The Pa. Department of Environmental Protection issued a Code Orange Air Quality Action Day for Monday.
The department says Monday's forecast -- high temperatures, sunny skies, light south winds -- combined with increased weekday emissions will result in higher ozone concentrations.
This Code Orange encompasses the counties of Bucks, Montgomery, Chester, Delaware, and Philadelphia in southeastern Pennsylvania. Various other counties are included in southcentral and southwestern parts of the state.
According to the U.S. Environmental Protection Agency air quality index, orange represents "unhealthy pollution levels for sensitive groups of people".
The DEP recommends young children, the elderly, and those with respiratory problems reduce outdoor activity.