On Dec. 5, 1952, a thick layer of fog settled over the streets of London, blanketing the city. This was no ordinary wintery mist, but rather a noxious haze of sulfur dioxide from coal-fired industrial factories and cookstoves in London homes. London's Great Smog hung in the air for five consecutive days; visibility was reduced to mere feet and cars were abandoned or led off the road by police with traffic flares. It was the “nation's worst air pollution disaster” and remains the deadliest smog event on record. According to *The Telegraph*, the devastation the smog wrought “only became apparent when undertakers reported that they were running out of coffins and florists had sold all their flowers.” In the following three months, an estimated 13,000 people died of respiratory complications.

The hazy scenes of London's Great Smog bear a striking resemblance to modern-day images of China's urban centers on their most polluted days. And though China has never had an event to match those four days in London, its pollution problem is persistent and pervasive. In 2010, air pollution contributed to 1.2 million deaths in China. Between 1981 and 2001, particulate levels in its major cities were five times greater than what the United States experienced before 1970. And the problem is worsening at an incredible rate. In 2009, the concentration of particulate pollution in the Chinese city of Harbin averaged 101 micrograms per meter, according to the World Health Organization. Four years later, in October 2013, levels were up tenfold, a new record.