SINGAPORE – Every 1 deg C rise in daily average temperature more than doubles an outdoor worker’s risk of heat stroke, a new study by Singapore researchers has found.
If this temperature increase is sustained over three days, the worker’s odds of having a heat injury nearly quadruple.
The study, led by Adjunct Assistant Professor Joel Aik from the Duke-NUS Medical School, also highlighted that between 2009 and 2023, around 40 outdoor workers suffered heat-related illnesses.
More than half of the reported cases were located in the central and east of Singapore, which had a slightly higher average temperature of around 28 deg C compared with other regions.
This is the first local study to quantify the number of outdoor workers with heat illness over an extended period.
But Prof Aik said the number of cases could have been under-reported as workers may be less likely to seek medical attention for milder cases of heat injuries, such as heat cramps.
The study was published in the scientific journal Urban Climate in early March, and comes as Singapore enters its hottest period of the year, from March to mid-year.
Of the roughly 40 cases reported to the Ministry of Manpower (MOM), about 85 per cent of them suffered from more severe illnesses such as heat stroke and heat exhaustion. Heat stroke symptoms include a high internal body temperature, delirium and seizures. If not treated quickly, heat stroke can rapidly damage the brain, heart, kidneys and muscles.
The remaining had moderate symptoms such as fainting, dizziness and muscle cramps – intense, painful muscle contractions when the body gets too hot.
Most of the cases involved workers from the construction sector, and more than two-thirds of them were foreigners. As at 2025, there are about 440,000 employees in the construction, marine shipyard and process sectors.
The study was also the first to map the cases to the weather conditions the workers were exposed to. There are few studies in tropical cities linking heat exposure and heat illnesses in outdoor workers.
Using data from 60 weather stations, Prof Aik’s team mapped each case to the average temperature and humidity of the day.
They also recorded the daily weather conditions on some days before each heat illness episode.
Through statistical analysis, they found that a 1 deg C rise in average temperature was associated with a 250 per cent increase in the likelihood a worker would suffer from heat illness on the same day.
They also found that a 1 deg C increase in average temperature – sustained over three days – nearly quadrupled a worker’s odds of experiencing a heat injury on the third day.
For example, if the average temperature on a Monday is 28 deg C, and it rises to 29 deg C on Tuesday and Wednesday, the chances of an employee suffering a heat-related illness on Wednesday increase by nearly four times.
“We’re not even talking about temperature extremes yet, like 35 to 36 deg C. This shows that even without considering maximum temperature, with just these fluctuations, people in outdoor occupations who do not sufficiently cool themselves on warmer days could be at risk,” said Prof Aik.
Singapore’s 2025 average temperature was 28.1 deg C.
With the El Nino climate phenomenon associated with drier, hotter weather expected to rear its head in the second half of 2026, the risk of a heatwave being declared could also rise.
In Singapore, a heatwave is declared when the highest daily temperature over three consecutive days is at least 35 deg C, with the mean temperature each day being at least 29 deg C.
When the body tries to cool down, it increases blood flow to the skin. The heat carried by the blood is then released to the air through radiation and convection.
The second method of releasing heat is through perspiration, when sweat evaporates from the skin.
But heat can shift only from a warm region to a cooler region.
“If the air is as warm as my body, I’m not going to be able to lose heat very well,” explained Prof Aik.
When the body is unable to sufficiently get rid of the heat, this can lead to heat-related illnesses.
The researchers also found that elevated moisture in the air takes a slightly longer time to manifest as a danger. When there is a lot of vapour in the air, the perspiration on the skin evaporates more slowly.
“If humidity is very high, perspiration can still evaporate, but it will be slower… A few consecutive days of high humidity without sufficient cooling can mess up your body system,” he explained.
Since 2023, MOM has mandated heat stress measures for outdoor workers. These include acclimatising new workers to Singapore’s weather by gradually increasing their exposure to heat over at least a week.
Workers should hydrate at least every hour, with 300ml of water or more. During periods of high heat stress, employees are urged to rest for at least 15 minutes in the shade for every hour of intense work.
While the MOM measures “lay a strong foundation” for worksites, Prof Aik hopes employers can make more cooling solutions and tailored measures available for at-risk workers.
The paper added that socio-economic factors can worsen the risk of heat illnesses, including migrant workers’ access to medical care and apprehensiveness in seeking medical attention.
Uncomfortable sleeping conditions can also be an obstacle.
Forty heat illness cases over 14 years is not a high number, which means it is often a minority of workers who are affected.
Prof Aik hopes wearables will be used to track core body temperature to identify outdoor workers at higher risk of heat illness, and enable early intervention.
He also suggested attaching cooling devices such as fans to the back of uniforms. But advancements in technology are needed to reduce the size and cost of such solutions, so that employers can access them.
Commenting on the study, Professor Marcus Ong, a senior consultant at Singapore General Hospital’s department of emergency medicine, said early recognition, cooling and immediate treatment reduce complications arising from heat illnesses.
“First response teams should be well-trained and equipped to initiate rapid cooling and first aid on-site.
“In terms of prevention, a review of occupational health policies and practices would be appropriate, especially in sectors with high risk of exposure,” he added.