The air pollution that's choking Asia

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(CNN)At the age of 13, Tan Yi Han could not see the edge of his schoolyard. It was 1998 in Singapore, the wealthy city-state known for its tidy streets and clean, green image. But for much of that particular school year, clouds of smoke shrouded the skyline. The record-setting air pollution, which had begun in 1997 and lasted for months, caused a 30% spike in hospital visits. It would later be remembered as one of southeast Asia's worst-ever "haze episodes."

Haze episodes have occurred in southeast Asia nearly every year since. Back in 1998, and for years afterwards, Tan didn't think too deeply about them. Yet at some point in his late 20s, he began to wonder: where did the haze come from? And why did it keep coming back?

Dirty air

Air pollution kills ar