UN: Rising air pollution in world's cities hurting millions

"Ambient air pollution, made of high concentrations of small and fine particulate matter, is the greatest environmental risk to health — causing more than 3 million premature deaths worldwide ever year," WHO said.

An accompanying news release said global urban air pollution levels rose 8 percent over that span "despite improvements in some regions," and noted that people face a higher risk of strokes, heart disease, lung cancer and respiratory diseases as the air quality worsens.

"Urban air pollution continues to rise at an alarming rate, wreaking havoc on human health," said Dr. Maria Neira, a WHO director for environment and public health.

The change coincides with a series of air-clearing measures including banning older cars and cargo trucks from the city, introducing steep fines for construction pollution or garbage burning, and shutting down an old coal-fired power plant.