Much of the U.S. is under extreme heat warnings or advisories while New York, Philadelphia and Washington could feel like 100 degrees or more.

At least 100 Americans are on alert for dangerous heat and humidity -- stretching from Texas to Maine -- on Friday.

In the U.S., hundreds of people succumb to heat-related illnesses each year. Older adults and people in areas that historically haven’t needed air conditioning tend to see the highest rates of ...