For an updated 2016 list, click here.

Air pollution already kills 3.3 million people a year.

And that number could double by 2050.

The most harmful pollutant to human health is called PM 2.5, short for particle matter that's less than 2.5 microns in diameter. It's found in soot, smoke, and dust and lodges in the lungs causing long-term health problems like asthma and chronic lung disease.

PM 2.5 starts to become a health problem when there is more than 35.5 micrograms of PM 2.5 per cubic meter (written like 35.5 µg/m3) of air, according to the Environmental Protection Agency. But the World Health Organizations recommends that PM 2.5 shouldn't even exceed 10 µg/m3.

The most polluted cities on Earth have anywhere from 9 to 15 times that amount – based on information from the WHO – and you might be surprised which make the top 10 list. Check them out: