The good news is the air quality in the US has been consistently improving since 1995, according to the US Environmental Protection Agency. The bad news is there are still a lot of cities where you literally shouldn’t even breathe. I find it surprising that California has the most polluted as well as the least polluted US cities.
Number of days metropolitan statistical areas failed to meet acceptable air-quality standards in 2005 (last year for which numbers are available):
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Bakersfield, CA: 87
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Baltimore, MD: 27
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Boston, MA: 1
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Chicago, IL: 23
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Cleveland, OH: 25
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Denver, CO: 1
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Fresno, CA:64
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Las Vegas, NV: 2
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Los Angeles/Long Beach, CA: 45
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New York, NY: 15
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Philadelphia, PA:26
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Riverside-San Bernardino, CA: 103
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Sacramento, CA: 39
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San Francisco, CA: 0
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Seattle, WA: 1