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PITTSBURGH (KDKA) — The Pennsylvania Department of Environmental Protection has issued a Code Orange Air Quality Action Day for 20 Pennsylvania counties.

The following counties are included: Allegheny, Butler, Westmoreland, Armstrong, Fayette, Beaver, Washington, Northampton, Lehigh, Berks, Cumberland, Dauphin, York, Lebanon, Lancaster, Philadelphia, Bucks, Delaware, Montgomery and Chester.

The DEP says fine particulate matter levels rose faster than expected over the weekend due to the warmer air.

“A strong inversion, when colder air is trapped below a warm air mass, with near calm winds and increased weekday emissions Monday will bring air quality for PM2.5 into the Code Orange range,” the DEP said in a release.

A cold front is expected to pass through western Pennsylvania Tuesday morning, then pass through the eastern areas of the state early Tuesday afternoon.

An “orange” air quality level signifies unhealthy air pollution levels for young children, the elderly and people with respiratory problems — such as asthma, emphysema and bronchitis.

On air quality action days, you should try to carpool, reduce vehicular travel, wait until after dusk to get gas and reduce the use of wood stoves and other sources of fine particulate matter.