As temperatures across the UK rocket this week, whether you're stuck in a sweltering office or running around after children on school holidays, all we're really dreaming about is a cool dip in the sea, river or lake.

But the RNLI and other safety organisations are warning of the dangers of leaping into cold open water without knowing the risks.

To coincide with the schools breaking up for summer and the current heatwave, the RNLI have stepped up their Float to Live campaign, urging people to Respect the Water.

It comes as a man drowned in a Gloucestershire lake this week while looking for his dog. And two bodies have been found in the River Thames after three swimmers went missing.

Last year, 128 people died in coastal waters around the UK, as 2018's hot summer saw a surge of visitors to beaches. In total, 263 people died from accidental drownings in 2018.

And the risks of cold water shock are probably higher than you thought. Anything below 15C is defined as cold water. Average UK and Ireland sea temperatures are around 12C, with rivers even colder - even when the outside temperature is soaring.

Cold water shock can lead to the blood vessels in the skin closing, which increases the resistance of blood flow and your heart rate.

As a result the heart has to work harder and your blood pressure goes up. Cold water shock can cause heart attacks, even in the relatively young and healthy.

A spokesperson for the RNLI said: "The sudden cooling of the skin by cold water also causes an involuntary gasp for breath. Breathing rates can change uncontrollably, sometimes increasing as much as tenfold. All these responses contribute to a feeling of panic, increasing the chance of inhaling water directly into the lungs.

"This can all happen very quickly: it only takes half a pint of sea water to enter the lungs for a fully grown man to start drowning. You could die if you don't get medical care immediately."

As part of the Float to Live campaign, the RNLI wants anyone entering open water to know what to do if they get in trouble.

Their advice includes:

  • Lean back, extend your arms and legs
  • If you need to, gently move your arms and legs to help you float
  • Float until you can control your breathing
  • Only then call for help or swim to safety
  • Clothes can help with buoyancy – during the first moments in water air is trapped between the layers. Moving less helps the air stay trapped, helping you float
  • Floating is not always something people are confident they can do, but most people can float
  • Practice floating in a safe environment like a swimming pool

The RNLI's video of former Royal Marine Ant Middleton, presenter of SAS Who Dares Wins, and an RNLI ambassador, demonstrates the effects of cold water shock on the body.

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