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Air Quality
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Air quality reflects the abundance of pollution present in air. Pollution is a mix of hazardous substances from both human-made and natural sources. Primary sources of human-made air pollution include vehicle emissions, fuel oils and natural gas to heat homes, by-products of manufacturing and power generation—particularly coal-fueled power plants—and fumes from chemical production.
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The Milwaukee Skyline in Smog, by Jeramey Jannene, via Flickr. Used via Creative Commons License CC BY 2.0.