Blame it on stressful jobs, salt-laden fast-food, smoking and lack of exercise.

CNN: Benefits by the Bunch

Boise, Idaho, and Salt Lake City, Utah, are other fast-growing high-tech centres.

ECONOMIST: A new Republican heartland

They point to cities such as Charlotte in North Carolina and Salt Lake City in Utah: places that have grown as fast as Houston and Dallas and yet have managed to improve their air.

ECONOMIST: The Lone Smog State