Nearly 80 million people in the United States remained under a heat advisory or warning Tuesday, with scorching temperatures and humidity expected through the Fourth of July.
There are reports of at least eight heat-related deaths in Canada and the United States, stretching back to the weekend, including six in Montreal.
Some places in the East may feel as hot as 100 to 110 degrees on Tuesday, when taking into account the humidity and heat, with advisories and warnings stretching from central Virginia to eastern Maine.
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.
A woman died of heat-related causes on Saturday 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.
Two possible heat-related deaths are 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.
Nearly 100 million people could see temperatures climb above 95 degrees Fahrenheit this week, with a majority of those residing in the eastern third of the nation.
Excessive heat warnings and advisories are posted for much of New York and New Jersey, with heat index values up to 105. Heat index is what it actually feels like outside when you take into account the humidity and the high temperature.
While temperatures topping 90 degrees are not too uncommon for New York City, it is rare to see the heat last more than a couple of days. The city could top 90 for up to seven consecutive days this week, something the National Weather Service in New York has said happens about once every 33 years.
Prolonged heat could be expected through Thursday, with heat index values up to 105 for northeast New Jersey during the peak of the heat on Tuesday, according to the New York weather service office.
"Heat-related illnesses possible through this time period, especially for at-risk populations such as the elderly, very young, and those with mental illnesses," the New York weather service office warned.
Residents are warned to "drink plenty of fluids, stay in an air-conditioned room, and stay out of the sun," the agency says.
This advice applies to pets, too.
A dog in Brick, New Jersey, died on Tuesday after being left in extreme heat over the weekend.
The dog's owner has been charged with failing to provide necessary care to an animal, exposing an animal to adverse weather conditions, failing to provide a shelter of a proper size, and failing to obtain a dog license, according to a news release from the Ocean County Prosecutors Office.
Temperatures are expected to be 10 degrees above normal across New England, according to the weather service. Relief could be days away, as the forecast is that temperatures will return to the mid-80s in the East near the end of the week.