Those conditions can be fatal when they don't relent for days on end. In Chicago, more than 700 people died when a heat wave gripped the city in 1995.
Here's what makes prolonged hot weather so dangerous:
Temperatures don't cool off enough at night to give your body a break, which is why air conditioning is so important.
But Dr. Steven Englender with the Cincinnati Health Department points out many homes and apartments in Cincinnati are old, so they don't have central air or window units.
Then factor in the urban heat island: Pavement, buildings and other structures bake in the sun, trapping the heat and radiating it back out.
"You're basically in an oven on low," Englender said.
Normally, sweating would help reduce our body temperature. But when it's this humid, sweat doesn't evaporate and can't cool you off.
Those who are young and elderly are most at risk. The heat also can be risky for people who have trouble regulating their body temperature because of medications.
If you don't have access to a cool place, Englender recommends taking a cool bath or cool shower to bring down your core body temperature.
Other tips include making sure you drink plenty of water, limiting your outdoor activity time and wearing loose, light-weight clothing.
Cincinnati Recreation Commission centers will be open during their normal Monday-through-Friday business hours for heat relief. Recreation centers will provide a cool, comfortable place to sit and rest that will allow people to get out of the heat.