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.

Millions of Americans are under heat alerts Monday as a heat wave brings triple-digit temperatures or heat indices to nearly half the continental United States, according to the National Weather Service—with the heat expected to impact the parts of the Southeast and the Mississippi river the hardest.

Extreme heat is in the forecast for the Chicago area Monday. There's also a Severe Thunderstorm Warning for Cook County until 1:30 p.m. ABC7 Accuweather Meteorologist Mark McGinnis said temperatures will climb into the low to mid 90s in the afternoon and it will feel like between 105-109 degrees.

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

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.

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.

St. Louis faces an Extreme Heat Warning with a high near 97°F and heat index values up to 107, along with potential severe thunderstorms.