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In this Jan. 9, 2013, photo, an inversion cloud covers downtown Salt Lake City. A group of Utah doctors is declaring a health emergency over the Salt Lake City area's lingering air pollution problem. Utah Physicians for a Healthy Environment planned to deliver a petition Wednesday, Jan. 23, 2013, demanding immediate action by elected officials. (AP Photo/The Deseret News, Ravell Cal)

In this Jan. 9, 2013, photo, an inversion cloud covers downtown Salt Lake City. A group of Utah doctors is declaring a health emergency over the Salt Lake City area's lingering air pollution problem. Utah Physicians for a Healthy Environment planned to deliver a petition Wednesday, Jan. 23, 2013, demanding immediate action by elected officials. (AP Photo/The Deseret News, Ravell Cal)

Toxic fog settles over Salt Lake City

January 24, 2013

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

people.