Vendors selling drinks stand beside vehicles near the India Gate war memorial on a smoggy day in New Delhi February 1, 2013. REUTERS/Adnan Abidi Last week, the World Health Organization reported that an estimated 8.2 million deaths a year are linked to air pollution.

And that number is on the rise.

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 WHO recommends that PM 2.5 shouldn't even exceed 10 µg/m3.

The most polluted cities on Earth have anywhere from nine 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: