Comment by buro9

Interesting link... some world cities compared (current Air Quality Index at time of writing before link):

VERY UNHEALTHY

298: http://aqicn.org/city/chicago/

UNHEALTHY

170: http://aqicn.org/city/usa/washington/seattle/beacon-hill/

UNHEALTHY FOR SENSITIVE GROUPS

122: http://aqicn.org/city/beijing/

MODERATE

96: http://aqicn.org/city/usa/oregon/portland/

70: http://aqicn.org/city/tokyo/

66: http://aqicn.org/city/berlin/

65: http://aqicn.org/city/london/

61: http://aqicn.org/city/hongkong/

54: http://aqicn.org/city/newyork/

GOOD

33: http://aqicn.org/city/zurich/

31: http://aqicn.org/city/stockholm/

24: http://aqicn.org/city/paris/

13: http://aqicn.org/city/california/san-francisco/san-francisco...