More than 148 million Americans suffered scorching heat through the weekend and into Monday. The extreme heat has settled in from the Midwest to the East Coast. Many cities have advised people to stay indoors under an extreme heat warning. The warmth and humidity are likely to last through the week. The heat wave could break records and endanger health. "Hot temperatures, oppressive humidity and very light winds are expected today," the National Weather Service (NWS) posted on X. "It may feel like the low 110s in some areas. This is a dangerous level of heat, with no significant relief at night!" That “feels like” temp is known as the heat index. It's a combo of the air temp and humidity. Chicago logged a heat index of 103 degrees Fahrenheit on Sunday. The NWS predicts that number could hit 110 degrees on Tuesday and Wednesday. Local and national leaders have issued health warnings and canceled outdoor events. And sports teams have taken measures to protect players. Several pro athletes fell ill on the field during baseball games over the weekend. Climate change has made extreme heat more common in the US. But this heat wave has surprised experts with its size and scope. “It’s basically everywhere east of the Rockies,” Mark Gehring told The Associated Press. “That is unusual.” Gehring works for the NWS. Reflect: What’s one way really hot weather can change your daily routine, and how do you stay safe or comfortable?