Air pollution warning in effect for Clairton, Liberty areas of Allegheny County

An air pollution warning is in effect Thursday for the Clairton and Liberty areas of Allegheny County.

The National Weather Service had said the “code orange” alert applied to five counties in Southwestern Pennsylvania and parts of two other counties, but corrected its alert around midday Thursday to apply to the smaller area.

A “code orange” alert means that air pollution concentrations in the region may become unhealthy for sensitive groups including children, the elderly and people suffering from asthma, heart disease or other lung diseases.

Those groups should avoid outdoor exercise or strenuous activity, according to the alert.

A high-pressure system was expected to produce light to calm winds early in the day Thursday, leading to a temperature inversion, according to a statement from the state Department of Environmental Protection.

This could trap fine particulate matter close to the ground.

The U.S. Environmental Protection Agency’s air quality index uses colors to report daily air quality. Green signifies good; yellow means moderate; orange represents unhealthy pollution levels for sensitive people; and red warns of unhealthy pollution levels for all.

Thursday’s alert comes about a month after parts of Allegheny County and surrounding areas of Southwestern Pennsylvania experienced several days of elevated particulate matter, or PM 2.5, pollution in late December.

Those elevated pollution levels also occurred during a temperature inversion.

Soon after, the Allegheny County Health Department announced plans to develop new pollution regulations and forecasting mechanisms for weather events like inversions or periods of heavy fog and light wind.

Note: This story has been updated to reflect corrections to the discrepancies between the DEP and National Weather Service’s alerts.