By Caroline Vandergriff

FORT WORTH (CBSDFW.COM) – Sunday was officially the hottest day of the year.

With the heat index at about 105 degrees, the temperatures aren’t just unpleasant – they can cause serious illness.

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MedStar implemented its extreme weather response procedure because of the high temperatures.

“Even if it’s not technically 105 degrees, with the humidity and everything counted in, that’s when it becomes dangerous,” said Brian White, operations supervisor for MedStar.

The hottest ticket in town on Sunday was the All-Star Race at Texas Motor Speedway.

Fans say the temperature wouldn’t keep them away, whether they planned to watch from the stands or out in the campgrounds.

“We would be here no matter what,” said NASCAR fan Lisa Zavala.

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But after being inside the speedway yesterday, she and her friends chose to stay out by their RV today.

“You have everything you need,” she said. “You have your drinks, you have your bathroom, you have your food…. A little A/C, if you get too hot, you go in, yea. You don’t get that in the stands.”

Large gatherings and the hot Texas sun can be a dangerous mix.

MedStar treated four people for heat-related emergencies at a concert at Panther Island Pavilion last night.

Paramedics were on standby for fans at Texas Motor Speedway, too.

“They need to keep drinking water,” White said. “I know popular beverages are always fun to drink at those events, but space them out with some water too, that way you don’t de-hydrate.”

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It’s advice everyone can use now that the summer heat is officially here in North Texas.

Caroline Vandergriff