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Utah fog

Smog from an inversion hangs over downtown Salt Lake City Wednesday, Jan. 23, 2013. The geography that makes Utah one of the worldís most beautiful places also brings the nationís dirtiest air in winter, when an icy fog smothers mountain valleys for days or weeks at a time. A group of doctors is declaring a health emergency over northern Utah's lingering pollution problem. Utah Physicians for a Healthy Environment planned to deliver a petition Wednesday demanding immediate action by elected officials. The group wants Gov. Gary Herbert and mayors of northern Utah cities to cut the pollution. (AP Photo/Rick Bowmer)

Toxic fog settles over Salt Lake City

The pollution has turned so bad that more than 100 Utah doctors called Jan. 23 on authorities to immediately lower highway speed limits, curb

industrial activity and make mass transit free for the rest of winter.

Doctors say the microscopic soot — a shower of combustion particles from

tailpipe and other emissions — can tax the lungs of even healthy