From the National Weather Service:
"Heat is the number one weather-related killer in the United States.The National Weather Service statistical data shows that heat causes more fatalities per year than floods, lightning, tornadoes and hurricanes combined. Based on the 10-year average from 2000 to 2009, excessive heat claims an average of 162 lives a year. By contrast, hurricanes killed 117; floods 65; tornadoes, 62; and lightning, 48".
As noted on the attached map, the heat index reached values exceeding 120 F on Sunday August 8, 2010. Lincoln's high temperature was 98 F but the heat index was 114 F and Omaha had a high temperature of 96 F but had a heat index of 115 F.
The official Heat Index table is included as well. Note that the values were calculated based upon being in the shade. If you are in the open with exposure to the direct rays of the sun, the values would be significantly higher. High wind speeds actually make it worse as well since they increase the amount of hot air striking your body.