Heatwave deaths are set to more than triple in the UK by 2050, research suggests.
With scorching summers becoming the norm, fatalities are predicted to rise from 2,000 to 7,000 a year over the next few decades.
Scientists behind a new Lancet report worry older people and those with heart or kidney disease may be particularly vulnerable, with them being unable to “thermoregulate”.
A record 220 million more over 65-year-olds worldwide were reportedly exposed to heatwaves last year than in 2000.
Even compared to 2017, 63 million more older people were at risk last year.
The world is 1°C (33.8°F) warmer than before the industrial revolution.
And eight of the 10 hottest years on record occurred in the past decade.
The UK’s heatwave last year made headlines as the joint second hottest, shared with 1995, since records began in 1884. It was beaten only by 1976, according to the Met Office.
Balmy temperatures may signal BBQs and picnics on the beach, but experts warn they can also trigger heat stress.
This occurs when the body’s internal “temperature regulator” starts to fail, according to the Health and Safety Executive.
The body may compensate by excessively sweating, causing dehydration.
A sufferer’s heart rate also rises, putting an additional strain on the body.
If someone becomes so dehydrated they stop sweating, heat stress can develop into heatstroke.
This occurs when they are unable to cool their body and is a medical emergency.
Overheating can also affect the kidneys.
Dehydration lowers blood pressure and inhibits kidney function, according to the National Kidney Foundation.
This can ultimately lead to kidney failure, when the organs stop working altogether.
Older people may be particularly vulnerable, with both heart and kidney complications becoming more common with age.
“I worry about people over 65,” report author Dr Nicholas Watts, from University College London, said.
“If their heart or kidneys give out, they lose the ability to thermoregulate”.
Europe is predicted to be worst hit, with the continent experiencing temperatures some likened to “hell” this past summer.
“Europe is heating up but it also has an ageing population, which makes it more vulnerable,” Dr Watts said.
Last year, nearly one fifth (19%) of EU dwellers were 65 or over, according to Eurostat.
The scientists warn city dwellers may also be more at risk.
“Cities lock in carbon and the world is becoming more urbanised,” Dr Watts said.
“As cities warm, it exacerbates the trapping of air pollution.
“There is a bubble over London.”
How to stay safe during a heatwave
While a heatwave may seem a long way off as we batten down the hatches for winter, it could pay to be prepared for the summer months.
It may sound obvious, but keeping out of the sun could ensure you don’t overheat when the seasons change.
Public Health England also recommends wearing loose clothes to keep you cool in soaring temperatures.
To avoid sunburn, wear sunscreen, sunglasses and a cap or shawl if you go out.
Opt for cool drinks and food, like salad, and avoid alcohol.
If you feel yourself getting too hot, have a cool bath or shower.
Placing bowls of cool water around the house can also bring down the temperature of a room.
Closing curtains during the day and turning lights off that are not needed is another good tip.
Opening windows at night can bring cooler air inside.
Keep an eye on older people or those who live alone, and never leave the elderly or young children in a parked car on a hot day.
Signs you are overheating include headache, dizziness, confusion, nausea, loss of appetite, clammy skin, cramps, rapid pulse, panting, fever and excessive thirst, according to the NHS.
Children may also become floppy and sleepy.
If any of the above occurs, lie down in a cool place with your feet slightly raised.
Drink plenty of water and cool your skin with wet sponges or fans.
If symptoms do not improve after half-an-hour, it may be a sign of heatstroke.
This requires immediate attention.
Other signs include not sweating, seizures, loss of consciousness and failing to respond.