The urban US Northeast baked in record-breaking, 38-degree Celsius temperatures and steambath humidity combined with the heat-trapping effects of asphalt and concrete to make millions of people miserable.
The extreme heat extended north into Canada across southern Ontario and Quebec.
The mercury Friday in Newark, New Jersey, reached 42degC, the highest temperature ever recorded in the city. Airports near Washington and Baltimore hit 40.5degC.
Philadelphia hit 40degC. Boston 39.5degC, Portland, Maine, and Concord, New Hampshire, 38.5degC, and Providence, Rhode Island, 38degC.
New York City hit 40degC, just 1 degree C short of its all-time high, and with the oppressive humidity, it felt like 45degC.
The heat wave wafted in from the Midwest - it began last weekend and did not break until Friday in Chicago - and is a suspected or confirmed cause in more than a dozen deaths around the country.
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 landscaper who died Thursday night in Louisville, Kentucky, had a temperature of 43degC, the coroner said.
Jake Crouch, a climatologist at the National Climatic Data Center in Asheville, North Carolina, said the heat wave is taking its place in duration alongside deadly hot spells in 1988 and 1995 that lasted a week or more.
In Canada, an extreme heat alert remained in effect Friday in several Canadian cities, a day after at least 17 cities and towns in Ontario and Quebec broke their previous single-day records for July 21, Environment Canada said. In Toronto, Canada's most populous city, the temperature set a single-day record of 38 degrees Celsius on Thursday.
Some New Yorkers were unable to take a dip to cool off at some beaches in Brooklyn and Staten Island after millions of litres of raw sewage spilled from a wastewater treatment plant.
On Friday, power supplies were stretched, and utilities were hoping that some businesses would close early for the weekend. Con Edison in New York set a record for power demand at 1pm, breaking a mark set August 2, 2006, utility spokesman Bob McGhee said.
Several hundred homes and businesses were hit with blackouts, but power was restored by midafternoon. Voltage was deliberately reduced in several neighborhoods in the city and suburbs to keep underground cables from overheating.
Dangerous-heat advisories and air quality alerts were sent out for most of the Northeast on Friday. Richard Ruvo, section chief in New York for the Environmental Protection Administration, said: ''Today is a very bad day.''
Lauren Nash, a meteorologist with the National Weather Service, said the cities are experiencing the ''urban heat island'' effect.
''All the concrete and the blacktop warms up faster, so it keeps the city hotter and it stays hotter longer,'' she said. Overnight temperatures did not get below 80 (26 C) in some areas.