Climate change-fuelled heat stress could cut productivity and fertility, costing billions: NUS study

SINGAPORE — Heat stress can cause lower fertility, worker productivity and cognitive capacity, a study by the National University of Singapore (NUS) has found.

The study was done as climate change results in an increased number of hotter days and warmer nights.

It found that heat stress is set to reduce the total average annual productive working time in four sectors in Singapore surveyed – services, manufacturing, agriculture, and construction – by 14 per cent in 2035.

This could cause an economy-wide output loss of S$2.22 billion, the study concluded.

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Project HeatSafe is the first large-scale study in Singapore and the region aimed at investigating the impact of rising heat levels on the health, productivity and well-being of occupational workers in tropical climates such as Singapore.

Led by the NUS Yong Loo Lin School of Medicine (NUS Medicine), the research project spanned a period between October 2020 and March 2024. Its findings were shared with the media in a press release on Monday (March 18).

The research project comprised nine study areas — including environmental monitoring at workplaces, a fertility and pregnancy study, and an economic analysis — each led by different researchers and spanning different study durations. 

WHY IT MATTERS

Last July, United Nations chief Antonio Guterres warned that the world has moved past global warming and entered an era of “global boiling”.

In January, the Centre for Climate Research Singapore released findings from its third national climate change study, which included projections that Singapore could possibly face very hot days by 2100, where daily maximum temperatures exceed 35°C for up to 351 days in a year.

That study also found that occurrences of high heat stress would be significantly more frequent than occurrences of moderate heat stress.

Studies have found that rising temperatures, extreme heat and high heat stress could lead to several adverse effects.

These include growing health risks, worsening mental health, heat waves, flooding, crop failures, and even the possibility that current buildings and infrastructure may not be designed to withstand intense and prolonged heat.

Project HeatSafe aimed to understand the threats that extreme heat exposure poses to human health, well-being and work productivity in tropical countries such as Singapore — and to find sustainable and scalable solutions to mitigate and reduce the growing impacts of this heat stress.

Associate Professor Jason Lee, the study’s lead principal investigator and director of the Heat Resilience and Performance Centre at NUS Medicine, outlined the role Singapore could play in addressing the problem.

“Given our strategic location, and expertise in heat stress management, Singapore is well-placed to lead the way in developing and deploying solutions to counter the complexities of increasing heat stress and help individuals, communities and society as a whole thrive in this warming world,” he said.

IMPACT OF HEAT STRESS ON FERTILITY AND PREGNANCY

The impact of extreme heat on fertility outcomes for women and men here has not been well-studied in tropical countries such as Singapore, compared to countries with four seasons, NUS Medicine said in its media release.

The NUS researchers collected sperm samples from 818 men, and found that extreme heat exposure between two weeks and two months before semen sample collection was associated with a 40 per cent higher risk of low sperm count.

Extreme heat exposure between one and three months before sample collection was associated with a 40 per cent increased risk of low sperm concentration and a 46 per cent higher risk of low sperm count.

Even though the majority of the men surveyed were facing fertility and other health issues, or undergoing treatments that could already result in lowered sperm quality, the researchers believe that the results would be the same among men with healthy sperm.

The researchers therefore said that men who are planning to conceive in one to three months should be discouraged from unnecessarily going outdoors on extremely hot days.

Those who have no option but to expose themselves to these extreme outdoor conditions should be encouraged to cool their testes, either by staying indoors, wearing looser pants and underwear, and sleeping in cooler temperatures, in the days immediately before they and their wife try conceiving.

The team also analysed the birth records of more than 30,000 mothers over the three pregnancy trimesters to determine the risk of pre-term birth and small-for-gestational-age birth.

The researchers then surveyed the responses of 342 pregnant women to understand behavioural changes they make during pregnancy in response to heat.

They found that exposure to extreme heat during the second trimester was associated with a lower risk of small-for-gestational-age birth in Chinese women, but a higher risk of the same in Malay women.

Suggesting possible reasons for this during a media conference on Monday, the researchers said that women of different ethnicities could respond differently to different temperatures or have different heat stress thresholds.

Another factor could be that pregnant women with more children were more likely to go outdoors on hot days such as taking their children to play, they added.

IMPACT OF HEAT STRESS ON PRODUCTIVITY AND THE ECONOMY

The higher the physical and mental exertion faced in a job role, the higher the productivity and economic losses due to heat stress, the study found.

The team surveyed 355 workers and 214 employers during Singapore’s hottest months of the year — between April and August — and found that economic loss significantly increased with workers’ exposure to adverse environmental conditions at the workplace. 

These can include working under the sun, in semi-outdoor spaces, or being exposed to added sources of heat such as machinery.

The study estimates that for every hot day, the workers’ reduced productivity during working hours translates to a median income loss of S$21 a worker, or about a quarter (24 per cent) of the surveyed workers’ daily median salary.

In a separate Project HeatSafe study, researchers from NUS Medicine surveyed 79 indoor and 76 outdoor construction workers and found that about two-thirds (65 per cent) felt that their productivity was reduced and they tended to work at a slower pace on hot days.

The most common heat-related symptom was fatigue, which was felt by 60 per cent of respondents, while 37 per cent suffered headaches after prolonged workplace heat exposure.

Possible intervention measures to combat the effects of heat stress include enforcing work-rest cycles, regular heat stress education and training for both workers and managers, and water breaks, the research team said.

The researchers also found that a combination of high body core temperature, high skin temperature and increased heart rate affected workers’ posture during attention-demanding tasks such as welding and plank-walking. 

This could impair their decision-making and lead to a higher risk of injury in workers.

Project HeatSafe is funded by Singapore’s National Research Foundation.

Other key partners of the research project include the Singapore-ETH Centre, Singapore’s Ministry of Manpower, and Workplace Safety and Health Institute, and overseas institutions such as the Health and Environment International Trust, Tsinghua University, Vietnam Military Medical University, Institute of Technology of Cambodia and Seoul National University.