1. Cheyenne, Wyoming, USA
PM 2.5 annual mean, ug/m3: 3.65
The capital of Wyoming is famous for its clean, mountain air. According to ALA report, Cheyenne tied the number one spot with Farmington, as the city with the cleanest air in the U.S. It is interesting that top three cities from our list of 15 cities with the cleanest air in the world are located in the USA. What else to say but: Great job!