Only 20 years ago, Linfen, China, a city in Shanxi province, went by the nickname the "floral city." It was a nod to the region's clear spring water, lush greenery and abundant agriculture. This was before the factories and the coal mines moved in, churning out goods and electricity and a thick blanket of industrial smog. These days, Linfen is more likely to be referred to as one of the most polluted cities in the world, and researchers estimate that a day spent breathing its air is akin to a day spent smoking three packs of cigarettes.