The Pacific Northwest was hit by a record-breaking heat wave over the weekend, with the highest temperatures seen at this point in the season.
Portland, Oregon, reached 93 degrees on Saturday -- breaking a record set in 1973 -- and 92 degrees on Sunday, according to National Weather Service Portland. Meanwhile, in Seattle, temperatures soared to a record-high 82 degrees, according to National Weather Service Seattle.
Although some regions are expected to see some relief at the beginning of this week, temperatures are expected to rise again.
The Centers for Disease Control and Prevention says 618 people die from heat-related illnesses every year in the U.S. and heat waves can lead to excess deaths. More than 70,000 people died in a heat wave in Europe in 2003.
Experts say people can suffer severe health effects beyond the well-known heat-related illnesses in extreme heat.
"It's more than a heat wave or just heatstroke," Dr. Kai Chen, an assistant professor of epidemiology at the Yale School of Public Health, told ABC News. "That's not the whole story, it's the tip of the iceberg."
Normally during extreme heat -- meaning temperatures that are hotter and/or more humid than average -- the body tries to cool itself by sweating.
However, if a person does not replenish with fluids, that can lead to dehydration. The body temperature can then continue to rise, which can then lead to other heat-related illnesses.
"Heat illness is a spectrum of disease," Dr. John Purakal, an assistant professor of emergency medicine at Duke University School of Medicine, told ABC News. "And it starts with kind of very low and mild symptoms and then works its way up to much more critical and concerning symptoms. It's usually a result of your body's difficulty with thermo-regulation and that's because of both dehydration and your salt stores being depleted."
The mild symptoms come in the form of a sunburn or a heat rash or heat cramps, with signs including muscle pain and spasms.
This can progress to heat exhaustion -- which includes symptoms of headaches, dizziness, muscle cramps, nausea and vomiting -- and, if the body reaches extremely high temperatures, heat stroke.
"It doesn't necessarily have to be a very long time outside with exposure to get this to happen," said Purakal. "It just depends on how hot it is outside, what sort of medical conditions you have and whether or not you're exerting yourself."
Anyone can be impacted by heat-related illness but some populations -- including the elderly, infants and young children, outdoor workers and people with low socioeconomic status -- are at higher risk.
Being exposed to extreme heat can cause even more damage to the body. Chen said extreme heat can increase the risk of heart attacks and strokes. Additionally, a recent study found extreme heat exposure is linked to an increase in kidney disease-related ER visits.
"But also, the impact goes beyond just the physical health, it can also impact our mental health," Chen said.
Research has shown that extreme heat has been linked to increases in irritability and symptoms of depression.
Additionally, people on certain medications, including some antidepressants and antipsychotics, can have trouble regulating temperature.
To stay safe, experts recommend using air-conditioning or finding your nearest cooling center if air conditioning is not available. They also recommend people wear loose, light-fitting clothes, limit time outside and drink plenty of water.
Purakal said it's also important to recognize something might be wrong and to call for help if needed.
"One of the things that folks get themselves into trouble with is just kind of not knowing the signs to look for and not thinking to call sooner than later, because the longer you expose yourself, the more risk that's involved," he said.