INDIANAPOLIS (AP) — Hot weather could make it hard for some people to breathe Friday in Indiana.

An air quality action day has been posted for at least 20 counties in central and southeastern Indiana. Officials say people who are sensitive to air quality might feel it when ozone levels are high, especially the elderly and anyone with a heart condition.

The sun and high temperatures can combine to turn vehicle exhaust and factory emissions into ground-level ozone. In Indianapolis, there will be free access Friday to the city’s bike share program to get people out of their cars.

Here are the counties where ozone could be high: Marion, Bartholomew, Boone, Brown, Delaware, Hamilton, Hancock, Hendricks, Johnson, Madison, Monroe, Morgan, Shelby, Clark, Floyd, Harrison, Jackson, Jefferson, Scott, and Washington.