(CNN) — The unprecedented heat wave that’s scorching parts of Northern America isn’t letting up just yet.

More than 50 million people are under heat alerts Wednesday across the West and Northeast US, while high temperatures are also continuing to bake parts of western Canada.

Lytton, British Columbia, hit 121 degrees Fahrenheit on Tuesday — setting the Canadian heat record for the third day in a row. Spokane, Washington state, recorded its hottest day ever on Tuesday, at 109 degrees. On the other side of the coast, Boston, Massachusetts tied and Hartford, Connecticut, broke their daily record — topping at 99 degrees.

The sweltering heat is dangerous — between 2004 and 2018, an average of more than 700 Americans died of heat-related causes each year, according to a US Centers for Disease Control and Prevention report. Roughly 90% of those deaths were between the months of May to September, the report said.

And experts say climate change is making heat waves more common and more powerful.

Here’s what happens to your body in extreme heat, what you need to watch out for and how to stay safe.

What happens to your body

Normally, your body is used to a certain range of temperatures, usually between 97 to 99 degrees Fahrenheit. When your brain senses a change — either lower or higher than that — it attempts to help your body cool down or heat up, according to Dr. Judith Linden, executive vice chair of the department of emergency medicine at Boston Medical Center.

“There are a number of different ways in which (the brain) attempts to cool the body down. One way, the most common way we think of, is that you sweat,” Linden said. “The pores open, the body sweats and the sweat evaporates, that cools the body.”

The second way your body cools itself down is by dilating vessels and upping your heart rate, which helps bring heat and blood to the surface of your body and helps releases that excess heat.

When you’re exposed to high temperatures, it becomes harder for your body to try and keep up with cooling itself down. And if your environment is hot and humid, sweat doesn’t evaporate as easily — which pushes your body’s temperature even higher, according to the Mayo Clinic.

“The higher the humidity, the lower temperatures you need for extreme heat,” Linden said.

High body temperatures can lead to damage to the brain and other vital organs, the CDC says. They can also lead to several heat-related illnesses.

Types of heat-related illnesses

Mild-heat related illnesses, including heat cramps, are most common, Linden said. Heat cramps can develop in people who sweat a lot, including during exercising. The excessive sweating uses up all of the body’s salt and moisture and can lead to muscle pains or spasms, usually in the abdomen, arms or legs, according to the CDC.

A heat rash can also develop. That’s a skin irritation caused by too much sweating in hot and humid weather, and is most common in young children, the CDC says. It is usually a red cluster of pimples or blisters, and tends to be in places including the neck, upper chest or in elbow creases.

When your body’s beginning to exceed its ability to cool itself down, you can develop what’s known as heat exhaustion.

“In this case you’re going to see excessive sweating because your body is really going to try and keep up with that extra heat. You’re going to feel light-headed, you may feel dizzy, often people present with nausea, headaches and their skin often looks pale and clammy and their pulse is often fast,” Linden said.

“This is the body’s last attempt to cool itself before it really goes into a point of no return.”

A heat stroke is the most serious heat-related illness, and, if left untreated, can lead to death.

“That’s where your body’s temperature goes above 104 to 105 degrees or so, and this is where your mechanisms are starting to fail,” Linden said.

Warning signs may include extremely high body temperatures, red and dry skin, a rapid pulse, headache, dizziness, nausea or loss of consciousness, according to the CDC.

The hallmark of a heat stroke is confusion and agitation, Linden said.

“So when somebody’s in the heat and they become confused and agitated, that’s heat stroke until proven otherwise and you need to call 911 for that or get help immediately and get the person out of the heat.”

Who’s at highest risk

Elderly, people with chronic medical conditions as well as children are at higher risk for severe heat-related illnesses.

The elderly and people with chronic medical conditions may be less likely to sense and respond to temperature changes and may be taking medication that make the heat effects worse, the CDC said.

“Very young (people) as well, because they’re less likely to recognize heat-related illness and they’re less likely to get out of the heat if they’re starting to feel overheated,” Linden said.

She added that student-athletes and pets are also at higher risk.

“In this weather, you must never, ever, ever leave a child or a pet in the car for even a minute,” Linden added.

How to stay safe

When your community is facing extreme heat, there are several things you can do to keep yourself and others safe.

First, keep an eye out for symptoms of heat exhaustion or other illnesses.

“If somebody starts feeling light-headed, dizzy, nausea or headache, that is the time to act immediately,” Linden said. “That means getting them out of the heat and into a cool environment.”

Putting water on someone who may be experiencing symptoms and giving them fluids can help cool them down. If someone is starting to lose consciousness or has nausea or vomiting, call 911.

“If you see anybody with any type of confusion, that’s an immediate red flag,” Linden added.

When it’s hot outside, try to avoid outdoor activities — especially between the hours of 11 a.m. and 3 p.m., according to Linden. If you have to go outside, wear light-colored clothing, cover your head and drink plenty of fluids.

Don’t wait until you’re thirsty to drink water — as that can be a sign of dehydration. Linden recommends drinking at least one glass of water — or more — an hour.

“If you do start to feel light-headed, dizzy, sweating, fast pulse, get out of the heat immediately,” Linden said.

Try to find air conditioning, or places in your area where you can go to stay cool, according to Ready.gov. Even spending a few hours in a shopping mall or public library can help.

When you’re home, fans can help, but don’t rely on them as your only way of cooling down — while it may feel more comfortable, they won’t help prevent heat-related illness.

“If you’re in a super hot room, if you’ve got a fan, is it helpful? No. I think, if you’ve got a fan, and you’re able to mist yourself … then fans can be helpful,” Linden said. “Fans are not foolproof.”

Finally, make sure you’re checking on your neighbors, parents and friends — especially older individuals who may be living alone or are isolated, Linden said.

This is what happens to your body during extreme heat