An air pollution alert is in effect for five Pennsylvania regions, including Berks County and the Lehigh Valley.
A Code Orange ozone alert is in effect Friday for Berks County/Lehigh Valley, Liberty-Clairton, Pittsburgh, the Susquehanna Valley, and Mercer County, the Department of Environmental Protection said in a release.
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.
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.