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Utah fog

Smog from an inversion hangs over the Utah State Capitol Wednesday, Jan. 23, 2013, in Salt Lake City. The U.S. Environmental Protection Agency has singled out the greater Salt Lake region as having the nation's worst air for much of January, when an icy fog smothers mountain valleys for days or weeks at a time and traps lung-busting soot. (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