A heatwave warning has been issued for parts of the UK as temperatures are forecast to hit 41C (106F).
The hot weather will continue on Tuesday - with overnight temperatures warned to be in the mid twenties - before cooling on Wednesday.
Heat can affect anyone, but some people run a greater risk of serious harm. Check on those who may be more vulnerable, such older people and babies.
Here's what you need to know about the effects of heat on the body and how to stay cool.
What does extreme heat do to our bodies?
As the body gets hotter, blood vessels open up. This leads to lower blood pressure and makes the heart work harder to push the blood around the body.
This can cause mild symptoms such as an itchy heat rash or swollen feet as blood vessels become leaky.
At the same time, sweating leads to the loss of fluids and salt and, crucially, the balance between them in the body changes.
This, combined with the lowered blood pressure, can lead to heat exhaustion. Symptoms include:
If blood pressure drops too far, the risk of heart attacks rises.
Why do our bodies react this way?
Our bodies strive to keep a core temperature of about 37C whether we're in a snowstorm or a heatwave.
It is the temperature our bodies have evolved to work at.
But as the weather gets hotter, the body has to work harder to keep its core temperature down.
It opens more blood vessels near the skin to lose heat to our surroundings and starts sweating.
As the sweat evaporates, it dramatically increases the heat lost from the skin.
How can I stay safe in the heat?
The UK Health Security Agency has some tips:
Look out for those who may struggle to keep cool, such as older people, those with underlying conditions and and those who live alone
Stay cool indoors by closing curtains on rooms that face the sun
Drink plenty of fluids and don't drink too much alcohol
Don't leave anyone, especially babies, young children and animals, in a locked vehicle
Keep out of the sun between 11am and 3pm when the sun's rays are strongest
Keep in the shade, use sunscreen with a high SPF and UVA rating, and wear a wide-brimmed hat
Avoid physical exercise in the hottest part of the day
Take water with you if travelling
Be aware of hidden dangers in rivers and open water if tempted to cool off
How can I get a good night's sleep?
Use thin sheets, cool your socks in the fridge before putting them on and stick to your usual bedtime routine, experts say.
10 tips for sleeping in hot weather
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What should I do if I see someone with heat exhaustion?
If they can be cooled down within half an hour, then heat exhaustion is not normally serious.
NHS advice says:
Move them to a cool place.
Get them to lie down and raise their feet slightly
Get them to drink plenty of water - sports or rehydration drinks are also OK
Cool their skin - spray or sponge them with cool water and fan them. Cold packs around the armpits or neck are good too
However, if they do not recover within 30 minutes, then what follows is heat stroke.
It is a medical emergency and you should call 999.
People with heat stroke may stop sweating even though they are too hot. Their temperature could go over 40C and they might have seizures or lose consciousness.