According to a WHO study of pollution in world capitals Stockholm/Wellington, Canberra and Ottawa occupy the top three spots. Riyadh, Delhi and Doha are at the bottom. Kiruna is Swedens cleanest city while Malmö and Gothenburg are the most polluted.
Read more:
https://www.thelocal.se/20170313/stockholm-tied-for-worlds-least-polluted-capital-who
http://www.who.int/gho/phe/outdoor_air_pollution/exposure/en/