http://www.usatoday.com/news/nation/2004-11-16-smog-usat_x.htm
"A study of 95 U.S. urban areas found that a small increase in the average smog level over seven days can lead to a 0.5% rise in deaths, many from heart and lung disease"
"Smog, which is formed from chemicals emitted by vehicles and power plants, is the nation's most widespread air-quality problem."