Mill odors wafting throughout the South Hills this morning could have been the result of a temperature inversion coupled with an atypical wind direction.

That's the best guess of Darrell Stern, the air quality manager for the Allegheny County Health Department, who responded to queries from the Post-Gazette after it received complaints about mill odors from readers in Dormont and Mt. Lebanon.

Mr. Stern said slight winds from the southeast between 5 and 10 a.m., then from the south, could have carried odors from U.S. Steel's Clairton Coke Works or other Mon Valley mills into the South Hills area, where such odors usually do not occur.

A temperature inversion, involving cold air along the ground covered by warmer air above it, can keep pollution at ground level for a longer period of time than usual. Slight winds from the southeast, too weak to disturb the inversion, could have pushed the odors into the South Hills.

Once the sunrise heats the cold air, it rises and helps dissipate the concentrations of pollution, Mr. Stern said.

"I think it was a slight odor from the Mon Valley because of the way the wind was blowing -- low-speed winds, not enough to break up the inversion from the general direction of the mill in the Mon Valley," he said.

The health department received no complaints about the odors, probably due to the fact that South Hills residents aren't as aware of the 24-hour health department complaint line, where such problems can be reported.

That number is 412-687-2243.

The health department will investigate complaints of odors or pollution that it receives, he said, especially if it receives multiple complaints.

Typically mill odors involve hydrogen sulfides that smell like rotten eggs. People, Mr. Stern said, have different tolerance levels for such odors, which can come from vehicles, mills and sewage treatment plants.

The Clairton Coke Works is the major producer of such odors in the Mon Valley, he said.

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