SINGAPORE – It will take only 1 deg C of further global warming for intense, humid heat to endanger the health and imperil the livelihoods of hundreds of millions of outdoor workers in the tropics, a United States-led study has found.

The planet has already become hotter by about 1.1 deg C on average since pre-industrial times.

And if that extra warming by a single deg C happens, about 800 million people in the tropics will be living in areas where heavy physical work becomes dangerous for more than half the hours in a year, according to a review paper published in early March in the Cell Press journal One Earth.

The 2023 United Nations climate change conference warned that the world had not done enough to keep global warming to 1.5 deg C from pre-industrial times – a landmark goal under the Paris Agreement.

“Over a billion outdoor workers live in the tropics, where nearly a fifth of all hours in the year are hot and humid enough to exceed recommended safety thresholds for heavily toiling workers,” the report said, warning about rising heat-related illnesses such as heat stroke and chronic kidney disease.

The paper’s authors called for individuals, communities and governments to step up efforts to bolster the heat resilience of outdoor workers in the tropics, including those in agriculture, construction, forestry and fisheries.

They also highlighted research gaps related to outdoor workers in humid heat, including the need for more fieldwork to better measure heat impacts for each outdoor work sector and demographic, since factors such as age, health and working conditions shape each person’s risk of heat strain.

The authors also noted that existing studies rarely delved into solutions, which means workers’ adaptations to heat remain understudied.

The 15 authors of the review paper – titled “Impacts of warming on outdoor worker well-being in the tropics and adaptation options” – concluded that there is additional danger for 800 million people in the tropics after studying data from various sources, including the latest weather statistics and climate models.

The tropics as defined in the paper are the regions within 30 degrees north and south of the Equator, which include dry deserts, coastal regions dotted with mangroves, and tropical forests in Africa, South Asia and South-east Asia.

There is high humidity in the areas examined, where working in sweaty, sticky and thermally uncomfortable conditions is a reality for outdoor workers.

Humidity and heat are a lethal combination because the high moisture in the air makes it hard for sweat to evaporate from the skin, which is the main mechanism the body employs to cool down.

Dr Yuta Masuda, director of science at the Paul G. Allen Family Foundation in Seattle, who led the review paper, noted: “In extreme humid heat conditions, sweating is not as efficient, and a person must sweat more to achieve the same heat loss, which contributes to heat strain.”

He added: “How dangerous the conditions are (for those affected) depends on the environment, level of exertion, clothing, personal factors, heat acclimatisation status and protective infrastructure.”

On top of heat illnesses, the report noted that heat strain can worsen workers’ productivity and cause accidents. Heat and dehydration can affect complex cognitive function and motor performance. And when blood flows to the skin to help cool the body, that could weaken the muscles’ ability to perform physical work.

Workers who do not receive fixed wages but are paid based on productivity tend to ignore heat exhaustion symptoms to earn more.

A separate project by the National University of Singapore (NUS) called Project HeatSafe found that delivery workers in South Korea who were paid for every job showed higher physiological strain and were at risk of compromising their health.

Project HeatSafe is examining how rising temperatures affect the health and productivity of people here and in the region.

Dr Richard Horton, editor-in-chief of top medical journal The Lancet, said there is a need to take a less catastrophist approach to climate and health.

“Creating an ambience of anxiety, dread and pessimism is precisely the wrong way to provoke governments and society to address climate change more seriously. (We need) more energising thinking than believing that we’re all going to fry,” added Dr Horton, who was speaking at a plenary by the NUS Yong Loo Lin School of Medicine’s Centre for Sustainable Medicine on March 13.

Echoing the paper’s suggestions, he said: “Employers have a duty of care to their workers. The periods of exposure of workers to high heat stress environments should be reduced. The responsibility is on the employers, and also politicians who need to legislate protections for workers.”

The study published in One Earth also scrutinised several solutions to prevent heat-related illnesses in workers such as acclimatisation, work-rest cycles, and pushing heavy labour to cooler parts of the day.

But if rising temperatures make the cooler early-morning hours of the day hotter and humid, there may be a need to mechanise some outdoor work in construction and agriculture to keep employees safe, it said.

The authors also warned about maladaptation in solutions. A common culprit of maladaptation is air-conditioning. Increasing demand for air-conditioning will raise energy demand, which could release more planet-warming greenhouse gases and waste heat.

The Singapore authorities have been paying more attention to construction workers’ well-being in hot weather.

It is now mandatory for outdoor workers involved in heavy physical activity to get 15 minutes’ rest every hour when Wet Bulb Globe Temperature (WBGT) readings are 33 deg C or higher. WBGT is a measure of heat stress.

Project HeatSafe also trialled education and cooling interventions at a worksite in mid-2023. These included enforcing breaks under shelter, carrying cool water in insulated bottle sleeves, and wearing uniforms that are breathable and ultraviolet-protective.

Construction workers The Straits Times spoke to could easily recall recent instances of colleagues feeling light-headed or fainting due to heat exhaustion.

A worker who does pipeline work for Housing Board blocks and roads, who declined to be named, said: “If it is very hot the whole day, workers will get headaches. Sometimes, there are no restrooms near the roadside. If we are having a hard time, we will rest for 10 to 15 minutes. If there are no HDB buildings nearby, we will rest under trees.”

A 41-year-old migrant worker who works as a safety coordinator for a project in Sin Ming Avenue, said his employer in January gave workers super cooling towels that, upon soaking in water and wrapping around the neck, provide quick relief. He declined to reveal his name due to confidentiality.

Associate Professor Jason Lee, who leads Project HeatSafe and is director of the Heat Resilience and Performance Centre at NUS Medicine, said people in the tropics are like “frogs being boiled slowly” in the humid heat.

He added: “I hope we can scale proven solutions. The scenario of older folk who are already more vulnerable to heat and staying in the workforce for longer is also becoming real. Let’s not ignore indoor workers, too.”