The recent heat index in Illinois is reaching dangerous levels, with many areas experiencing heat index values between 100-115 degrees Fahrenheit. Specifically, the National Weather Service is reporting heat index values between 105-110 degrees in many northern Illinois locations through Tuesday.

Temperatures climbed into the low to mid-90s in the afternoon, and it will feel like between 105-109 degrees, ABC7 Chicago AccuWeather Meteorologist Mark McGinnis said. But the heat is not stopping from coming to enjoy Monday night's Summer Series concert at Millennium Park.

Major cities under heat alerts Monday include Dallas, Oklahoma City, St. Louis, Chicago, Memphis, Atlanta, Tampa, Charleston, New York, Philadelphia and Boston.

Smartphones have optimal operating temperatures, and exceeding them can damage battery life. If a phone overheats, turn it off, move it to a cooler location, and let it cool down.

Extreme heat is considered the deadliest weather-related hazard in the U.S., according to the Fifth National Climate Assessment. About 2,000 Americans die each year on average from extreme heat, according to the CDC.

With extreme heat warnings in effect across major U.S. cities, officials urge residents to prioritize safety and hydration during this dangerous heat wave.

Record-high minimum temperatures, in which temperatures don't drop in the nighttime hours, are likely across much of the U.S.