The Pennsylvania Department of Environmental Protection issued a Code Orange Air Quality Action Day Forecast for ozone for Friday, June 21.

The forecast encompasses the greater Pittsburgh area, including Butler, Allegheny, Armstrong, Beaver, Fayette, Indiana and Washington counties.

According to Airnow.gov, ozone figures are expected to reach into the orange range, which is considered “unhealthy for sensitive groups.”

This means that while most of the public should be unaffected by the high levels of ground-level ozone, more sensitive groups may be at risk. These groups include children, the elderly, and people with heart or lung ailments.

Meteorologist Colton Milcarek of the National Weather Service said the high concentrations of ozone and the heat wave enveloping Butler County are due to a temperature inversion, in which a stable layer of warm air is trapped within a region for an extended period.

“When you have an inversion ... it does a good job of keeping all the surface air trapped,” Milcarek said. “So any pollution that happens near the surface will stay near the surface.”

The concentrations of fine particulate matter — PM2.5 for short — which are also reported on Airnow.gov, are expected to be “low to moderate” on Friday.