The capital of Wyoming is famous for its mountain vistas and clean air, which puts it high on the WHO's list. But take care, citizens of Cheyenne — there may be trouble on the horizon. Drilling and fracking for natural gas has led to increased air pollution in parts of western Wyoming — in some places, rivaling the smog of Los Angeles. Cheyenne has remained mostly untouched so far, but gas drilling has a way of spreading.