Coal

"It takes five to 10 days for the pollution from China's coal-fired plants to make its way to the United States, like a slow-moving storm.
It shows up as mercury in the bass and trout caught in Oregon's Willamette River. It increases cloud cover and raises ozone levels. And along the way, it contributes to acid rain in Japan and South Korea and health problems everywhere from Taiyuan to the United States.
[...] The growth of coal-burning is also contributing to global warming, and is linked to environmental and health issues including acid rain and asthma. Air pollution kills more than 2 million people prematurely, according to the World Health Organization"

From"World's coal dependency hits environment" from Yahoo News.