Air pollution killed more than 6 million people in 2012, according to the World Health Organization (WHO), making it the world's single biggest environmental health risk. Air pollution kills more people than malaria and AIDS combined.

And it’s only getting worse.

A new study released by the United Nations agency on Wednesday found that air pollution has increased since the last survey in 2011, resulting in heightened risk of cancer, stroke and heart disease for urban denizens. Last year, the WHO's International Agency for Research on Cancer (IARC), classified outdoor air pollution as carcinogenic to humans. Specifically, the IARC links air pollution to lung cancer as well as an increased risk of bladder cancer.

The 2014 report looks at ambient (outdoor) air pollution data over the last five years for almost 1,600 cities in 91 countries, as measured by fine particulate matter. The study notes that about half of the world’s urban population live in cities that exceed by 2.5 times or more the recommended levels of fine particulate matter set out by WHO Air Quality Guidelines. Only around 12 percent of the total urban population lives in cities where the air quality complies with recommended levels.  

The cities with the worst air pollution are as follows:

1. Delhi, India 

2. Patna, India

3. Gwalior, India

4. Raipur, India

5. Karachi, Pakistan

6. Peshwar, Pakistan

7. Rawalpindi, Pakistan

8. Khoramabad, Iran

9. Ahmedabad, India

10. Lucknow, India

The WHO cites a number of sources; some of the major contributors include exhaust from vehicles and emissions from factories and power generation. In addition, household emissions in cities where people use coal and wood for cooking and heating also play a significant role.

Cities with the lowest level of pollution were located in Canada, the United States, Finland, Iceland and Sweden.

For more information, see the full report.

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