"In those conditions high heat and high exercise, the body will overheat no matter how well hydrated. At at body temp of 105..."

"... Heat Stroke occurs and all manner of body systems fail. Bottom line, do not exercise in high heat days. Above 80 degrees F and 80 percent humidity and equivalent, the body cannot lose the heat it generates in exercise and will overheat. One function that goes first is judgement. We get hot and can not realize we are in danger."

A comment written by a doctor on "Hyperthermia contributed to death of woman, 32, hiking Billy Goat Trail, officials say" (WaPo). The Billy Goat trail is a 2-mile trail, and the woman had difficulty one half mile into it. She was given plenty of water — 4 bottles — one of the companions was a nurse, and 911 was called quickly. The temperature was in the high 90s, with a "feels like" temperature (including humidity) of 110 degrees.

Another commenter: "The park warned people. I was there that day and there were big signs at the Visitor Center as well as the trailheads saying to avoid the Billy Goat A trail due to the heat. Signs also advised carrying 2L water per person. A volunteer stationed at the trailhead advised people about the conditions. He said only 20% of hikers carried water. No matter how much the park warns visitors, some will walk around barricades, step over ropes, ignore signs, and blaze new trails to get around any kind of obstacle."