April 22 is Earth Day. The event began in 1970 to get people thinking about cleaning up the planet. Smog is one of Earth's nastiest problems. A hazy blend of smoke and fog, it causes breathing problems in industrial areas. These five cities--the world's smoggiest--need to clean up their act!
1. Mexico City, Mexico
2. Sao Paulo, Brazil
3. Cairo, Egypt
4. New Delhi, India
5. Shanghai, China