More than 185 million Americans are on alert for life-threatening heat and humidity from South Dakota to Florida on Monday.
Parts of Mississippi and Louisiana are on alert for heat indices -- what the temperature feels like with humidity -- of up to 120 degrees.
Extreme heat warnings are in place for parts of South Dakota, Nebraska, Iowa and Illinois, where it could feel like over 110 degrees.
From Boston to Philadelphia, it'll feel like the mid-90s to lower 100s through Wednesday.
Extreme heat is considered the deadliest weather-related hazard in the U.S., according to the Fifth National Climate Assessment. About 2,000 Americans die each year on average from extreme heat, according to the Centers for Disease Control and Prevention.
Here are heat safety tips from the CDC:
Take precautions to prevent sunburn, which can make you dehydrated and affect your ability to cool down.
Use sunscreen that's SPF 15 or higher 30 minutes before going outside. Sunscreens that say "broad spectrum" or "UVA/UVB protection" are best.
Drink extra fluids, and don't wait until you're thirsty.
Avoid very sugary drinks and alcohol, which can cause your body to lose more fluid, and be wary of extra-cold drinks that may cause stomach cramps.
Avoiding hot and heavy meals also can reduce your body's overall temperature.
Cut down on exercise during heat waves and rest often and in shady areas.
Try to limit your time outside to when it is cooler, like in the early morning and evening.
Never leave children in a parked car -- even if windows are cracked open.
MORE: Hot car safety tipsAnyone can suffer from heat-related illness at any time, but these people are at greater risk:
-- Babies and young children
-- Overweight people
-- Those 65 years old or older
-- People who overexert during work or exercise
-- Those who suffer from heart disease or high blood pressure and those who take certain medications, including for depression, insomnia or poor circulation
Symptoms of heat stroke include:
-- Body temperature of 103 degrees or higher
-- Hot, red, dry or damp skin
-- Fast, strong pulse
-- Headache
-- Dizziness
-- Nausea
-- Confusion
-- Passing out
-- No longer sweating
Symptoms of heat exhaustion include:
-- Heavy sweating
-- Cold, pale, clammy skin
-- Fast, weak pulse
-- Nausea or vomiting
-- Muscle cramps
-- Feeling tired or weak
-- Headache
-- Passing out
If someone shows symptoms of heat stroke or heat exhaustion, call 911, move them somewhere cooler and use towels to cool down their body.
Don't forget about your furry friends!
Here are some tips from the American Society for the Prevention of Cruelty to Animals for how to keep your pets safe in the heat: provide plenty of fresh water so they don't get dehydrated; don't over-exercise pets; never leave pets alone in a parked car; and watch for symptoms of overheating, which include excessive panting, difficulty breathing, increased heart and respiratory rate and drooling.
Animals with flat faces, like pugs, can't pant as well and are more at risk of heat stroke. These pets, as well as older and overweight pets, should be kept inside as much as possible.
A heat wave is a continuous period of abnormally hot weather, generally lasting over two days, according to the National Weather Service.
However, the definition of a heat wave varies by region.
In the Midwest and the Northeast, a heat wave is considered three or more days in a row of temperatures of at least 90 degrees. But in Phoenix, for example, temperatures are regularly well above 90 degrees and into the triple digits without it being from a heat wave. So a heat wave is relative to the local average temperature and may require temperatures to be significantly above normal for several days, according to the NWS.
ABC News' Kyle Reiman contributed to this report.
