SINGAPORE — A worker died and two others are in intensive care after collapsing during a routine tank cleaning at Choa Chu Kang Waterworks on Thursday morning (May 23).

In a statement on Thursday, national water agency PUB said that the three men are sub-contractors and were found unconscious at around 11.15am at Singapore's second-largest waterworks along Nanyang Drive.

"Preliminary investigations indicate that the workers had inhaled hydrogen sulphide, a gas produced from sludge which is a by-product of the water treatment process," PUB added.

The agency said that all three workers were taken unconscious to Ng Teng Fong General Hospital.

One of them, a 40-year-old Indian national who worked as a cleaning operations manager, died in hospital.

The other two, a pair of Malaysian general workers aged 24 and 39, were still in intensive care.

Hydrogen sulphide is a colourless gas that is extremely flammable and highly toxic. It is known for its strong odour of rotten eggs.

In response to CNA's queries, the Ministry of Manpower (MOM) said that it is investigating the incident and has instructed PUB to stop all tank cleaning works.

"As a general safety measure, proper ventilation must be provided to prevent accumulation of toxic vapours," MOM said.

"Workers must also be protected from excessive exposure to toxic vapours by equipping them with gas monitoring devices, and the appropriate personal protective equipment such as respirator mask and breathing apparatus."

PUB said that it is in touch with the contractors and will render support to the workers' family members.

"PUB expresses our deepest condolences to the deceased's family," the agency said.

"Safety and health at all our workplaces are of utmost importance to PUB. We will cooperate fully with the investigation.

"As a precaution, PUB has called a safety timeout for similar operations in confined spaces and is reviewing the associated safety measures."

In response to TODAY's queries, the Singapore Civil Defence Force (SCDF) said that it was alerted to the incident at about 11.25am on Thursday.

"SCDF firefighters and HazMat (Hazardous Material) specialists wearing personal protective equipment, including breathing apparatus sets, entered the premises and identified the presence of a hazardous vapour — hydrogen sulphide."

Two water jets were used to disperse the vapour. 

The three workers were unconscious en route to the hospital and cardiopulmonary resuscitation (CPR) had to be performed on one of them.

SCDF did not state if the worker who died was the same one who received CPR.

No hydrogen sulphide was detected at the site after the water jets were used for an hour, SCDF added.