Record high temperatures are possible Wednesday, as many areas may feel as hot as 95 to 110 degrees on the heat index, according to the National Weather Service. Some scattered showers and thunderstorms are also in the forecast.
Heat advisories and heat warnings are in effect in parts of the Northeast as well as the Midwest, affecting more than 94 million people. While the weather is expected to ease up by the end of the week, the Southwest is heating up.
Parts of Arizona, Nevada and Southern California will also see extreme heat -- although it's a dry kind of heat. But temperatures in certain areas may get as high as 110 to 120 degrees.
There have been reports of at least nine heat-related deaths in Canada and the United States, stretching back to the weekend, including six in Montreal.
In Montreal, Dr. Mylene Drouin, regional director of public health, told CNN partner CTV News that the deaths occurred over the weekend. Most were people who were living alone. Temperatures there have been in the mid-90s since Sunday, and the heat will continue through Thursday, to highs around 93, before dropping back to average, or 77 degrees, on Friday.
At least three heat-related deaths have been reported in the US.
One was an elderly man who died in New York City on Friday, the city's medical examiner office told CNN.
On Saturday, a woman died of heat-related causes in Pennsylvania while working in her garden, according to the Blair County coroner's office. The woman went into cardiac arrest at her home and died at a hospital.
On Sunday, a 30-year-old man died after he collapsed on a mountain trail while running a race in Wilmington, New York, the Essex County coroner said. At the emergency room the man's internal temperature reached 108 degrees, damaging his brain.
"When your brain becomes overheated like that, it can't function any more," Essex County Coroner Frank Whitelaw said.
In addition, two possible heat-related deaths are also being investigated in Kansas City, Missouri. The deaths involve a man in his 80s, who died Monday, and a woman in her 40s, who died last week, according to the Kansas City Health Department.
When temperatures rise inside the body, it can cause damage to the central nervous system, the brain and organs.
The Centers for Disease Control and Prevention advises the following ways to reduce heat-related illnesses:
Slow down or reduce strenuous activities until the coolest time of day Stay in air-conditioned places as much as possible, such as a mall or public library Stay hydrated Wear lightweight, light-colored, loose clothing Don't leave kids or pets in a parked car
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