People are being warned not to exercise over the coming days, as record temperatures are forecast to create deadly conditions.
Ireland has recorded its hottest temperature in more than a century at 33C with a status yellow warning in place across the country.
A lot of heat is produced by the body during exercise and this combined with the temperatures could see even the fittest people suffering from heat exhaustion, experts have said.
Scientists say everyone is at risk from extreme heat, and are warning people to take precautions and simply not view the heatwave as normal summer days.
Dr Eunice Lo, climate scientist at the University of Bristol Cabot Institute for the Environment, said: “Everyone is at risk and we do need to be aware and take precautions and definitely not view these as normal summer days or something to go out and have fun in.”
Mike Tipton, professor of human and applied physiology, University of Portsmouth, said a person can go from emitting as much heat as a 90 watts lightbulb when at rest, to emitting as much heat as a two kilowatt fire when exercising.
He added: “We produce a lot of heat when we exercise.
“And we’re now at a temperature where … actually people will warm up, just doing their day-to-day activities in the house, or outside, and cooking, that sort of thing.
“And so fitter people undoubtedly do better in the heat, but we still see fit people who suffer from heat exhaustion in particular.
“So the recommendation would be just for the next couple of days, when we’re now in unprecedented temperatures, is just to stop exercising.”
Prof Tipton explained that people overheating through exercise risked putting “additional load on the health system”.
Data suggests the vast majority of deaths associated with heatwaves occur on those aged 65 and above, and are primarily related to cardiovascular problems as a result of the strain placed on that system by the heat.
Researchers have also identified those with psychiatric illness as a group vulnerable to extreme temperatures.
Dr Laurence Wainwright, departmental lecturer at the Smith School of Enterprise and the Environment, University of Oxford, said: “In recent years, there’s been an increasingly large body of research showing us that heatwaves worsen outcomes for those with underlying psychiatric illnesses – rates of suicides go up, levels of mortality go up, for those with existing conditions, symptoms can worsen.
“For people with certain conditions, say bipolar disorder, extreme heats can trigger people into certain aspects or certain phases for that condition. For bipolar, for instance, mania.”
He added: “A couple of nights of broken sleep can be a trigger for the onset of a depressive phase.”
Dr Wainwright said that various medications for a number of treatments also have an impact as they may have an effect of hydration or thirst.
He concluded: “There’s a lot we don’t know, but what is very clear is that heatwaves have implications for those with underlying medical conditions.”
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