Life-threatening temperatures are enveloping the East Coast, with 29 states from Maine to Texas under heat alerts.
Tuesday could be the hottest day of the year for parts of the East Coast and Interstate 95 corridor.
MORE: How zoos in the hottest parts of the country keep animals healthy during extreme heatActual temperatures may climb to the triple digits in Philadelphia, Washington, D.C., Virginia and North Carolina.
The heat index -- what temperature it feels like -- is forecast to skyrocket to 98 degrees in New York City; 97 degrees in Boston; a scorching 108 degrees in Washington, D.C., and Charleston, South Carolina; 103 in Orlando, Florida; and 105 in Houston.
New York City is under a heat advisory while Philadelphia officials have issued a heat health emergency.
The heat and humidity will last until Wednesday for the I-95 corridor. A cold front will bring relief by the end of the week.
MORE: 5-year-old boy dies after being left in hot car for 7 hours; foster mom arrested: PoliceDoctors recommend taking excessive heat warnings seriously. There are hundreds of deaths each year in the U.S. due to excessive heat, according to CDC WONDER, an online database, and scientists caution that the actual number of heat-related deaths is likely higher.
Heat is the country's top cause of weather-related deaths, according to the National Oceanic and Atmospheric Administration.
Click here for what to know about staying safe in the heat.