A transport event in April 2001 brought substantial quantities of mineral dust from Asian deserts to the U.S. atmospheric boundary layer (ABL). The dust was seen in large amounts throughout the ABL in the U.S., with almost no reduction in concentrations. It was estimated that the amount of Asian dust in the continental U.S. ABL in mid-April 2001 was 1.1 E5 metric tons, a value comparable to the daily emission flux of all U.S. sources of particulate matter (PM) less then 10 μm diameter (PM 10). In some regions, the Asian dust, combined with local pollution, elevated urban PM to levels associated with adverse health effects. The April 2001 event appears to be the largest Asian dust event ever observed in the U.S., and its effects provide evidence that air pollution issues must be viewed in a global context.