On Monday afternoon, August 19, 2013, the Yosemite Emergency Communications Center received a 911 call from a hiker on the Upper Yosemite Fall Trail. The caller stated that his brother, a 62 year-old male, had collapsed on the trail below Columbia Rock (approximately one mile from the trailhead). It was a warm, sunny day, with high temperatures predicted at 95°F. The two had been hiking since 6:30am and were returning to the Valley from the top of Yosemite Falls when the subject became dizzy, short of breath, and unable to walk or speak. Yosemite Search & Rescue (YOSAR) dispatched a hasty team—a paramedic and two other team members—to gain further information and provide medical care.
http://www.nps.gov/yose/blogs/Heat-stroke-on-a-hot-summer-day.htm