The highest death toll, 11 people, was registered in Webb County, Texas, near the Mexican border.

“As of Wednesday, there has been 11 total deaths,” local officials said in a statement to AFP. “Ten are Webb County residents, the eleventh death was from a neighbouring county that was brought to a local hospital and unfortunately passed away.”

A 14-year-old died last week when he was hiking in Big Bend National Park in Texas, where temperatures reached 48° Celsius. Tragically, the victim’s stepfather died in a car accident as he was rushing to the boy’s rescue.

And a 62-year-old woman died in the neighbouring state of Louisiana last week, after a storm left thousands of families without power and thus without air conditioning, according to local officials.

In recent days, temperatures in some southern US cities have felt like 45°C, with the pavement cracking in Houston, Texas and authorities setting up cooling centres in the city of with 2.3 million.

Meanwhile, Canada continued to battle the worst forest fire season in its history, a phenomenon that scientists say is exacerbated by human-induced climate change.

As smoke drifted south, large parts of the US that are home to more than 120 million people, from the Midwest to the east coast, remained under air quality alerts.

In New York and Philadelphia, the air was considered unhealthy, according to the government platform AirNow.

Air quality alerts were also issued in the Canadian province of Ontario, as well as for much of the Great Lakes and parts of Minnesota, North Carolina and Georgia.

Smoke from the wildfires has also drifted across the Atlantic Ocean and over European countries, including Portugal and Spain.