Which cities face most, least air pollution according to new WHO data

Photos: The Nigerian city covered in soot
Port Harcourt, Nigeria – The Nigerian city of Port Harcourt used to be known as "The Garden City." Black soot is now everywhere.
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Photos: The Nigerian city covered in soot
Port Harcourt, Nigeria – The city is in the oil-producing Niger Delta region.
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Photos: The Nigerian city covered in soot
Port Harcourt, Nigeria – A social media campaign has been launched by radio host Sandra Ezekwesili, #StopTheSoot.
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Photos: The Nigerian city covered in soot
Port Harcourt, Nigeria – The cause of the soot is has been a mysterious to residents, but many think it's related to the destruction of illegal oil refineries.
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Photos: The Nigerian city covered in soot
Port Harcourt, Nigeria – People say the pollution is affecting their health.
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