Thanks to improved satellite sensors, scientists have begun to track pollution produced by East Asian forest fires, industrial waste, and urban exhaust. A study released last year estimated that an amount of pollution equivalent to 15 percent of emissions from the U.S. and Canada had traveled to North America from East Asia between 2002 and 2005.

This image shows the pollution over Indonesia and the Indian Ocean in 1997. The white pixels represent the aerosols (smoke) leftover from the fires, while the green, yellow, and red pixels represent rising amounts of tropospheric ozone (smog).