>> after ten minutes, my temperature changes. but it's not what you think. >> my temperature has gone down 96.8. that's lower than normal because my body is compensating for the heat by overcooling it. imt 21 minutes in, i just started feeling a headache come on. one of the first signs of heat exhaust wrong. >> 60 minutes later. >> i'm feeling lightheaded. it's a telltale sign of heat exhaustion. fast pulse, heat exhaustion to watch for. before-threatenilife-threatenin. >> louis jimenez was on his way to work when he ended up in the e.r. i felt dizzy, wobbly and a lot of sweat. he now know what is to look for. >> i thought it was a heart attack. >> it was the heat causing it. it was the heat. the elderly, especially those on medication and the very young are of course, the most vulnerable to heat. what happened behind the