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Working out in the open on farms and building sites or indoors in factories and hospitals could expose people to heat stress.
Dr Jimmy Lee, an emergency medic Singapore treating Covid-19 patients say the personal protective equipment, essential for avoiding infection, is making things worse by creating a sweltering 'micro-climate'.
“It's really uncomfortable over a whole shift of eight hours - it affects morale," he said, noting that overheating can slow down their ability to make quick decisions which is vital for medical staff.
If they keep working ignoring warning signs of heat stress, such as faintness and nausea, they risk collapsing.
Heat stress when the body is unable to cool down properly so its core temperature keeps rising to dangerous levels and key organs can shut down.
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It happens when the main technique for getting rid of excess heat - the evaporation of sweat on the skin - can't take place because the air is too humid, the BBC report says.
Dr Rebecca Lucas, who researches physiology at the University of Birmingham, said the symptoms can escalate from fainting and disorientation to cramps and failure of the guts and kidneys.
A system known as the Wet Bulb Globe Temperature (WBGT) is used to measure not only heat but also humidity and other factors. When the WBGT reaches 29C, for example, the recommendation is to suspend exercise for anyone not acclimatised.
Yet that's the level Dr Lee and his colleagues are regularly experiencing at Singapore's Ng Teng Fong General Hospital.
When the WBGT registers 32C, the US says strenuous training should stop because the risk becomes "extreme". But levels that high have recently been recorded inside hospitals in Chennai by Prof Vidhya Venugopal of the Sri Ramachandra University.
‘Drink lots of fluid and take rest’
Prof Richard Betts of the UK Met Office has run computer models which suggest that the number of days with a WBGT above 32C is set to increase but it depends on whether greenhouse gas emissions are cut.
He warns that if prompt steps are not taken to tackle climate change early, then sooner or later “the hottest parts of the world could start to see conditions that are simply too hot for us."
Dr Jimmy Lee advises people to drink plenty of fluid before they start work, take regular breaks and then drink again when they rest.
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Dr Jason Lee, an associate professor in physiology at the National University of Singapore, says that as well as measures like rest and fluids - and shade for outdoor workers - a key strategy for resisting heat stress is to be fit.
"By keeping yourself aerobically fit, you're also increasing your heat tolerance, and there are so many other benefits too," he says.
Dr Lee is a leading member of a group specialising in the dangers of excessive heat, the Global Heat Health Information Network, which has drawn up guidelines to help medics cope with Covid-19. It is spearheaded by the World Health Organization (WHO), the World Meteorological Organization (WMO) and the US weather and climate agency Noaa.
"This climate change will be a bigger monster and we really need a coordinated effort across nations to prepare for what is to come,” he says. "If not, there'll be a price to be paid."