About 80% of all cities have worse air quality than what's considered healthy - here are the 15 with the worst air pollution

Air pollution in urban areas is getting worse.

Between 2008 and 2013, global urban air pollution levels rose by 8%.

Some 80% of all urban areas have air pollution levels above what's considered healthy by the World Health Organization, a new report said Thursday. The rate is even more dismal for cities with more than 100,000 people in low- and middle-income countries: The report found that 98% of those areas had unhealthy air.

The most harmful pollutant to human health is called PM 2.5, particle matter smaller than 2.5 microns in diameter that's found in soot, smoke, and dust. PM 2.5 is especially dangerous because it can get lodged in the lungs and cause long-term health problems like asthma and chronic lung disease.

PM 2.5 starts to become a major health problem when there is more than 35.5 micrograms (µg) of PM 2.5 per cubic meter of air, according to the Environmental Protection Agency. But the World Health Organizations recommends keeping yearly average PM 2.5 levels three times lower than that.

The most polluted cities on Earth have anywhere between 11 and 20 times that amount - based on the new WHO data from cities that collect it - and you might be surprised which make the top 15 list this year.