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Plunging into cold water during hot weather can cause heart attacks even in young, fit and healthy individuals, according to new research by scientists at Portsmouth University.
They're warning that entering cold water suddenly, without taking time to acclimatise, may cause abnormal heart rhythms that can be fatal.
The research comes after several people died after going into water during the recent heatwave, although the cause of death in these cases is not yet known.
Professor Mike Tipton, who runs the Extreme Environments Laboratory at the University of Portsmouth, said entering cold water should be done with caution.
''As the recent sad spate of immersion deaths confirm, we have entered the most dangerous time of the year for water-related deaths.
''As air temperatures rise dramatically, people start to go into water that remains dangerously cold.
''The body's responses to immersion in cold water are profound, uncontrollable and can result in drowning and heart problems within seconds.''
In the study, published in the Journal of Physiology, Prof Tipton and Prof Mike Shattock of King's College London explain how rapid submersion in cold water, combined with holding one's breath, automatically activates two powerful responses in the body which may interact and cause conflict for the heart.
The body's cold shock response speeds up the heart rate and causes hyperventilation which can conflict with the diving response, which does the opposite and which acts to conserve oxygen.
This can lead to autonomic conflict, causing the heart to go into abnormal rhythms and, on occasions, causing sudden death.
Prof Tipton said:
''Those wanting to enter the water should do so in a slow and controlled fashion to minimise these hazardous responses.
''Individuals should also realise the water they felt comfortable in at the end of last year is colder, and they are less prepared for it at the start of the summer.
''The prevalence of heart problems on immersion in water tends to be underestimated because electrical disturbances to the heart are undetectable post-mortem.
''The incapacitation caused by cardiac arrest, such as gasping for breath and breathing in water, means that death is often ascribed to drowning, but we believe a significant number of these cases could have a basis in autonomic conflict.''
Immersion-related deaths are the second most common cause of accidental death in the UK with one child a week lost from this cause, according to a spokeswoman for the University of Portsmouth.
''There is a peak in these deaths during early summer as people return to water-based activities.
''The scientists say that a few simple precautions, such a slow entry and taking time to habituate to the cold water, will save lives at this critical time.''
She added that 67% of immersion deaths occur in strong swimmers and 55% of these are rapid and within three metres of a safe haven and 42% within two metres.
Worldwide, WHO estimates that 1.2 million immersion-related deaths occur each year and more children die from immersion-related deaths than malaria, TB, HIV or polio.