An official with the Philadelphia Fire Department tells Action News that 17 people have been taken to local hospitals because of the heat.
So far, no deaths have been reported.
This comes as summer school programs are closing and cooling centers are opening as a dangerous heat wave continues to blanket much of Pennsylvania.
Temperatures are expected to be near triple digits Thursday in Philadelphia, while oppressive humidity will make it feel even hotter.
The Philadelphia school district ordered an early dismissal for summer school programs on Thursday and canceled them on Friday because of the heat.
In Allentown, the city began waiving fees at its four public pools to help residents cool off. The pools will be free for the remainder of the season whenever the weather service issues an excessive heat warning for the area, Mayor Ed Pawlowski said. Two spray parks are also operating.
"We are happy to provide this relief for all city residents to ensure their safety and give everyone a chance to escape from the heat," Pawlowski said in a statement.
Forecasters expect Friday to be even hotter, with Philadelphia and Harrisburg both expected to be over 100 degrees. High temperatures will stick around through the weekend.