Pollution from road traffic, particularly diesel fumes, is the most significant cause of poor air quality in most cities.

BBC: Court reserves UK air quality ruling

People are heavily dependent on cars, the air quality is poor and access to green space is haphazard.

ECONOMIST: Greening Houston

They lean toward making poor quality ads and then saturating the airwaves with them, he said, resulting in an air war that drains campaign coffers and turns voters off.

CNN: latimes.com: A Nader action figure? Ad maverick signs on to team