18. World Health Organization.

9 Out of 10 people worldwide breathe polluted air, but more countries are taking action 2018 

[cited 2018 October 2018]. 

Available from: https://www.who.int/news-room/detail/02-05-2018-9-out-of-10-people-worldwide-breathe-polluted-air-but-more-countries-are-taking-action.