http://en.wikipedia.org/wiki/2013_Harbin_smog
http://en.wikipedia.org/wiki/Donora_Smog_of_1948
http://en.wikipedia.org/wiki/St._Louis_smog_episode_(1939)
http://www.webmd.com/asthma/ss/slideshow-worst-smog-cities
http://envirohistorytimeline.wikispaces.com/1948+Donora,+PA+(1948)
http://www.corbisimages.com/stock-photo/rights-managed/U1099797INP/donora-smog-of-1948
http://cdrcmissoula.org/cdrc-library-books/
http://www.stltoday.com/look-back-smoky-st-louis-nov/image_4a934d0c-f5c9-11df-a2e4-00127992bc8b.html
http://www.businessinsider.com/instagram-users-in-harbin-document-smog-2013-10
http://en.wikipedia.org/wiki/Donora_Smog_of_1948
http://en.wikipedia.org/wiki/St._Louis_smog_episode_(1939)
http://www.webmd.com/asthma/ss/slideshow-worst-smog-cities
http://envirohistorytimeline.wikispaces.com/1948+Donora,+PA+(1948)
http://www.corbisimages.com/stock-photo/rights-managed/U1099797INP/donora-smog-of-1948
http://cdrcmissoula.org/cdrc-library-books/
http://www.stltoday.com/look-back-smoky-st-louis-nov/image_4a934d0c-f5c9-11df-a2e4-00127992bc8b.html
http://www.businessinsider.com/instagram-users-in-harbin-document-smog-2013-10