Extreme heat kills more than 700 people in the United States every year, according to the Centers for Disease Control and Prevention (CDC), and thousands more suffer from nonfatal heat-related illness, or hyperthermia. Infants, young children, and older adults have a higher risk of developing health problems under extreme heat, and overheating is especially dangerous for people living with an underlying heart condition. But any person who is trapped in an overheating home or car, exercising or working outside in the heat, or exposed to extreme heat for a prolonged time is at risk of developing heat exhaustion and heatstroke. The two heat-related illnesses can look similar from the outside, but have very different effects on the body. Whereas heat exhaustion is related to dehydration and a cardiovascular system struggling to pump blood through the body, heatstroke occurs when a person’s body temperature gets so dangerously high that it begins overheating their tissues and organs, says William Roberts, MD, a professor of family medicine and community health at the University of Minnesota in Minneapolis. Heat exhaustion can usually be treated with cold water, air, and rest, but heatstroke is an emergency medical condition that requires immediate care. Although heat exhaustion can appear to be a stepping stone to heatstroke, Dr. Roberts says a person does not have to first show signs of heat exhaustion to experience heatstroke. “You can go right there without the intervening step,” he says. In any instance, swift attention is key to avoiding complications of both heatstroke and heat exhaustion. RELATED: Here’s What You Need to Know About Dehydration

Recognizing the Warning Signs

Heat exhaustion and heatstroke share many of the same warning signs, with a few key differences. “To the naked eye, it’s difficult to distinguish between heat exhaustion and heatstroke. To differentiate between the two, you need to know if the person’s brain is working well and what their body temperature is,” says Roberts. According to the Mayo Clinic, the most serious instances of heatstroke occur when a person’s body temperature reaches 104 degrees Fahrenheit. People experiencing heatstroke may have an altered mental state, become aggressive, delusional, confused, irritable, or have slurred speech. According to the CDC, signs of heatstroke also include:

Hot, red, dry, or damp skin, if the person has been exercisingFast, strong pulseHeadacheDizzinessNauseaConfusionPassing out

Signs of heat exhaustion include:

Heavy sweatingCool, pale, clammy skinFast, weak pulseNausea or vomitingMuscle crampsTiredness or weaknessDizzinessHeadachePassing outA body temperature less than 103 degrees Fahrenheit

How to Treat Heat Exhaustion or Heatstroke

Since heat exhaustion and heatstroke are different medical conditions, the proper treatments differ slightly. If you suspect that you or someone you know is suffering from heat exhaustion, there are a few steps you can take to start cooling off and rehydrating while you’re waiting for medical help. If possible, the CDC recommends drinking cold, nonalcoholic beverages, cooling off in the shade, staying indoors with air-conditioning or a fan, taking a cool bath or shower, and removing excess clothing. Rest is also key because it will put less strain on the cardiovascular system and help stabilize body temperature. Heatstroke, on the other hand, is a life-threatening condition that requires emergency treatment. If left untreated, the condition can quickly cause brain, heart, kidney, and muscle damage or failure, and can be fatal. When caring for someone you suspect has heatstroke, the first thing you should do is call an ambulance. While you wait for medical professionals to arrive, you can begin to try to lower the person’s body temperature in the following ways:

Move the person to a shaded or air-conditioned area to cool down until their temperature is steadily below 102 degrees F.If possible, place them in a cool bath or shower, or cool them with water from a garden hose. According to a meta-analysis of 63 studies on heatstroke treatment methods, published in January 2020, in the journal Resuscitation, submerging a person in cold water — about 34 to 63 degrees F — is the most effective method to lower core body temperature.

Importantly, the CDC advises against giving a person with heatstroke anything to drink, because most people with heatstroke have an altered state of consciousness, making it unsafe for them to swallow fluids. Research has shown that people who experience heat-related illness may be at a greater risk for eventually developing some cardiovascular diseases. According to a small study published in February 2019 in the journal PLoS One, people who had experienced heat-related illness previously were associated with a greater risk of having a heart attack, stroke, and chronic kidney disease.

Heatstroke and heat exhaustion are completely preventable, according to Barrak Alahmad, MBBS, MPH, a physician and a Mission Scholar in the faculty of public health at Kuwait University and a PhD student at Harvard University’s T.H. Chan School of Public Health in Cambridge, Massachusetts. “Both are triggered by high temperature, high humidity, and extensive physical activity,” he says, noting that as a first step, early warning systems can be used to alert the public of extreme-heat days so they can prepare to stay out of potentially dangerous heat. Knowing the heat index, which combines heat and humidity for a “real feel” temperature, is key to understanding how well your body will be able to cool itself, Dr. Alahmad explains. Sweating is one of the main ways the body expels excess heat. When sweat evaporates from the skin, the body cools. If the air already contains a lot of moisture, less sweat can evaporate. Essentially, humidity takes away the body’s natural way of cooling itself off, says Alahmad. Roberts says it’s important that everyone respect their personal limits regarding heat. “Something that complicates this is if you are being asked to perform, as is the case sometimes in football; you may push yourself because you don’t want to get cut from the team,” he says. While some may tolerate the heat or exercise more than others, no one is immune to heatstroke and heat exhaustion — multiple professional athletes have lost their lives to heatstroke. If you do have to be outside on a hot day, be sure to wear light clothing, both in color and fabric, a hat, and sunscreen, and take frequent breaks. “Getting into air-conditioning for as little as 30 minutes a day can make the difference between fatal heatstroke and not,” says Roberts, who also recommends exercising with a buddy so you can look out for each other. Be sure to check on relatives, friends, or neighbors who have a difficult time leaving their home and do not have air-conditioning. Roberts says these people are at a higher risk of serious complications related to heatstroke and heat exhaustion since they may be stuck in a hot home. Pay attention to the way your body feels if an extreme heat warning is issued, and never leave people or pets in a closed car. Since alcohol dehydrates the body, it’s also best not to drink booze in extreme heat. “It’s important to maintain good hydration, and don’t let yourself get thirsty,” says Roberts, who says there is no specific amount he recommends for everyone. “Recommending a certain amount is a recipe for overhydration, which can also be dangerous. Just drink when you are thirsty and don’t let your urine get dark yellow,” he says. RELATED: How Diarrhea Can Cause Dehydration