Extreme heat is posing growing challenges in the workplace, the UN's health and climate agencies said, as they issued guidance for governments, employers and health authorities to mitigate the risks.

"Immediate action is needed to address the worsening impact of heat stress on workers worldwide," they said.

Many workers are regularly exposed to dangerous heat conditions, the World Health Organization and the World Meteorological Organization said.

But the WHO and WMO said the frequency and intensity of those extreme heat events had risen sharply, increasing the risks for both outdoor and indoor workers alike.

Manual workers in sectors such as agriculture, construction and fisheries are particularly hard hit, they said in a report.

The agencies said worker productivity dropped by two to three percent for every degree above 20C.

The related health risks include heatstroke, dehydration, kidney dysfunction, and neurological disorders.

- Economic factor -

"Occupational heat stress has become a global societal challenge, which is no longer confined to countries located close to the Equator," said WMO Deputy Secretary-General Ko Barrett.

"Protection of workers from extreme heat is not just a health imperative but an economic necessity."

The agencies called for occupational heat action plans, tailored to specific industries and regions.

The guidance drew on findings by the UN's International Labour Organization (ILO), highlighting that more than 2.4 billion workers are exposed to excessive heat globally -- 71 percent of the world's working population.

It results in more than 22.85 million occupational injuries each year and almost 19,000 fatalities.

"Investing in effective, preventive and protective strategies would save the world several billion dollars every single year," said Joaquim Pintado Nunes, the ILO's chief of occupational safety and health and the working environment.

"Without bold, coordinated action, heat stress will become one of the most devastating occupational hazards of our time, leading to a significant loss of life and productivity."

- Adapt or die -

The guidance called for a focus on middle-aged and older workers, and those with chronic health conditions or lower physical fitness who would be more susceptible to heat stress.

It said workers, trade unions, health experts and local authorities should work together to come up with heat-health strategies that would then be widely supported.

It said heat stress symptoms were often misdiagnosed, with people failing to recognise the signs.

The last WHO technical report and guidance on workplace heat stress dates from 1969, "when the world looked very different in terms of climate change", said the WHO's environment chief Ruediger Krech.

"What's changed is the severity," he said, with each of the past 10 years being in the warmest 10 ever recorded.

WMO services division senior director Johan Stander added: "We must face up to the future of extreme heat. It's a reality for many: a case of adapt or die."