© BBCOne New Yorker says being outside is like "sitting in a sauna all day long"
The mercury in Newark, New Jersey, reached 108F (42C) on Friday, the highest ever recorded in the city.
In Canada, an extreme heat alert remained in effect, a day after two dozen cities and towns broke their previous single-day heat records.
At least 22 deaths have been blamed on the heat.
Across the US alone, where nearly half of the population was under a heat advisory, more than 220 heat records have tumbled.
Sewage spillage
Many regions in the central US and parts of the eastern seaboard have seen heat indexes - a combination of temperature and humidity - topping 43C.
© BBCThis is the weather forecast for North America.
Philadelphia - where bathers at public swimming pools were asked to leave every half hour to allow a new crowd to enjoy a cooling dip - saw temperatures of 40C (104F).
New York City also hit 40C, just a degree short of its all-time high, although with the oppressive humidity, it felt like 45C (113F).
As New Yorkers roasted in the heat, health officials warned them to stay out of the water at four beaches on New York Harbor after a sewage treatment plant damaged by fire began pumping raw waste into the Hudson River.
Several hundred homes and businesses in New York were hit with temporary blackouts.
Voltage was reduced in several neighbourhoods in the city and suburbs to keep underground cables from overheating.
Teenager Dies
On Friday, the medical examiner's office in Chicago listed heat stress or heat stroke as the cause of death for seven people.
An 18-year-old landscape gardener who died on Thursday night in Louisville, Kentucky, had a temperature of 43C (110F), a coroner said.
In Canada, temperature records were broken in two dozen cities across Ontario and Quebec on Thursday, including the hottest ever July temperature in Toronto, at 37.9C (100.2F).
Asphalt and concrete pavements and buildings in cities were "re-radiating" the heat, forecasters said.
Eli Jacks, a meteorologist with the National Weather Service, told the BBC: "This is an exceptionally strong ridge of high pressure that really has an exceptional scope and duration."
The combination of high heat and humidity make it hard for the human body to cool itself - because sweat does not evaporate efficiently, he added.
Officials in the central state of Missouri say 13 people have died, and there have also been fatalities in neighbouring Oklahoma, including a three-year-old boy.
In the south, more than three-quarters of Texas is suffering from drought amid the worst dry spell in the state for decades.
High temperatures - the number one weather-related killer in the US - claim 162 lives on average in the country each year.
The most severe heatwave in modern North American history took place during the Great Depression in 1936. The heat that summer was blamed for more than 5,000 deaths in the US and Canada.
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