By now, many of us recognise that we are confronting a climate emergency on a vast scale, and that rising temperatures will threaten the lives of millions across the planet. Severe heat waves have already killed many thousands of people over the past decade, but what is less recognised is that rising temperatures are also, slowly but surely, bringing more dangerous heat stress into our daily lives.

Millions of people work outside or in uncooled indoor environments every day. People working in construction, agriculture, fishing, forestry or the military often work intensively in direct sun for extended periods of time. Millions of workers in indoor factories, warehouses and workshops are also exposed to excessive workplace heat. A study of a garment factory in Cambodia predominantly employing young women showed indoor temperatures as high as 37C.

Working in high temperatures is dangerous because extreme heat prevents the body from adequately cooling, causing the core body temperature to rise. The longer the body remains overheated, the greater the risk of heat-related illnesses such as heat cramps, heat exhaustion, heat stroke and, in extreme circumstances, death.

Heat fatalities are already happening. This summer, I collaborated on research that analysed the deaths of Nepalese construction workers in Qatar and showed the fatal effect of heat stress during the hottest months. We found that hundreds of deaths could have been avoided if workers had not been forced to toil for long hours outside in searing heat.

On the other side of the world, hundreds of young sugar cane harvesters in Central America have already died from chronic kidney failure linked to excessive sweating and dehydration during heavy outdoor work.

As the number and intensity of hot days increases, more and more working people will face even greater challenges to avoid heat stress, particularly the two-thirds of the global population who live in tropical and sub-tropical areas. Heat exhaustion threatens the livelihoods of millions and undermines efforts to reduce poverty.

I was first confronted by the issue of hot working conditions while visiting factories in Vietnam where excessive heat seriously affected production during the hottest months of the year. Around the same time, I was part of an Intergovernmental Panel on Climate Change (IPCC) meeting reporting on the effects of climate change on population health. I listened as successive speakers reported on the effects of climate change on agriculture, forestry and other industries. Not one mentioned the effects of heat on working people.

Revealed: hundreds of migrant workers dying of heat stress in Qatar each year Read more

Years later and the threat to human health and economic survival from excessive heat exposure still receives little attention in the debate about the global climate emergency. Yet the few analyses available on the negative economic effects of heat on working people and their labour productivity indicate major impacts. The International Labour Organization estimates global productivity losses equivalent to 80 million full-time jobs could occur by 2030 in the hottest countries.

It is not enough to say that air conditioning or changing our working patterns will overcome the health threat of excessive heat. For many of the most affected, this is simply not possible. A migrant worker in Qatar or a sewing machine operator in Vietnam often have little control over their working environment or the potentially deadly levels of heat stress to which they may be exposed. Global heating is a serious threat both to workers’ lives and the livelihoods of millions of people. Emerging policies on climate must take this into account if we are to have any chance of getting to grips with what is ahead.