A "heat dome" has led to record high temperatures in the US states of Washington and Oregon.
Dry ice is being used to cool water, people have been sleeping in "cooling shelters", and a swimming pool in Seattle was closed because the surrounding deck area was "dangerously" hot.
A heat dome is when high pressure parks itself over an area, acting like a lid on a saucepan, trapping heat.
A record temperature of 104F (40C) was set in Seattle on Sunday - the highest since 2009.
At 7pm, thermometers were still registering 99F (37.2C), the National Weather Service said. Monday could see a high of 110F (43.3C).
That figure was reached in Portland, Oregon, on Sunday, while in the state capital, Salem, the mercury rose to 112F (44.4C), breaking the previous record by four degrees Fahrenheit.
In Ontario, Oregon, forecasters are predicting at least a week of triple-digit temperatures, including a high of 109F (42.8 C) on Wednesday.
The heatwave is also moving into Idaho, where temperatures above 100F (38C) are forecast for the state capital, Boise, for at least seven days from Monday.
Some residents have been losing power, with about 3,000 customers in Portland and 3,400 in the greater Seattle area affected.
King County closed several COVID-19 testing centres and trains may have to slow down because of excessive heat on the tracks.
Despite the breathless conditions, about 3,000 athletes participated in an Ironman Triathlon in Coeur d'Alene, Idaho, on Sunday, starting at 5am to try to avoid the worst of the heat.
It has also been very hot in British Columbia, Canada, where the mercury reached 115F (46.1C) in the village of Lytton on Sunday.