The National Weather Service issued a heat advisory for the region through Friday night with warnings of a heat index near 108 on Friday.
The heat advisory for Cook, Will, Lake, Newton, Jasper, Porter and LaPorte counties is in effect from 11 a.m. Thursday through 7 p.m. Friday.
Forecasters said temperatures are expected in the low- to mid-90s Thursday and the mid- to upper-90s Friday.
Overnight lows are expected to only dip into the mid-70s.
The National Weather Service warned that extreme heat can be dangerous for anyone, especially children, the elderly and those with pre-existing medical conditions.
People in the heat advisory area are being asked to stay inside in air conditioning, seek cooling shelters, drink plenty of water and stay out of the sun.
The Indiana Department of Environmental Management also warned of more high ozone levels Thursday that could cause health problems for children, the elderly and those with respiratory difficulties.
IDEM issued an Air Quality Action Day for ozone in Lake, Porter, LaPorte, Newton and Jasper counties.
Ozone is created on hot, still days when pollutants in gasoline, automobile emissions and other combustion engines, such as those that burn coal and other fossil fuels, are cooked by the sun.
Environmental and health agencies issue alerts when ozone levels are expected to be high and urge people to reduce vehicle trips, mow lawns early in the morning or later in the evening and reduce energy use in businesses and homes.
Thursday marks the fourth Air Quality Action Day of the week and the sixth of the year.