CNA Explains: Is it safe to swim at Sentosa, after water quality issues at World Aquatics Championships?

CNA

16-07-2025

SINGAPORE: Open water swimming races at the World Aquatics Championships finally went ahead on Wednesday afternoon (Jul 16) after they were postponed twice due to poor water quality. The cause was high levels of E. coli bacteria in the waters off the Sentosa island venue, though the source of the contamination has not been determined.
What is E. coli?
E. coli, or Escherichia coli, is a type of bacteria that naturally lives in the intestines of humans and animals, where it usually poses no threat.
But when found outside the gut, it can cause a range of infections, said Dr Nares Smitasin, a senior consultant with the National University Hospital's (NUH) infectious diseases division.
For example, E. coli commonly causes urinary tract infections, especially in women. The bacteria can also lead to abdominal infections, he added.
While most strains are harmless, certain types can be highly pathogenic. For example, E. coli O157:H7 infection can cause severe abdominal cramps, bloody diarrhoea, and in some cases, hemolytic uremic syndrome – a serious complication that can lead to kidney failure, especially in young children and older adults, said Dr Smitasin.
Outbreaks of this strain of E. coli have been documented around the world and are frequently linked to contaminated food products such as undercooked ground beef, raw vegetables or unpasteurised juices, he added.
In 2023, concerns in Singapore spiked after a viral video claimed E. coli was present in almost half of chicken rice stalls visited by a YouTuber.
Republic Polytechnic lecturer Heng Kiang Soon said at the time that the severity of illness can depend on one's tolerance threshold and health status.
"Research has shown that individuals with a better microbiota (range of microorganisms) balance tend to have less chances of being infected with E. coli as the good bacteria in their gut fight against the bad ones," he said.
How would E. coli get into the sea?
According to World Aquatics' latest set of competition regulations, enterococcus - another type of bacteria - and E. coli are both 'reliable indicators' of faecal contamination from animals, human sewage or effluent, which refers to liquid waste or sewage discharged into the sea.
Large rainstorms may wash animal faeces into the water, carrying pathogens with them.
Ageing sanitary sewer pipes and other wastewater infrastructure can also leak sewage, which then makes its way into water bodies.
Research has found that E. coli can also survive and reproduce in settings such as sand on the beach.
CNA has reached out to Sentosa Development Corporation with questions on why the water quality exceeded acceptable thresholds, the cause of the elevated E. coli levels, and whether it has any advice for the general public which might be wondering if it is safe to swim at Sentosa's beaches.
How does World Aquatics measure water quality?
The international water sports governing body lays out standards in its competition regulations. These take reference from limits proposed by the World Health Organization (WHO), the United States Environmental Protection Agency, the European Environmental Agency, the New Zealand Ministry of Health and the Australian National Health and Research Council.
They state that sea water with less than 250 colony-forming units of E. coli per 100ml is considered of 'excellent' quality, while sea water with less than 500 of such units is 'good'. Anything above 500 units is "unacceptable".
For enterococcus bacteria, below 100 colony-forming units per 100ml is excellent; below 200 is good; and more than 200 is unacceptable.
Water samples are collected from three different locations on along the course, starting from two months before the competition and up to raceday itself.
For World Aquatics, the recommended tolerance levels are not more than 100 colony-forming units per 100ml
for enterococcus and not more than 250 for E. coli.
The federation notes that open-water swimmers may be exposed to contamination from waterborne bacteria through inhalation, swallowing water and by direct skin contact. Bacteria may also infect them through sensitive mucosal linings of the eyes, ears, nose, mouth and upper respiratory tract during prolonged immersion.
This prolonged exposure faced by athletes - for instance, a 10km race could take about two hours or more - explains why water quality standards are higher than those for recreational swimmers.
How does Singapore measure beach water quality?
The National Environment Agency (NEA) adopts WHO's 2021 guidelines for recreational activity – the levels of enterococcus in at least 95 per cent of 100 samples must not exceed 200 counts per 100ml.
Counts per ml account for both viable and non-viable bacterial cells, while colony-forming units per ml focus on the viable, culturable portion only.
E.coli is not part of the parameters NEA uses to assess water quality.
NEA releases weekly updates on its website on enterococcus levels at beaches around Singapore.
Only beaches classified as 'good' and above are suitable for primary contact activities, where the swimmer's whole body, face and trunk are frequently immersed, and they are likely to swallow some water.
All three beaches at Sentosa were graded as good.
In February 2024, NEA advised people against swimming at Pasir Ris Beach and Sembawang Park Beach due to more frequent readings of elevated enterococcus bacteria levels in the water.
As of January 2025, Pasir Ris Beach has been graded "good". But for Sembawang Park Beach, members of the public are still advised to not swim and to not do other primary contact water activities.
NEA however cautioned that beach users should avoid entering the water immediately after heavy rainfall, regardless of water quality.
Those with open sores, skin infections, or who are unwell should also refrain from water activities.
Should the public worry about swimming at Sentosa?
Based on NEA's latest update, no.
The weekly recreational water quality banding for Sentosa's beaches is "normal", with fewer than 200 counts of enterococcus per 100ml in at least 95 per cent of the samples. This means it's safe to enter the water and all beachgoers can continue with water activities.
Latest samples taken by World Aquatics on Tuesday evening also showed levels of E. coli falling between the ranges of "good" to "excellent".
NUH's Dr Smitasin advised members of the public to avoid contact with waters where advisories have been issued about high bacterial levels.
Direct exposure may lead to infections such as conjunctivitis, or eye infections, as well as skin infections, especially if individuals have open cuts or abrasions.