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Shanghai, China
That's because the air is thick with pollutants, including a particularly dangerous type of pollutant called PM2.5. These tiny particles, which measure less than 2.5 microns in diameter (about 28 times smaller than a human hair) are found in soot, smoke, and dust. Cigarette smoke also contains a large amount of PM2.5.
Because PM2.5 particles are so small, they're particularly bad for your lungs and respiratory system, where they can easily get lodged and accumulate, eventually causing respiratory illnesses like asthma and chronic lung disease.
In a recent report by the nonprofit Berkeley Earth, researchers found that the average person in China was exposed to 52 micrograms of PM2.5 per unit of cubed air (the standard way of measuring the pollutant). For comparison, a study of the long-term effects of air pollution found that an annual exposure to just five micrograms of PM2.5 per unit of cubed air (10 times less than the amount discovered in the air in some areas of China) increased the risk of developing heart problems by 13%.
China's rapid industrialization has left the country without a great way of monitoring coal-powered plants or the growing amount of car exhaust in the air.
About a year and a half ago, the country declared a "war on pollution." The government set up about 1,000 monitoring stations to check for pollutants such as nitrogen dioxide, ozone, carbon monoxide, and - most importantly - the PM2.5. But these efforts are just beginning to figure out how bad the situation is.
Here's a chart from The Economist showing where the air is most deadly.
Fortunately, some areas aren't all that bad, especially areas farther way from Beijing and Shanghai. But even so, The Economist notes, about half of China's 1.3 billion population lives in areas with PM2.5 levels above the EPA's highest tolerance level.
Looks like China's "war on pollution" still has a ways to go.