"China and the U.S. are two of the world's top polluters of air [source: AFP]. Los Angeles, long thought to be one of the world's worst cities for air quality, has been surpassed by Beijing [source: Wired]. Prior to the opening of the 2008 Olympics, Beijing's air had five times the upper limit of the World Health Organization's safe level of ozone and particles emitted from diesel engines [source: IHT]." (Quoted from article)