Speaking on BBC News, Dr Thomas Waite warns the hot weather about to hit Britain with temperatures as high as 31C could put old people and young children at risk of falling ill.
The Public Health England doctor explained the human body tries to protect itself from overheating by pushing blood closer to the skin putting extra strain on the heart and lungs.
He said: “It’s going to be warm all week, which is great news for many of us, but there are some common sense precautions everybody can take to make sure they stay well and they enjoy this weather.
“Because some of us are at greater risk than others.
“People who are older, people with long-term health conditions like heart diseases or lung conditions.
“But also very young children can be at risk of illness in this sort of weather.
“So common sense steps to stay cool, to stay hydrated and to keep your home cool.
“It’s a little different with cold weather when it’s hot people notice it very very quickly.
“And that’s down to how the body tries to stay cool.
“You push a lot of blood closer to the skin so that you can reduce heat that way and then on top of that you also sweat which means you lose a lot of moisture, you lose fluids.
“That’s why you need to stay hydrated and the extra strain it puts on your heart is why people notice a number of differences in side effects.
“Swollen ankles, a heat rash, which is a quite fine itching rash on the skin, and potentially also feeling dizzy.
“We see a rise in heart attacks and in strokes. And people contacting 111 and their GPs about a number of different types of conditions related to the heat.”
The Met Office has also warned Britons could “pay the price” of widespread sunshine as UV and pollen levels “will be very high” in regions across the UK this week.
After a “chilly start” across some rural spots, clear skies are expected to pave the way for “plenty of sunshine” with temperatures hitting 29C in parts of the UK during the day.
Meteorologist Marco Petagna said: “As we go through the day, the rest of Scotland, Northern Ireland and indeed much of England and Wales set fare lots of clear weather round to start with.
“So plenty of sunshine and temperatures already starting to climb up to 15 degrees in the London area.
“We hold onto a good a good deal of sunshine, hardly a cloud in the sky in fact across the bulk of the UK.
“But the very warm conditions if not hot conditions are reserved for England and Wales with 28C or 29C likely in the London area.”
But the meteorologist insisted Britons will “pay the price” for the gorgeous weather conditions as UV and pollen levels soar.
Mr Petagna warned: “We will pay the price though because UV levels and pollen levels will be very high in place.”
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