The gold standard of air pollution research is the IQAir survey. It looks at air pollution levels in every country and large cities worldwide. It looks at PM2.5 data. PM2.5 are particles or droplets that measure two and one-half microns or less in width. The U.S. Environmental Protection Agency uses them as a standard for air pollution. (These are the most congested cities in the world.)
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The IQAir study examines PM2.5 information across 131 countries and 7,323 cities. The data are pulled from over 30,000 air quality monitors. Poor air quality causes over 6 million deaths per year. It causes 93 billion days of people who “live with illness.”

Air quality is ranked on a scale of zero to 100. The closer a rating is to 100, the worse the air quality. Among countries, Chad has the worst score at 89.7. Iraq follows it at 80.1, and Pakistan at 70.9. The United States ranks 99th with a rating of 8.9, the same as the United Kingdom.

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The countries with the cleanest air are Guam at 1.3, followed by French Polynesia at 2.5, the U.S. Virgin Islands at 2.9 and Bermuda at 3.0.

The regional capital city with the worst air is N’Djamena, Chad’s largest city, at 89.7. New Delhi follows it at 89.1 and Bagdad at 86.7.

The most polluted city in the United States is Columbus, Ohio, at 13.1. Atlanta follows it at 12.1.

These are the 10 most polluted cities in the United States based on air quality:

• Columbus, Ohio (13.1)
• Atlanta, Ga. (12.1)
• Chicago, Ill. (11.8)
• Indianapolis, Ind. (11.3)
• Dallas, Texas (10.9)
• Baltimore, Md. (10.7)
• Los Angeles, Calif. (10.5)
• Sacramento, Calif. (10.4)
• Houston, Texas (10.1)
• Detroit, Mich. (10.0)

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