Air pollution in the Mon Valley exceeded federal standards over Christmas, the Allegheny County Health Department reported.

Based on preliminary results, the air quality monitor in Liberty showed PM2.5 pollution above the federal threshold of 35 micrograms per cubic meter from Saturday through Wednesday, the health department said. PM2.5 refers to particulate matter with a diameter of less than 2.5 micrometers.

“As a result, ACHD’s Air Quality Division has been in daily communication with U.S. Steel and other facilities in the Mon Valley to monitor all activity and ensure compliance with their permits,” the health department said.

The state Department of Environmental Protection advised residents of poor air quality repeatedly this week, twice issuing a Code Orange Air Quality Action Day for Tuesday and Wednesday. The warning was thought to be due to several days of heavy fog, light winds and temperature inversions.

Temperature inversions — when warm air traps cold air and pollutants close to the ground — caused spikes in particulate matter in parts of Allegheny County. The health department reported Monday that the highest spike took place at the Liberty air quality monitor, which measures air quality in the Mon Valley.

Lee Hendricks, a meteorologist at the National Weather Service in Moon Township, said the region is experiencing the affects of a storm system moving out of the southwestern part of the country. The warm air is trapping moisture and pollutants, he said.

“We need a cold front to come through to clean it out,” Hendricks said.

Air quality was predicted to be unhealthy for people in sensitive groups, including children, older adults and people with lung or heart disease.

Health officials looked to U.S. Steel and other facilities in the Mon Valley to make sure they were operating within their permit limits.

Nothing out of the ordinary was detected.

“As ACHD has stated in past alerts, it is impossible to determine the sole source of emissions during strong inversions,” U.S. Steel said in a statement Friday. “Our Clairton Plant is operating normally, with no issues to report. Clairton has been operating at a reduced production rate of approximately 9,000 tons per day due to market conditions. Clairton is on pace to set a record for fewest stack exceedances in December.”

The health department said the air quality is expected to improve in the coming days.

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