A Code Orange air quality warning has been issued for Monday in central Pennsylvania.

The Pennsylvania Department of Environmental Protection has issued an orange air quality warning for Monday due to high ozone levels in the Harrisburg area ahead of expected sunny and hot weather.

The orange warning on the DEP and U.S. Environmental Protection Agency color scale indicates that air quality may be unhealthy for particularly vulnerable or sensitive people. Young children, the elderly and people with asthma or other lung diseases should limit their outdoor activities, according to the authorities.

The warning applies to Dauphin, Cumberland, Lebanon, York and Lancaster counties, where bright sunshine and highs of 35 degrees are expected on Monday, according to the National Weather Service.

Ozone is created when precursor chemicals such as carbon monoxide and nitrogen oxide react with sunlight, according to the DEP. Peak ozone levels occur most often on summer days with long periods of sunshine. Precursor chemicals typically come from car exhaust and industrial emissions.

According to the EPA, ozone concentrations in the Harrisburg area are expected to reach an index value of 105 on Monday.

An index value of 100 corresponds to a maximum safe 8-hour average ozone concentration of 70 parts per billion, according to the agency’s latest standards.