HARRISBURG, Pa. -

An air pollution alert has been expanded and extended in Pennsylvania.

The Department of Environmental Protection said in a release Friday afternoon that a Code Orange ozone alert will continue through the weekend for Berks County and the Lehigh Valley, Philadelphia, Liberty-Clairton, Pittsburgh, the Susquehanna Valley, Mercer County, and Indiana County.

Young children, the elderly and those with respiratory problems such as asthma should limit outdoor activities.

Ground-level ozone, or smog, forms during warm weather when pollution from vehicles, power plants and other sources heats up, making it hard for some people to breathe.

To help keep the air healthy, the DEP encourages residents and businesses to voluntarily limit certain pollution-producing activities by:

  • Refueling cars and trucks after dusk;
  • Conserve energy in your home and business; and
  • Limit engine idling.

The alert system uses colors to report air quality. Green signifies good; yellow is moderate; orange means unhealthy pollution levels for sensitive people; and red warns of unhealthy pollution levels for everyone.