Just 1 deg C hotter but outdoor workers’ heat stroke risk spikes 250%: S’pore study

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.
ST PHOTO: LIM YAOHUI

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.

Linking temperature rise to heat stress

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.