Thanks to a Calif. law implemented in 2008 requiring ocean-going ships to switch from high-sulfur bunker fuel to cleaner fuels within 24 miles of the coast, Bay Area air quality significantly improved by reducing deadly particulate emissions.
Oct 9, 2013 BAAQMD News Release
In Beijing, the level of air pollution is the highest the monitors at the U.S. embassy have ever recorded since put in place in 2008. The pollution results from a combination of weather conditions and particulate matter - most from coal burning.
Jan 15, 2013 The New York Times - Environment