The Fourth of July holiday is going to be a hot one across much of the United States with nearly half of all Americans baking under sweltering extreme heat.Nearly 170 million people are under heat watches and advisories Thursday, as two separate high-pressure heat domes will send heat indexes spiking into the triple digits from California all the way to parts of the lower Mississippi River Valley. The National Weather Service said severe thunderstorms are also expected across parts of the upper Mississippi River Valley and Ozarks, as well as the southern and central plains. Scripps News Meteorologist Scott Withers said the excessive heat is expected to last several days and urged people to take precautions for the July Fourth holiday in order to avoid any heat-related injuries or illness.Related story: At least 7 dead as hurricane Beryl storms through Caribbean en route for MexicoSymptoms of heat exhaustion include heavy sweating, cold, pale and clammy skin, muscle cramps, tiredness or weakness and dizziness. Meanwhile, when things progress into heat stroke territory, health risks can turn dangerous or even deadly. Symptoms of heat stroke include a body temperature of 103 degrees Fahrenheit or higher, hot, red or damp skin, headache, dizziness and nausea.It's important to take steps to stay hydrated and cool, as the soaring temperatures are expected to stick around for a prolonged period of time. Cooling centers will also be open in several cities for those in need of relief.