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Twenty Pennsylvania counties are under a state-imposed Code Orange Air Quality alert, including Allegheny, Beaver, Westmoreland and Washington counties. This is due to higher than normal levels of fine particulate pollution, which are tiny particles that can be inhaled and cause health problems.

The state Department of Environmental Protection attributes the poor air quality to warmer temperatures and a lack of wind, which prevents the particles from blowing away.

The Allegheny County Health Department is also warning residents that air pollution locally will likely spike Tuesday.

“Under the influence of a high-pressure system, Allegheny County experienced multiple strong surface temperature inversions Saturday morning, Sunday morning and Monday morning,” said Jim Kelly, the Health Department’s Deputy Director for Environmental Health, in an email Monday. “This is very unusual, and seems to have impacted a large region, including western Ohio and central and eastern Pennsylvania.”

LISTEN to more about inversions:

Air quality in Pittsburgh was at an Air Quality Index of “moderate” Saturday and Sunday, meaning air quality is acceptable but could pose a danger for select groups especially sensitive to air pollution. However, air quality in Clairton was categorized as “unhealthy” over the weekend. At the “unhealthy” range, the government says all people may start to experience adverse health effects.

“The pollutant of concern is PM 2.5, which is emitted from multiple sources that burn fossil fuels, including US Steel’s Clairton Coke Works, automobiles, and all other sources of combustion,” Kelly said. “During these types of inversions, it is impossible to distinguish a sole source of pollutants.”

Poor air quality has been a concern in Clairton because of December’s Clairton Coke Works fire, which damaged pollution controls.

“The issue is particularly acute in the Mon Valley, but it affects all of the air in Allegheny County,” said Matt Mehalik, executive director of the Breathe Project. “This is just an unfortunate piling on of issues.”

U.S. Steel has said most of the repairs on the Clairton Coke Works could be done by May. The facility has changed its operations in the meantime to lower its emissions.

In a statement Tuesday morning, U.S. Steel said: “As communicated by the Allegheny County Health Department in alerts during recent days, fluctuating weather conditions can impact air quality. We are aware of these alerts, as well as those issued across the state of Pennsylvania. We continue to evaluate the performance of our area operations.”

This story was updated at 9:52 a.m. Tuesday, Feb. 5 to include statements by the Allegheny County Health Department and U.S. Steel Corp.