Ron Falk collapsed outside a shop in Texas during a heatwave last year. As well as losing his right leg, he needed extensive skin grafts on the other. The 62-year-old fell unconscious due to heatstroke, and ended up losing his job and home.
Almost 90 million people are under heat alerts in the US – and conditions are set to worsen as the country marks Independence Day.
California has been especially hard hit, with temperatures of up to 46.1C (115F) forecast between now and Sunday night.
The dry conditions also elevate the risk of wildfires, with crews already working to put out a swiftly growing blaze in scorching conditions.
Firefighters have been lining roads in Butte County, about 70 miles (112km) north of Sacramento, in a desperate bid to stop the flames from reaching homes.
More than 13,000 have been evacuated from the area – and in 10 other counties across the state, power has been shut off to reduce the risk of further wildfires.
The outages come amid concerns that strong winds could topple power lines or cause trees to fall on them.
Californians are also being told to be mindful of wildfires while celebrating Independence Day.
“We want to urge everyone to use extreme caution when participating in activities that cause sparks, like using fireworks,” meteorologist Brad Schaaf said.
The hidden dangers of heatwaves
Estimates from the AP news agency suggest excessive heat has killed a record-breaking 2,300 people in the US over the past year – a figure that’s likely undercounted.
Americans are being urged to look out for signs of heatstroke, as the consequences can be severe.
Sizzling pavements and unshaded playgrounds increase the risk of surface burns – with young children, older adults and homeless people most at risk.
Ron Falk collapsed outside a shop in Texas during a heatwave last year. As well as losing his right leg, he needed extensive skin grafts on the other.
The 62-year-old was unconscious due to heatstroke, and ended up losing his job and home.
“If you don’t get somewhere to cool down, the heat will affect you. Then you won’t know what’s happening, like in my case,” Mr Falk said.
Contact burns can happen in seconds when skin touches a surface of 82C (10F).
Dr Kevin Foster, who works at the Southwest’s largest burn centre, says at least 50 people have been treated in hospital for such injuries since the start of June alone.
“Last year’s record heatwave brought an alarming number of patients with life-threatening burns,” he added.
The recovery process can take months – with Bob Woolley suffering second and third-degree burns to his hands, arms, leg and torso after falling in his back garden.
“The ordeal was extremely painful, it was almost unbearable,” the 71-year-old said.
Parents have also been warned small children are particularly vulnerable because they are not fully aware of the harm that touching a hot surface can cause.