A relentless heat wave is expanding across the South. Heat alerts are in effect from southeast Arizona through most of Texas to Mobile, Alabama and Pensacola, Florida.
Nearly 40 million people are under alerts total.
Temperatures could get as high as 113 degrees in Tucson, Arizona. In Mobile, Alabama, the heat index is up to 110 degrees. In Abilene, Texas, it's so hot cookies bake in the back of a car.
And the heat hazard is expected to expand as the Fourth of July approaches.
The first heat wave of the summer claimed at least two lives just days ago in Texas.
Officials say on June 23rd an emergency call came in for a 14-year-old in distress at Big Bend National Park in Texas.
The dad took off to find help while the other brother attempted to carry his little brother to the trailhead.
An hour and a half after the emergency call, rescue personnel found the teen dead on the Marufo Vega trail, about a mile from the trailhead.
30 minutes later, the boys' stepfather died in a car crash about half a mile away at a canyon overlook.
SEE MORE: Father, stepson die at Texas national park as temperature reached 119
Big Bend National Park Ranger Tom Vandenberg says an investigation is underway, but it's believed heat reaching 119 degrees at the park played a factor in the stepfather's death.
He says this year four visitors have died at the park, adding that the 14-mile trail the Florida family embarked on is dangerous in the summer, with rugged terrain and no cell service, shade or water.
Experts and officials say in temperatures like these, any strenuous activity should be done early in the morning or late in the evening once things cool off.
"The message is, don't think you can just push through the heat. Pay attention to your bodies and be aware and be very careful," said Kirk Watson, mayor of Austin, Texas.
SEE MORE: Why is Texas so hot right now? A heat dome — and it could grow
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