of Maryland says in a video released in conjunction with the study. In another surprise, most of the arriving aerosols were found to be naturally occurring dust, not man-made pollution such as emissions produced by burning fossil fuels, researchers said. Nearly 90 percent is dust carried across the Pacific Ocean, the scientists said, while the rest is either Saharan dust moving across the Atlantic Ocean or man-made aerosols, mostly from Asia but some from Europe. Because it arrives high in the atmosphere and not closer to the ground, it\'s not the threat to health and the environment that North America\'s home-grown pollution represents, Remer said.\"As it turns out, we should still keep focused on our own pollution in order to keep our air healthy, because the particles that come from other places come high,\" she says in the video released by the American Association for the Advancement of Science.