The eastern US and parts of Canada are in the grip of a heatwave which is putting power grids under strain and forcing some people to seek shelter in city "cooling centres".
Soaring temperatures are afflicting areas from Quebec to Virginia, hitting 39.5C (103F) in some places.
Power cuts have occurred as energy usage soars, leaving thousands of households without power.
The heat is already being blamed for at least two deaths in the US.
A 92-year-old woman, who did not have air conditioning, was found dead in her home in Philadelphia, Pennsylvania.
The body of a homeless woman was also found lying next to a car on Sunday in Detroit.
The heat is also said to be causing sharp increases in hospital visits in Canada.
Officials told local media 158 people had visited the emergency room at Montfort Hospital in Ottawa on Monday - the highest number of visits the hospital has seen in one day.
Electricity companies are urging consumers to cut back on energy use to relieve the stress on the power system.
"In addition to putting stress on people and their health, the intense heat will also place a higher demand on power grids due to increased usage of air conditioning," AccuWeather.com meteorologist Heather Buchman told the AFP news agency.
She added: "Power outages could result in some communities, putting people at an even greater risk for developing heat-related illnesses."
But heavy usage has already taxed some power plants and distributors, leaving residents in cities including New York, Philadelphia and Washington DC without power.
In Philadelphia alone, 8,000 people lost power due to increased demand on Tuesday.
Power companies said demand on Tuesday had approached the record highs set during a heatwave in 2006.
New York City broke the 38.3C (101F) record set on 6 July 1999. The National Weather Service measured 39.5C (103F) in mid-Manhattan on Tuesday.
Philadelphia, in Pennsylvania, also broke its previous record for the time of year when temperatures rose to 38.9C (102F).
Temperatures in Baltimore reached 100F (38C) for a third day in a row on Wednesday.
Some residents in both the US and Canada have been seeking refuge from the heat in air-conditioned churches and municipal buildings.
Keepers at the National Zoo in Washington have been giving some animals, including the zoo's famous pandas, homemade fruit ice lollies to help them stay cool. Most animals also have access to air-conditioned indoor enclosures.
The soaring temperatures are expected to continue throughout much of the week.