From the Introduction: "Heat is an underrated danger, with an average of 175 Americans losing their lives annually from heat-related causes. According to the Centers for Disease Control and Prevention, from 1979-2003 excessive heat exposure caused 8,015 deaths in the United States. During this period, more people died from extreme heat than from hurricanes, lightning, tornadoes, and floods combined. [...] This study will attempt to show the frequency of various heat index values for a section of the United States, focusing on the Plains, upper and middle Mississippi River Valleys, and the western Great Lakes. Also, the physiological response to heat will be briefly investigated, including a review of how heat acclimatization affects the human body's biology. This protective biological response is an important consideration when evaluating the impact of the heat on those that are, or are not, acclimatized to the heat."
U.S. National Weather Service: https://www.weather.gov/