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Utah fog

A poor air quality sign is posted over a highway Wednesday, Jan. 23, 2013, in Salt Lake City. 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. (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